© MOTOROLA INC., 1992
MC68020
MC68EC020
MICROPROCESSORS
USER’S MANUAL
First Edition
Motorola reserves the right to make changes without further notice to any products herein to improve reliability, function or design. Motorola does not assume any
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Equal Opportunity/Affirmative Action Employer.
MOTOROLA M68020 USER’S MANUAL iii
PREFACE
The
M68020 User’s Manual
describes the capabilities, operation, and programming of the
MC68020 32-bit, second-generation, enhanced microprocessor and the MC68EC020 32-
bit, second-generation, enhanced embedded microprocessor.
Throughout this manual, “MC68020/EC020” is used when information applies to both the
MC68020 and the MC68EC020. “MC68020” and “MC68EC020” are used when
information applies only to the MC68020 or MC68EC020, respectively.
For detailed information on the MC68020 and MC68EC020 instruction set, refer to
M68000PM/AD,
M68000 Family Programmer’s Reference Manual
.
This manual consists of the following sections:
Section 1 Introduction
Section 2 Processing States
Section 3 Signal Description
Section 4 On-Chip Cache Memory
Section 5 Bus Operation
Section 6 Exception Processing
Section 7 Coprocessor Interface Description
Section 8 Instruction Execution Timing
Section 9 Applications Information
Section 10 Electrical Characteristics
Section 11 Ordering Information and Mechanical Data
Appendix A Interfacing an MC68EC020 to a DMA Device That Supports a Three-Wire
Bus Arbitration Protocol
NOTE
In this manual,
assert
and
negate
are used to specify forcing a
signal to a particular state. In particular,
assertion
and
assert
refer to a signal that is active or true;
negation
and
negate
indicate a signal that is inactive or false. These terms are used
independently of the voltage level (high or low) that they
represent.
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TABLE OF CONTENTS
Paragraph Page
Number Title Number
Section 1
Introduction
1.1 Features .................................................................................................. 1-2
1.2 Programming Model ................................................................................ 1-4
1.3 Data Types and Addressing Modes Overview ........................................ 1-8
1.4 Instruction Set Overview ......................................................................... 1-10
1.5 Virtual Memory and Virtual Machine Concepts....................................... 1-10
1.5.1 Virtual Memory .................................................................................... 1-10
1.5.2 Virtual Machine.................................................................................... 1-12
1.6 Pipelined Architecture ............................................................................. 1-12
1.7 Cache Memory........................................................................................ 1-13
Section 2
Processing States
2.1 Privilege Levels....................................................................................... 2-2
2.1.1 Supervisor Privilege Level ................................................................... 2-2
2.1.2 User Privilege Level............................................................................. 2-3
2.1.3 Changing Privilege Level..................................................................... 2-3
2.2 Address Space Types ............................................................................. 2-4
2.3 Exception Processing.............................................................................. 2-5
2.3.1 Exception Vectors................................................................................ 2-5
2.3.2 Exception Stack Frame ....................................................................... 2-6
Section 3
Signal Description
3.1 Signal Index ............................................................................................ 3-2
3.2 Function Code Signals (FC2–FC0)......................................................... 3-2
3.3 Address Bus (A31–A0, MC68020)(A23–A0, MC68EC020) .................... 3-2
3.4 Data Bus (D31–D0)................................................................................. 3-2
3.5 Transfer Size Signals (SIZ1, SIZ0) ......................................................... 3-2
3.6 Asynchronous Bus Control Signals......................................................... 3-4
3.7 Interrupt Control Signals.......................................................................... 3-5
3.8 Bus Arbitration Control Signals ............................................................... 3-6
3.9 Bus Exception Control Signals................................................................ 3-6
3.10 Emulator Support Signal ......................................................................... 3-7
3.11 Clock (CLK)............................................................................................. 3-7
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3.12 Power Supply Connections..................................................................... 3-7
3.13 Signal Summary...................................................................................... 3-8
Section 4
On-Chip Cache Memory
4.1 On-Chip Cache Organization and Operation .......................................... 4-1
4.2 Cache Reset ........................................................................................... 4-3
4.3 Cache Control ......................................................................................... 4-3
4.3.1 Cache Control Register (CACR) ......................................................... 4-3
4.3.2 Cache Address Register (CAAR) ........................................................ 4-4
Section 5
Bus Operation
5.1 Bus Transfer Signals............................................................................... 5-1
5.1.1 Bus Control Signals............................................................................. 5-2
5.1.2 Address Bus........................................................................................ 5-3
5.1.3 Address Strobe.................................................................................... 5-3
5.1.4 Data Bus.............................................................................................. 5-3
5.1.5 Data Strobe ......................................................................................... 5-4
5.1.6 Data Buffer Enable .............................................................................. 5-4
5.1.7 Bus Cycle Termination Signals............................................................ 5-4
5.2 Data Transfer Mechanism....................................................................... 5-5
5.2.1 Dynamic Bus Sizing ............................................................................ 5-5
5.2.2 Misaligned Operands........................................................................... 5-14
5.2.3 Effects of Dynamic Bus Sizing and Operand Misalignment ................ 5-20
5.2.4 Address, Size, and Data Bus Relationships........................................ 5-21
5.2.5 Cache Interactions .............................................................................. 5-22
5.2.6 Bus Operation ..................................................................................... 5-24
5.2.7 Synchronous Operation with DSACK1/DSACK0............................... 5-24
5.3 Data Transfer Cycles .............................................................................. 5-25
5.3.1 Read Cycle.......................................................................................... 5-26
5.3.2 Write Cycle .......................................................................................... 5-33
5.3.3 Read-Modify-Write Cycle..................................................................... 5-39
5.4 CPU Space Cycles ................................................................................. 5-44
5.4.1 Interrupt Acknowledge Bus Cycles...................................................... 5-45
5.4.1.1 Interrupt Acknowledge Cycle—Terminated Normally...................... 5-45
5.4.1.2 Autovector Interrupt Acknowledge Cycle......................................... 5-48
5.4.1.3 Spurious Interrupt Cycle .................................................................. 5-48
5.4.2 Breakpoint Acknowledge Cycle........................................................... 5-50
5.4.3 Coprocessor Communication Cycles .................................................. 5-53
5.5 Bus Exception Control Cycles................................................................. 5-53
5.5.1 Bus Errors ........................................................................................... 5-55
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5.5.2 Retry Operation ................................................................................... 5-56
5.5.3 Halt Operation...................................................................................... 5-60
5.5.4 Double Bus Fault................................................................................. 5-60
5.6 Bus Synchronization................................................................................ 5-62
5.7 Bus Arbitration......................................................................................... 5-62
5.7.1 MC68020 Bus Arbitration .................................................................... 5-63
5.7.1.1 Bus Request (MC68020) ................................................................. 5-66
5.7.1.2 Bus Grant (MC68020)...................................................................... 5-66
5.7.1.3 Bus Grant Acknowledge (MC68020) ............................................... 5-66
5.7.1.4 Bus Arbitration Control (MC68020).................................................. 5-67
5.7.2 MC68EC020 Bus Arbitration ............................................................... 5-70
5.7.2.1 Bus Request (MC68EC020) ............................................................ 5-71
5.7.2.2 Bus Grant (MC68EC020)................................................................. 5-71
5.7.2.3 Bus Arbitration Control (MC68EC020)............................................. 5-73
5.8 Reset Operation ...................................................................................... 5-76
Section 6
Exception Processing
6.1 Exception Processing Sequence ............................................................ 6-1
6.1.1 Reset Exception................................................................................... 6-4
6.1.2 Bus Error Exception............................................................................. 6-4
6.1.3 Address Error Exception...................................................................... 6-6
6.1.4 Instruction Trap Exception................................................................... 6-6
6.1.5 Illegal Instruction and Unimplemented Instruction Exceptions ............ 6-7
6.1.6 Privilege Violation Exception ............................................................... 6-8
6.1.7 Trace Exception................................................................................... 6-9
6.1.8 Format Error Exception ....................................................................... 6-10
6.1.9 Interrupt Exceptions............................................................................. 6-11
6.1.10 Breakpoint Instruction Exception......................................................... 6-17
6.1.11 Multiple Exceptions.............................................................................. 6-17
6.1.12 Return from Exception......................................................................... 6-19
6.2 Bus Fault Recovery................................................................................. 6-21
6.2.1 Special Status Word (SSW)................................................................. 6-21
6.2.2 Using Software to Complete the Bus Cycles....................................... 6-23
6.2.3 Completing the Bus Cycles with RTE.................................................. 6-24
6.3 Coprocessor Considerations................................................................... 6-25
6.4 Exception Stack Frame Formats............................................................. 6-25
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Section 7
Coprocessor Interface Description
7.1 Introduction ............................................................................................. 7-1
7.1.1 Interface Features ............................................................................... 7-2
7.1.2 Concurrent Operation Support ............................................................ 7-2
7.1.3 Coprocessor Instruction Format.......................................................... 7-3
7.1.4 Coprocessor System Interface ............................................................ 7-4
7.1.4.1 Coprocessor Classification .............................................................. 7-4
7.1.4.2 Processor-Coprocessor Interface.................................................... 7-5
7.1.4.3 Coprocessor Interface Register Selection ....................................... 7-6
7.2 Coprocessor Instruction Types ............................................................... 7-7
7.2.1 Coprocessor General Instructions....................................................... 7-8
7.2.1.1 Format ............................................................................................. 7-8
7.2.1.2 Protocol............................................................................................ 7-9
7.2.2 Coprocessor Conditional Instructions.................................................. 7-10
7.2.2.1 Branch on Coprocessor Condition Instruction ................................. 7-12
7.2.2.1.1 Format .......................................................................................... 7-12
7.2.2.1.2 Protocol........................................................................................ 7-12
7.2.2.2 Set on Coprocessor Condition Instruction ....................................... 7-13
7.2.2.2.1 Format .......................................................................................... 7-13
7.2.2.2.2 Protocol........................................................................................ 7-14
7.2.2.3 Test Coprocessor Condition, Decrement, and Branch Instruction... 7-14
7.2.2.3.1 Format .......................................................................................... 7-14
7.2.2.3.2 Protocol........................................................................................ 7-15
7.2.2.4 Trap on Coprocessor Condition Instruction ..................................... 7-15
7.2.2.4.1 Format .......................................................................................... 7-15
7.2.2.4.2 Protocol........................................................................................ 7-16
7.2.3 Coprocessor Context Save and Restore Instructions ......................... 7-16
7.2.3.1 Coprocessor Internal State Frames................................................. 7-17
7.2.3.2 Coprocessor Format Words............................................................. 7-18
7.2.3.2.1 Empty/Reset Format Word........................................................... 7-18
7.2.3.2.2 Not-Ready Format Word .............................................................. 7-19
7.2.3.2.3 Invalid Format Word ..................................................................... 7-19
7.2.3.2.4 Valid Format Word ....................................................................... 7-20
7.2.3.3 Coprocessor Context Save Instruction ............................................ 7-20
7.2.3.3.1 Format .......................................................................................... 7-20
7.2.3.3.2 Protocol........................................................................................ 7-21
7.2.3.4 Coprocessor Context Restore Instruction........................................ 7-22
7.2.3.4.1 Format .......................................................................................... 7-22
7.2.3.4.2 Protocol........................................................................................ 7-23
7.3 Coprocessor Interface Register Set........................................................ 7-24
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Paragraph Page
Number Title Number
7.3.1 Response CIR ..................................................................................... 7-24
7.3.2 Control CIR.......................................................................................... 7-24
7.3.3 Save CIR ............................................................................................. 7-25
7.3.4 Restore CIR ......................................................................................... 7-25
7.3.5 Operation Word CIR ............................................................................ 7-25
7.3.6 Command CIR..................................................................................... 7-25
7.3.7 Condition CIR ...................................................................................... 7-26
7.3.8 Operand CIR ....................................................................................... 7-26
7.3.9 Register Select CIR ............................................................................. 7-27
7.3.10 Instruction Address CIR....................................................................... 7-27
7.3.11 Operand Address CIR ......................................................................... 7-27
7.4 Coprocessor Response Primitives .......................................................... 7-27
7.4.1 ScanPC ............................................................................................... 7-28
7.4.2 Coprocessor Response Primitive General Format.............................. 7-28
7.4.3 Busy Primitive ...................................................................................... 7-30
7.4.4 Null Primitive........................................................................................ 7-31
7.4.5 Supervisor Check Primitive ................................................................. 7-33
7.4.6 Transfer Operation Word Primitive ...................................................... 7-33
7.4.7 Transfer from Instruction Stream Primitive .......................................... 7-34
7.4.8 Evaluate and Transfer Effective Address Primitive ............................. 7-35
7.4.9 Evaluate Effective Address and Transfer Data Primitive..................... 7-35
7.4.10 Write to Previously Evaluated Effective Address Primitive.................. 7-37
7.4.11 Take Address and Transfer Data Primitive.......................................... 7-39
7.4.12 Transfer to/from Top of Stack Primitive ............................................... 7-40
7.4.13 Transfer Single Main Processor Register Primitive ............................. 7-40
7.4.14 Transfer Main Processor Control Register Primitive ........................... 7-41
7.4.15 Transfer Multiple Main Processor Registers Primitive......................... 7-42
7.4.16 Transfer Multiple Coprocessor Registers Primitive ............................. 7-42
7.4.17 Transfer Status Register and ScanPC Primitive.................................. 7-44
7.4.18 Take Preinstruction Exception Primitive .............................................. 7-45
7.4.19 Take Midinstruction Exception Primitive.............................................. 7-47
7.4.20 Take Postinstruction Exception Primitive ............................................ 7-48
7.5 Exceptions............................................................................................... 7-49
7.5.1 Coprocessor-Detected Exceptions...................................................... 7-49
7.5.1.1 Coprocessor-Detected Protocol Violations ...................................... 7-50
7.5.1.2 Coprocessor-Detected Illegal Command or Condition Words ......... 7-51
7.5.1.3 Coprocessor Data-Processing-Related Exceptions......................... 7-51
7.5.1.4 Coprocessor System-Related Exceptions ....................................... 7-51
7.5. 1.5 Format Errors ................................................................................... 7-52
7.5.2 Main-Processor-Detected Exceptions ................................................. 7-52
7.5.2.1 Protocol Violations ........................................................................... 7-52
7.5.2.2 F-Line Emulator Exceptions............................................................. 7-54
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7.5.2.3 Privilege Violations........................................................................... 7-55
7.5.2.4 cpTRAPcc Instruction Traps ............................................................ 7-55
7.5.2.5 Trace Exceptions ............................................................................. 7-55
7.5.2.6 Interrupts.......................................................................................... 7-56
7.5. 2.7 Format Errors................................................................................... 7-57
7.5.2.8 Address and Bus Errors................................................................... 7-57
7.5.3 Coprocessor Reset.............................................................................. 7-58
7.6 Coprocessor Summary ........................................................................... 7-58
Section 8
Instruction Execution Timing
8.1 Timing Estimation Factors ...................................................................... 8-1
8.1.1 Instruction Cache and Prefetch ........................................................... 8-1
8.1.2 Operand Misalignment ........................................................................ 8-2
8.1.3 Bus/Sequencer Concurrency............................................................... 8-2
8.1.4 Instruction Execution Overlap ............................................................. 8-3
8.1.5 Instruction Stream Timing Examples................................................... 8-4
8.2 Instruction Timing Tables........................................................................ 8-9
8.2.1 Fetch Effective Address ...................................................................... 8-13
8.2.2 Fetch Immediate Effective Address..................................................... 8-14
8.2.3 Calculate Effective Address ................................................................ 8-16
8.2.4 Calculate Immediate Effective Address............................................... 8-17
8.2.5 Jump Effective Address....................................................................... 8-19
8.2.6 MOVE Instruction ................................................................................ 8-20
8.2.7 Special-Purpose MOVE Instruction..................................................... 8-29
8.2.8 Arithmetic/Logical Instructions............................................................. 8-30
8.2.9 Immediate Arithmetic/Logical Instructions........................................... 8-31
8.2.10 Binary-Coded Decimal Operations...................................................... 8-32
8.2.11 Single-Operand Instructions................................................................ 8-33
8.2.12 Shift/Rotate Instructions ...................................................................... 8-34
8.2.13 Bit Manipulation Instructions ............................................................... 8-35
8.2.14 Bit Field Manipulation Instructions....................................................... 8-36
8.2.15 Conditional Branch Instructions........................................................... 8-37
8.2.16 Control Instructions.............................................................................. 8-38
8.2.17 Exception-Related Instructions............................................................ 8-39
8.2.18 Save and Restore Operations............................................................. 8-40
Section 9
Applications Information
9.1 Floating-Point Units................................................................................. 9-1
9.2 Byte Select Logic for the MC68020/EC020............................................. 9-5
9.3 Power and Ground Considerations......................................................... 9-9
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9.4 Clock Driver............................................................................................. 9-10
9.5 Memory Interface .................................................................................... 9-11
9.6 Access Time Calculations ....................................................................... 9-12
9.7 Module Support....................................................................................... 9-14
9.7.1 Module Descriptor................................................................................ 9-14
9.7.2 Module Stack Frame ........................................................................... 9-16
9.8 Access Levels ......................................................................................... 9-17
9.8.1 Module Call.......................................................................................... 9-18
9.8.2 Module Return ..................................................................................... 9-19
Section 10
Electrical Characteristics
10.1 Maximum Ratings ................................................................................. 10-1
10.2 Thermal Considerations ........................................................................ 10-1
10.2.1 MC68020 Thermal Characteristics and
DC Electrical Characteristics ........................................................... 10-2
10.2.2 MC68EC020 Thermal Characteristics and
DC Electrical Characteristics ........................................................... 10-4
10.3 AC Electrical Characteristics................................................................. 10-5
Section 11
Ordering Information and Mechanical Data
11.1 Standard Ordering Information.............................................................. 11-1
11.1.1 Standard MC68020 Ordering Information.......................................... 11-1
11.1.2 Standard MC68EC020 Ordering Information .................................... 11-1
11.2 Pin Assignments and Package Dimensions.......................................... 11-2
11.2.1 MC68020 RC and RP Suffix—Pin Assignment ................................. 11-2
11.2.2 MC68020 RC Suffix—Package Dimensions ..................................... 11-3
11.2.3 MC68020 RP Suffix—Package Dimensions...................................... 11-4
11.2.4 MC68020 FC and FE Suffix—Pin Assignment.................................. 11-5
11.2.5 MC68020 FC Suffix—Package Dimensions...................................... 11-6
11.2.6 MC68020 FE Suffix—Package Dimensions ...................................... 11-7
11.2.7 MC68EC020 RP Suffix—Pin Assignment.......................................... 11-8
11.2.8 MC68EC020 RP Suffix—Package Dimensions................................. 11-9
11.2.9 MC68EC020 FG Suffix—Pin Assignment.......................................... 11-10
11.2.10 MC68EC020 FG Suffix—Package Dimensions................................. 11-11
Appendix A
Interfacing an MC68EC020 to a DMA Device That
Supports a Three-Wire Bus Arbitration Protocol
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xiv M68020 USER’S MANUAL MOTOROLA
LIST OF ILLUSTRATIONS
Figure Page
Number Title Number
1-1 MC68020/EC020 Block Diagram ..................................................................... 1-3
1-2 User Programming Model ................................................................................ 1-5
1-3 Supervisor Programming Model Supplement .................................................. 1-6
1-4 Status Register (SR)........................................................................................ 1-7
1-5 Instruction Pipe ................................................................................................ 1-13
2-1 General Exception Stack Frame...................................................................... 2-6
3-1 Functional Signal Groups................................................................................. 3-1
4-1 MC68020/EC020 On-Chip Cache Organization .............................................. 4-2
4-2 Cache Control Register.................................................................................... 4-3
4-3 Cache Address Register.................................................................................. 4-4
5-1 Relationship between External and Internal Signals........................................ 5-2
5-2 Input Sample Window ...................................................................................... 5-2
5-3 Internal Operand Representation..................................................................... 5-6
5-4 MC68020/EC020 Interface to Various Port Sizes............................................ 5-6
5-5 Long-Word Operand Write to Word Port Example........................................... 5-10
5-6 Long-Word Operand Write to Word Port Timing.............................................. 5-11
5-7 Word Operand Write to Byte Port Example ..................................................... 5-12
5-8 Word Operand Write to Byte Port Timing......................................................... 5-13
5-9 Misaligned Long-Word Operand Write to Word Port Example ........................ 5-14
5-10 Misaligned Long-Word Operand Write to Word Port Timing............................ 5-15
5-11 Misaligned Long-Word Operand Read from Word Port Example .................... 5-16
5-12 Misaligned Word Operand Write to Word Port Example.................................. 5-16
5-13 Misaligned Word Operand Write to Word Port Timing..................................... 5-17
5-14 Misaligned Word Operand Read from Word Bus Example.............................. 5-18
5-15 Misaligned Long-Word Operand Write to Long-Word Port Example ............... 5-18
5-16 Misaligned Long-Word Operand Write to Long-Word Port Timing .................. 5-19
5-17 Misaligned Long-Word Operand Read from Long-Word Port Example........... 5-20
5-18 Byte Enable Signal Generation for 16- and 32-Bit Ports.................................. 5-23
5-19 Long-Word Read Cycle Flowchart................................................................... 5-26
5-20 Byte Read Cycle Flowchart.............................................................................. 5-27
5-21 Byte and Word Read Cycles—32-Bit Port ....................................................... 5-28
5-22 Long-Word Read—8-Bit Port ........................................................................... 5-29
5-23 Long-Word Read—16- and 32-Bit Ports.......................................................... 5-30
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LIST OF ILLUSTRATIONS (Continued)
Figure Page
Number Title Number
5-24 Write Cycle Flowchart ...................................................................................... 5-33
5-25 Read-Write-Read Cycles—32-Bit Port............................................................. 5-34
5-26 Byte and Word Write Cycles—32-Bit Port........................................................ 5-35
5-27 Long-Word Operand Write—8-Bit Port ............................................................ 5-36
5-28 Long-Word Operand Write—16-Bit Port........................................................... 5-37
5-29 Read-Modify-Write Cycle Flowchart................................................................. 5-40
5-30 Byte Read-Modify-Write Cycle—32-Bit Port (TAS Instruction) ........................ 5-41
5-31 MC68020/EC020 CPU Space Address Encoding............................................ 5-45
5-32 Interrupt Acknowledge Cycle Flowchart........................................................... 5-46
5-33 Interrupt Acknowledge Cycle Timing................................................................ 5-47
5-34 Autovector Operation Timing ........................................................................... 5-49
5-35 Breakpoint Acknowledge Cycle Flowchart ....................................................... 5-50
5-36 Breakpoint Acknowledge Cycle Timing............................................................ 5-51
5-37 Breakpoint Acknowledge Cycle Timing (Exception Signaled).......................... 5-52
5-38 Bus Error without DSACK1/DSACK0 ............................................................. 5-57
5-39 Late Bus Error with DSACK1/DSACK0 .......................................................... 5-58
5-40 Late Retry......................................................................................................... 5-59
5-41 Halt Operation Timing ...................................................................................... 5-61
5-42 MC68020 Bus Arbitration Flowchart for Single Request.................................. 5-64
5-43 MC68020 Bus Arbitration Operation Timing for Single Request...................... 5-65
5-44 MC68020 Bus Arbitration State Diagram......................................................... 5-67
5-45 MC68020 Bus Arbitration Operation Timing—Bus Inactive ............................. 5-69
5-46 MC68EC020 Bus Arbitration Flowchart for Single Request............................. 5-71
5-47 MC68EC020 Bus Arbitration Operation Timing for Single Request................. 5-72
5-48 MC68EC020 Bus Arbitration State Diagram .................................................... 5-73
5-49 MC68EC020 Bus Arbitration Operation Timing—Bus Inactive ........................ 5-75
5-50 Interface for Three-Wire to Two-Wire Bus Arbitration ...................................... 5-76
5-51 Initial Reset Operation Timing.......................................................................... 5-77
5-52 RESET Instruction Timing................................................................................ 5-78
6-1 Reset Operation Flowchart .............................................................................. 6-5
6-2 Interrupt Pending Procedure ............................................................................ 6-12
6-3 Interrupt Recognition Examples ....................................................................... 6-13
6-4 Assertion of IPEND (MC68020 Only)............................................................... 6-14
6-5 Interrupt Exception Processing Flowchart........................................................ 6-15
6-6 Breakpoint Instruction Flowchart...................................................................... 6-18
6-7 RTE Instruction for Throwaway Four-Word Frame .......................................... 6-20
6-8 Special Status Word Format ............................................................................ 6-22
7-1 F-Line Coprocessor Instruction Operation Word.............................................. 7-3
7-2 Asynchronous Non-DMA M68000 Coprocessor Interface Signal Usage......... 7-5
7-3 MC68020/EC020 CPU Space Address Encodings.......................................... 7-6
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LIST OF ILLUSTRATIONS (Continued)
Figure Page
Number Title Number
7-4 Coprocessor Address Map in MC68020/EC020 CPU Space .......................... 7-7
7-5 Coprocessor Interface Register Set Map......................................................... 7-7
7-6 Coprocessor General Instruction Format (cpGEN).......................................... 7-8
7-7 Coprocessor Interface Protocol for General Category Instructions.................. 7-10
7-8 Coprocessor Interface Protocol for Conditional Category Instructions ............ 7-11
7-9 Branch on Coprocessor Condition Instruction Format (cpBcc.W) ................... 7-12
7-10 Branch on Coprocessor Condition Instruction Format (cpBcc.L)..................... 7-12
7-11 Set on Coprocessor Condition Instruction Format (cpScc).............................. 7-13
7-12 Test Coprocessor Condition, Decrement, and Branch
Instruction Format (cpDBcc)........................................................................... 7-14
7-13 Trap on Coprocessor Condition Instruction Format (cpTRAPcc)..................... 7-15
7-14 Coprocessor State Frame Format in Memory.................................................. 7-17
7-15 Coprocessor Context Save Instruction Format (cpSAVE) ............................... 7-20
7-16 Coprocessor Context Save Instruction Protocol .............................................. 7-21
7-17 Coprocessor Context Restore Instruction Format (cpRESTORE)................... 7-22
7-18 Coprocessor Context Restore Instruction Protocol.......................................... 7-23
7-19 Control CIR Format .......................................................................................... 7-25
7-20 Condition CIR Format ...................................................................................... 7-26
7-21 Operand Alignment for Operand CIR Accesses .............................................. 7-26
7-22 Coprocessor Response Primitive Format ........................................................ 7-28
7-23 Busy Primitive Format...................................................................................... 7-30
7-24 Null Primitive Format........................................................................................ 7-31
7-25 Supervisor Check Primitive Format.................................................................. 7-33
7-26 Transfer Operation Word Primitive Format ...................................................... 7-33
7-27 Transfer from Instruction Stream Primitive Format .......................................... 7-34
7-28 Evaluate and Transfer Effective Address Primitive Format.............................. 7-35
7-29 Evaluate Effective Address and Transfer Data Primitive Format..................... 7-35
7-30 Write to Previously Evaluated Effective Address Primitive Format.................. 7-37
7-31 Take Address and Transfer Data Primitive Format.......................................... 7-39
7-32 Transfer to/from Top of Stack Primitive Format ............................................... 7-40
7-33 Transfer Single Main Processor Register Primitive Format ............................. 7-40
7-34 Transfer Main Processor Control Register Primitive Format ........................... 7-41
7-35 Transfer Multiple Main Processor Registers Primitive Format......................... 7-42
7-36 Register Select Mask Format........................................................................... 7-42
7-37 Transfer Multiple Coprocessor Registers Primitive Format.............................. 7-43
7-38 Operand Format in Memory for Transfer to –(An) ........................................... 7-44
7-39 Transfer Status Register and ScanPC Primitive Format.................................. 7-44
7-40 Take Preinstruction Exception Primitive Format.............................................. 7-45
7-41 MC68020/EC020 Preinstruction Stack Frame................................................. 7-46
7-42 Take Midinstruction Exception Primitive Format.............................................. 7-47
7-43 MC68020/EC020 Midinstruction Stack Frame................................................. 7-47
7-44 Take Postinstruction Exception Primitive Format............................................. 7-48
9/29/95 SECTION 1: OVERVIEW UM Rev 1
MOTOROLA M68020 USER’S MANUAL xvii
LIST OF ILLUSTRATIONS (Concluded)
Figure Page
Number Title Number
7-45 MC68020/EC020 Postinstruction Stack Frame................................................ 7-48
8-1 Concurrent Instruction Execution..................................................................... 8-3
8-2 Instruction Execution for Instruction Timing Purposes ..................................... 8-3
8-3 Processor Activity for Example 1 ..................................................................... 8-5
8-4 Processor Activity for Example 2 ..................................................................... 8-6
8-5 Processor Activity for Example 3 ..................................................................... 8-7
8-6 Processor Activity for Example 4 ..................................................................... 8-8
9-1 32-Bit Data Bus Coprocessor Connection........................................................ 9-2
9-2 Chip Select Generation PAL ............................................................................ 9-3
9-3 Chip Select PAL Equations.............................................................................. 9-4
9-4 Bus Cycle Timing Diagram............................................................................... 9-4
9-5 Example MC68020/EC020 Byte Select PAL System Configuration ................ 9-7
9-6 MC68020/EC020 Byte Select PAL Equations.................................................. 9-8
9-7 High-Resolution Clock Controller ..................................................................... 9-11
9-8 Alternate Clock Solution................................................................................... 9-11
9-9 Access Time Computation Diagram................................................................. 9-12
9-10 Module Descriptor Format................................................................................ 9-15
9-11 Module Entry Word .......................................................................................... 9-15
9-12 Module Call Stack Frame................................................................................. 9-16
9-13 Access Level Control Bus Registers ................................................................ 9-17
10-1 Drive Levels and Test Points for AC Specifications ....................................... 10-6
10-2 Clock Input Timing Diagram ........................................................................... 10-7
10-3 Read Cycle Timing Diagram .......................................................................... 10-11
10-4 Write Cycle Timing Diagram........................................................................... 10-12
10-5 Bus Arbitration Timing Diagram ..................................................................... 10-13
A-1 Bus Arbitration Circuit—MC68EC020 (Two-Wire) to DMA (Three-Wire) ......... A-1
9/29/95 SECTION 1: OVERVIEW UM Rev.1.0
xviii M68020 USER’S MANUAL MOTOROLA
LIST OF TABLES
Table Page
Number Title Number
1-1 Addressing Modes ........................................................................................... 1-9
1-2 Instruction Set .................................................................................................. 1-11
2-1 Address Space Encodings............................................................................... 2-4
3-1 Signal Index ..................................................................................................... 3-3
3-2 Signal Summary............................................................................................... 3-8
5-1 DSACK1/DSACK0 Encodings and Results .................................................... 5-5
5-2 SIZ1, SIZ0 Signal Encoding............................................................................. 5-7
5-3 Address Offset Encodings ............................................................................... 5-7
5-4 Data Bus Requirements for Read Cycles ........................................................ 5-8
5-5 MC68020/EC020 Internal to External Data Bus Multiplexer—
Write Cycles ................................................................................................... 5-9
5-6 Memory Alignment and Port Size Influence on Read/Write Bus Cycles.......... 5-20
5-7 Data Bus Byte Enable Signals for Byte, Word, and Long-Word Ports............. 5-22
5-8 DSACK1/DSACK0, BERR, HALT Assertion Results..................................... 5-54
6-1 Exception Vector Assignments ........................................................................ 6-3
6- 2 Tracing Control ................................................................................................ 6-9
6-3 Interrupt Levels and Mask Values.................................................................... 6-12
6-4 Exception Priority Groups ................................................................................ 6-18
6-5 Exception Stack Frames.................................................................................. 6-26
7-1 cpTRAPcc Opmode Encodings........................................................................ 7-16
7-2 Coprocessor Format Word Encodings............................................................. 7-18
7-3 Null Coprocessor Response Primitive Encodings............................................ 7-32
7-4 Valid Effective Address Field Codes................................................................ 7-36
7-5 Main Processor Control Register Select Codes............................................... 7-41
7-6 Exceptions Related to Primitive Processing .................................................... 7-53
8-1 Examples 1–4 Instruction Stream Execution Comparison............................... 8-8
8-2 Instruction Timings from Timing Tables ........................................................... 8-11
8-3 Observed Instruction Timings .......................................................................... 8-11
9/29/95 SECTION 1: OVERVIEW UM Rev 1
MOTOROLA M68020 USER’S MANUAL xix
LIST OF TABLES (Continued)
Table Page
Number Title Number
9-1 Data Bus Activity for Byte, Word, and Long-Word Ports.................................. 9-6
9-2 VCC and GND Pin Assignments—MC68EC020 PPGA (RP Suffix) ................. 9-10
9-3 VCC and GND Pin Assignments—MC68EC020 PQFP (FG Sufffix)................. 9-10
9-4 Memory Access Time Equations at 16.67 and 25 MHz ................................... 9-13
9-5 Calculated tAVDV Values for Operation at Frequencies
Less Than or Equal to the CPU Maximum Frequency Rating........................ 9-14
9-6 Access Status Register Codes......................................................................... 9-18
10-1 θJA vs. Airflow—MC68020 CQFP Package ................................................... 10-3
10-2 Power vs. Rated Frequency (at TJ Maximum = 110°C) ................................. 10-3
10-3 Temperature Rise of Board vs. PD—MC68020 CQFP Package ................... 10-3
10-4 θJA vs. Airflow—MC68EC020 PQFP Package .............................................. 10-4
MOTOROLA M68020 USER’S MANUAL v
MC68020/EC020 ACRONYM LIST
BCD — Binary-Coded Decimal
CAAR — Cache Address Register
CACR — Cache Control Register
CCR — Condition Code Register
CIR — Coprocessor Interface Register
CMOS — Complementary Metal Oxide Semiconductor
CPU — Central Processing Unit
CQFP — Ceramic Quad Flat Pack
DDMA — Dual-Channel Direct Memory Access
DFC — Destination Function Code Register
DM A — Direct Memory Access
DRAM — Dynamic Random Access Memory
FPCP — Floating-Point Coprocessor
HCMOS — High-Density Complementary Metal Oxide Semiconductor
IEEE — Institute of Electrical and Electronic Engineers
I SP — Interrupt Stack Pointer
LMB — Lower Middle Byte
LRAR — Limited Rate Auto Request
L S B — Least Significant Byte
MMU — Memory Management Unit
MPU — Microprocessor Unit
M S B — Most Significant Byte
M S P — Master Stack Pointer
NMO S — n-Type Metal Oxide Semiconductor
PAL — Programmable Array Logic
PC — Program Counter
PGA — Pin Grid Array
PMMU — Paged Memory Management Unit
PPGA — Plastic Pin Grid Array
PQFP — Plastic Quad Flat Pack
RAM — Random Access Memory
SF C — Source Function Code Register
S P — Stack Pointer
S R — Status Register
S S P — Supervisor Stack Pointer
S SW — Special Status Word
UMB — Upper Middle Byte
U S P — User Stack Pointer
VBR — Vector Base Register
VLSI — Very Large Scale Integration
MOTOROLA M68020 USER’S MANUAL 1-1
SECTION 1
INTRODUCTION
The MC68020 is the first full 32-bit implementation of the M68000 family of
microprocessors from Motorola. Using VLSI technology, the MC68020 is implemented
with 32-bit registers and data paths, 32-bit addresses, a rich instruction set, and versatile
addressing modes.
The MC68020 is object-code compatible with earlier members of the M68000 family and
has the added features of new addressing modes in support of high-level languages, an
on-chip instruction cache, and a flexible coprocessor interface with full IEEE floating-point
support (the MC68881 and MC68882). The internal operations of this microprocessor
operate in parallel, allowing multiple instructions to be executed concurrently.
The asynchronous bus structure of the MC68020 uses a nonmultiplexed bus with 32 bits
of address and 32 bits of data. The processor supports a dynamic bus sizing mechanism
that allows the processor to transfer operands to or from external devices while
automatically determining device port size on a cycle-by-cycle basis. The dynamic bus
interface allows access to devices of differing data bus widths, in addition to eliminating all
data alignment restrictions.
The MC68EC020 is an economical high-performance embedded microprocessor based
on the MC68020 and has been designed specifically to suit the needs of the embedded
microprocessor market. The major differences in the MC68EC020 and the MC68020 are
that the MC68EC020 has a 24-bit address bus and does not implement the following
signals: ECS, OCS, DBEN, IPEND, and BGACK. Also, the available packages and
frequencies differ for the MC68020 and MC68EC020 (see Section 11 Ordering
Information and Mechanical Data.) Unless otherwise stated, information in this manual
applies to both the MC68020 and the MC68EC020.
1-2 M68020 USER’S MANUAL MOTOROLA
1.1 FEATURES
The main features of the MC68020/EC020 are as follows:
• Object-Code Compatible with Earlier M68000 Microprocessors
• Addressing Mode Extensions for Enhanced Support of High-Level Languages
• New Bit Field Data Type Accelerates Bit-Oriented Applications—e.g., Video Graphics
• An On-Chip Instruction Cache for Faster Instruction Execution
• Coprocessor Interface to Companion 32-Bit Peripherals—the MC68881 and
MC68882 Floating-Point Coprocessors and the MC68851 Paged Memory
Management Unit
• Pipelined Architecture with High Degree of Internal Parallelism Allowing Multiple
Instructions To Be Executed Concurrently
• High-Performance Asynchronous Bus Is Nonmultiplexed and Full 32 Bits
• Dynamic Bus Sizing Efficiently Supports 8-/16-/32-Bit Memories and Peripherals
• Full Support of Virtual Memory and Virtual Machine
• Sixteen 32-Bit General-Purpose Data and Address Registers
• Two 32-Bit Supervisor Stack Pointers and Five Special-Purpose Control Registers
• Eighteen Addressing Modes and Seven Data Types
• 4-Gbyte Direct Addressing Range for the MC68020
• 16-Mbyte Direct Addressing Range for the MC68EC020
• Selection of Processor Speeds for the MC68020: 16.67, 20, 25, and 33.33 MHz
• Selection of Processor Speeds for the MCEC68020: 16.67 and 25 MHz
A block diagram of the MC68020/EC020 is shown in Figure 1-1.
MOTOROLA M68020 USER’S MANUAL 1-3
SEQUENCER AND CONTROL
CONTROL
STORE
CONTROL
LOGIC
INSTRUCTION
CACHE
STAGE
B
STAGE
C
STAGE
D
CACHE
HOLDING
R
EGISTE
R
(CAHR)
INTERNAL
D
ATA
B
US
INSTRUCTION PIPE
INSTRUCTION
ADDRES
S
BU
S
ADDRESS
SECTION
PROGRAM
COUNTE
R
SECTION
DATA
S
ECTIO
N
EXECUTION UNIT
MISALIGNMENT
MULTIPLEXER
SIZE
M
ULTIPLEXE
R
WRITE PENDING
BUFFER
PREFETCH PENDING
BUFFER
MICROBUS
CONTROL LOGIC
BUS CONTROLLER
BUS CONTROL
SIGNALS
ADDRESS
BU
S
ADDRESS
PADS
DATA
PADS
DATA
BUS
3
2-BI
T
ADDRESS
BUS
3
2-BI
T
*
*
24-Bit for MC68EC020
Figure 1-1. MC68020/EC020 Block Diagram
1-4 M68020 USER’S MANUAL MOTOROLA
1.2 PROGRAMMING MODEL
The programming model of the MC68020/EC020 consists of two groups of registers, the
user model and the supervisor model, that correspond to the user and supervisor privilege
levels, respectively. User programs executing at the user privilege level use the registers
of the user model. System software executing at the supervisor level uses the control
registers of the supervisor level to perform supervisor functions.
As shown in the programming models (see Figures 1-2 and 1-3), the MC68020/EC020
has 16 32-bit general-purpose registers, a 32-bit PC two 32-bit SSPs, a 16-bit SR, a 32-bit
VBR, two 3-bit alternate function code registers, and two 32-bit cache handling (address
and control) registers.
The user programming model remains unchanged from earlier M68000 family
microprocessors. The supervisor programming model supplements the user programming
model and is used exclusively by MC68020/EC020 system programmers who utilize the
supervisor privilege level to implement sensitive operating system functions. The
supervisor programming model contains all the controls to access and enable the special
features of the MC68020/EC020. All application software, written to run at the
nonprivileged user level, migrates to the MC68020/EC020 from any M68000 platform
without modification.
Registers D7–D0 are data registers used for bit and bit field (1 to 32 bits), byte (8 bit),
word (16 bit), long-word (32 bit), and quad-word (64 bit) operations. Registers A6–A0 and
the USP, ISP, and MSP are address registers that may be used as software stack
pointers or base address registers. Register A7 (shown as A7 in Figure 1-2 and as A7 ′
and A7″ in Figure 1-3) is a register designation that applies to the USP in the user
privilege level and to either the ISP or MSP in the supervisor privilege level. In the
supervisor privilege level, the active stack pointer (interrupt or master) is called the SSP.
In addition, the address registers may be used for word and long-word operations. All of
the 16 general-purpose registers (D7–D0, A7–A0) may be used as index registers.
The PC contains the address of the next instruction to be executed by the
MC68020/EC020. During instruction execution and exception processing, the processor
automatically increments the contents of the PC or places a new value in the PC, as
appropriate.
MOTOROLA M68020 USER’S MANUAL 1-5
0
7
8
15
16
31
D0
D1
D2
D3
D4
D5
D6
D7
D
ATA
R
EGISTER
S
0
15
16
31
A0
A1
A2
A3
A4
A5
A6
A
DDRESS
R
EGISTER
S
0
15
16
31
A
7 (USP
)
PC
C
C
R
C
ONDITION COD
E
R
EGISTER
7
8
0
31
15
0
P
ROGRAM
C
OUNTER
U
SER STACK
P
OINTER
0
Figure 1-2. User Programming Model
1-6 M68020 USER’S MANUAL MOTOROLA
0
7
8
15
16
31
SR
V
B
R
0
31
C
AC
R
C
AA
R
0
31
C
ACHE ADDRESS
R
EGISTER
C
ACHE CONTRO
L
R
EGISTER
15
16
15
0
0
0
31
(
CCR
)
0
2
3
31
31
S
F
C
A
7' (ISP
)
A
7'' (MSP
)
I
NTERRUPT STACK
P
OINTER
M
ASTER STACK
P
OINTER
S
TATUS
R
EGISTER
V
ECTOR BASE
R
EGISTER
D
F
C
A
LTERNATE
F
UNCTION CODE
R
EGISTERS
Figure 1-3. Supervisor Programming Model Supplement
MOTOROLA M68020 USER’S MANUAL 1-7
The SR (see Figure 1-4) stores the processor status. It contains the condition codes that
reflect the results of a previous operation and can be used for conditional instruction
execution in a program. The condition codes are extend (X), negative (N), zero (Z),
overflow (V), and carry (C). The user byte, which contains the condition codes, is the only
portion of the SR information available in the user privilege level, and it is referenced as
the CCR in user programs. In the supervisor privilege level, software can access the entire
SR, including the interrupt priority mask (three bits) and control bits that indicate whether
the processor is in:
1. One of two trace modes (T1, T0)
2. Supervisor or user privilege level (S)
3. Master or interrupt mode (M)
015
C
1
V
2
Z
3
N
4
X
5
0
6
0
7
0
8
I0
9
I1
10
I2
11
0
12
M
13
S
14
T0T1
SYSTEM BYTE USER BYTE
(
CONDITION CODE REGISTER
)
TRACE
E
NABL
E
INTERRUPT
PRIORITY MASK
SUPERVISOR/USER LEVE
L
MASTER/INTERRUPT MOD
E
C
ARR
Y
O
VERFLOW
Z
ER
O
N
EGATIV
E
E
XTEN
D
Figure 1-4. Status Register (SR)
The VBR contains the base address of the exception vector table in memory. The
displacement of an exception vector is added to the value in this register to access the
vector table.
The alternate function code registers, SFC and DFC, contain 3-bit function codes. For the
MC68020, function codes can be considered extensions of the 32-bit linear address that
optionally provide as many as eight 4-Gbyte address spaces; for the MC68EC020,
function codes can be considered extensions of the 24-bit linear address that optionally
provide as many as eight 16-Mbyte address spaces. Function codes are automatically
generated by the processor to select address spaces for data and program at the user
and supervisor privilege levels and to select a CPU address space for processor functions
(e.g., coprocessor communications). Registers SFC and DFC are used by certain
instructions to explicitly specify the function codes for operations.
The CACR controls the on-chip instruction cache of the MC68020/EC020. The CAAR
stores an address for cache control functions.
1-8 M68020 USER’S MANUAL MOTOROLA
1.3 DATA TYPES AND ADDRESSING MODES OVERVIEW
For detailed information on the data types and addressing modes supported by the
MC68020/EC020, refer to M68000PM/AD,
M68000 Family Programmer’s Reference
Manual
.
The MC68020/EC020 supports seven basic data types:
1. Bits
2. Bit Fields (Fields of consecutive bits, 1–32 bits long)
3. BCD Digits (Packed: 2 digits/byte, Unpacked: 1 digit/byte)
4. Byte Integers (8 bits)
5. Word Integers (16 bits)
6. Long-Word Integers (32 bits)
7. Quad-Word Integers (64 bits)
In addition, the MC68020/EC020 instruction set supports operations on other data types
such as memory addresses. The coprocessor mechanism allows direct support of floating-
point operations with the MC68881 and MC68882 floating-point coprocessors as well as
specialized user-defined data types and functions.
The 18 addressing modes listed in Table 1-1 include nine basic types:
1. Register Direct
2. Register Indirect
3. Register Indirect with Index
4. Memory Indirect
5. PC Indirect with Displacement
6. PC Indirect with Index
7. PC Memory Indirect
8. Absolute
9. Immediate
The register indirect addressing modes have postincrement, predecrement, displacement,
and index capabilities. The PC modes have index and offset capabilities. Both modes are
extended to provide indirect reference through memory. In addition to these addressing
modes, many instructions implicitly specify the use of the CCR, stack pointer, and/or PC.
MOTOROLA M68020 USER’S MANUAL 1-9
Table 1-1. Addressing Modes
Addressing Modes Syntax
Register Direct
Data
Address Dn
An
Register Indirect
Address
Address with Postincrement
Address with Predecrement
Address with Displacement
(An)
(An)+
–(An)
(d 16 , An)
Address Register Indirect with Index
8-Bit Displacement
Base Displacement (d 8, An, Xn)
(bd, An, Xn)
Memory Indirect
Postindexed
Preindexed ([bd, An], Xn, od)
([bd, An, Xn], od)
PC Indirect with Displacement (d16 , PC)
PC Indirect with Index
8-Bit Displacement
Base Displacement (d 8, PC, Xn)
(bd, PC, Xn)
PC Indirect
Postindexed
Preindexed ([bd, PC], Xn, od)
([bd, PC, Xn], od)
Absolute Data Addressing
Short
Long (xxx).W
(xxx).L
Immediate #<data>
NOTE:
Dn = Data Register, D7–D0
An = Address Register, A7–A0
d8, d16 = A twos complement or sign-extended displacement added as part
of the effective address calculation; size is 8 (d8) or 16 (d16) bits;
when omitted, assemblers use a value of zero.
Xn = Address or data register used as an index register; form is
Xn.SIZE*SCALE, where SIZE is .W or .L (indicates index register
size) and SCALE is 1, 2, 4, or 8 (index register is multiplied by
SCALE); use of SIZE and/or SCALE is optional.
bd = A twos-complement base displacement; when present, size can be
16 or 32 bits.
od = Outer displacement added as part of effective address calculation
after any memory indirection; use is optional with a size of 16 or 32
bits.
PC = Program Counter
<data> = Immediate value of 8, 16, or 32 bits
( ) = Effective Address
[ ] = Use as indirect access to long-word address.
1-10 M68020 USER’S MANUAL MOTOROLA
1.4 INSTRUCTION SET OVERVIEW
For detailed information on the MC68020/EC020 instruction set, refer to M68000PM/AD,
M68000 Family Programmer’s Reference Manual
.
The instructions in the MC68020/EC020 instruction set are listed in Table 1-2. The
instruction set has been tailored to support structured high-level languages and
sophisticated operating systems. Many instructions operate on bytes, words, or long
words, and most instructions can use any of the 18 addressing modes.
1.5 VIRTUAL MEMORY AND VIRTUAL MACHINE CONCEPTS
The full addressing range of the MC68020 is 4 Gbytes (4,294,967,296 bytes) in each of
eight address spaces; the full addressing range of the MC68EC020 is 16 Mbytes
(16,777,216 bytes) in each of the eight address spaces. Even though most systems
implement a smaller physical memory, the system can be made to appear to have a full 4
Gbytes (MC68020) or 16 Mbytes (MC68EC020) of memory available to each user
program by using virtual memory techniques.
In a virtual memory system, a user program can be written as if it has a large amount of
memory available, although the physical memory actually present is much smaller.
Similarly, a system can be designed to allow user programs to access devices that are not
physically present in the system, such as tape drives, disk drives, printers, terminals, and
so forth. With proper software emulation, a physical system can appear to be any other
M68000 computer system to a user program, and the program can be given full access to
all of the resources of that emulated system. Such an emulated system is called a virtual
machine.
1.5.1 Virtual Memory
A system that supports virtual memory has a limited amount of high-speed physical
memory that can be accessed directly by the processor and maintains an image of a
much larger virtual memory on a secondary storage device such as a large-capacity disk
drive. When the processor attempts to access a location in the virtual memory map that is
not resident in physical memory, a page fault occurs. The access to that location is
temporarily suspended while the necessary data is fetched from secondary storage and
placed in physical memory. The suspended access is then either restarted or continued.
The MC68020/EC020 uses instruction continuation to support virtual memory. When a
bus cycle is terminated with a bus error, the microprocessor suspends the current
instruction and executes the virtual memory bus error handler. When the bus error handler
has completed execution, it returns control to the program that was executing when the
error was detected, reruns the faulted bus cycle (when required), and continues the
suspended instruction.
MOTOROLA M68020 USER’S MANUAL 1-11
Table 1-2. Instruction Set
Mnemonic Description Mnemonic Description
ABCD Add Decimal with Extend MOVE USP Move User Stack Pointer
ADD Add MOVEC Move Control Register
ADDA Add Address MOVEM Move Multiple Registers
ADDI Add Immediate MOVEP Move Peripheral
ADDQ Add Quick MOVEQ Move Quick
ADDX Add with Extend MOVES Move Alternate Address Space
AND Logical AND MULS Signed Multiply
ANDI Logical AND Immediate MULU Unsigned Multiply
ASL, ASR Arithmetic Shift Left and Right NBCD Negate Decimal with Extend
Bcc Branch Conditionally NEG Negate
BCHG Test Bit and Change NEGX Negate with Extend
BCLR Test Bit and Clear NOP No Operation
BFCHG Test Bit Field and Change NOT Logical Complement
BFCLR Test Bit Field and Clear OR Logical Inclusive OR
BFEXTS Signed Bit Field Extract ORI Logical Inclusive OR Immediate
BFEXTU Unsigned Bit Field Extract ORI CCR Logical Inclusive Or Immediate to Condition Codes
BFFFO Bit Field Find First One ORI SR Logical Inclusive OR Immediate to Status Register
BFINS Bit Field Insert PACK Pack BCD
BFSET Test Bit Field and Set PEA Push Effective Address
BFTST Test Bit Field RESET Reset External Devices
BKPT Breakpoint ROL, ROR Rotate Left and Right
BRA Branch Always ROXL,ROXR Rotate with Extend Left and Right
BSET Test Bit and Set RTD Return and Deallocate
BSR Branch to Subroutine RTE Return from Exception
BTST Test Bit RTM Return from Module
CALLM Call Module RTR Return and Restore Codes
CAS Compare and Swap Operands RTS Return from Subroutine
CAS2 Compare and Swap Dual Operands SBCD Subtract Decimal with Extend
CHK Check Register Against Bound Scc Set Conditionally
CHK2 Check Register Against Upper and Lower Bound STOP Stop
CLR Clear SUB Subtract
CMP Compare SUBA Subtract Address
CMPA Compare Address SUBI Subtract Immediate
CMPI Compare Immediate SUBQ Subtract Quick
CMPM Compare Memory to Memory SUBX Subtract with Extend
CMP2 Compare Register Against Upper and Lower Bounds SWAP Swap Register Words
DBcc Test Condition, Decrement and Branch TAS Test and Set an Operand
DIVS, DIVSL Signed Divide TRAP Trap
DIVU, DIVUL Unsigned Divide TRAPcc Trap Conditionally
EOR Logical Exclusive OR TRAPV Trap on Overflow
EORI Logical Exclusive Or Immediate TST Test Operand
EXG Exchange Registers UNLK Unlink
EXT, EXTB Sign Extend UNPK Unpack BCD
ILLEGAL Take Illegal Instruction Trap
JMP Jump COPROCESSOR INSTRUCTIONS
JSR Jump to Subroutine Mnemonic Description
LEA Load Effective Address cpBcc Branch Conditionally
LINK Link and Allocate cpDBcc Test Coprocessor Condition, Decrement and Branch
LSL, LSR Logical Shift Left and Right cpGEN Coprocessor General Instruction
MOVE Move cpRESTORE Restore Internal State of Coprocessor
MOVEA Move Address cpSAVE Save Internal State of Coprocessor
MOVE CCR Move Condition Code Register cpScc Set Conditionally
MOVE SR Move Status Register cpTRAPcc Trap Conditionally
1-12 M68020 USER’S MANUAL MOTOROLA
1.5.2 Virtual Machine
A typical use for a virtual machine system is the development of software, such as an
operating system, for a new machine also under development and not yet available for
programming use. In a virtual machine system, a governing operating system emulates
the hardware of the new machine and allows the new software to be executed and
debugged as though it were running on the new hardware. Since the new software is
controlled by the governing operating system, it is executed at a lower privilege level than
the governing operating system. Thus, any attempts by the new software to use virtual
resources that are not physically present (and should be emulated) are trapped to the
governing operating system and performed by its software.
In the MC68020/EC020 implementation of a virtual machine, the virtual application runs at
the user privilege level. The governing operating system executes at the supervisor
privilege level and any attempt by the new operating system to access supervisor
resources or execute privileged instructions causes a trap to the governing operating
system.
Instruction continuation is used to support virtual I/O devices in memory-mapped
input/output systems. Control and data registers for the virtual device are simulated in the
memory map. An access to a virtual register causes a fault, and the function of the
register is emulated by software.
1.6 PIPELINED ARCHITECTURE
The MC68020/EC020 contains a three-word instruction pipe where instruction opcodes
are decoded. As shown in Figure 1-5, instruction words (instruction operation words and
all extension words) enter the pipe at stage B and proceed to stages C and D. An
instruction word is completely decoded when it reaches stage D of the pipe. Each stage
has a status bit that reflects whether the word in the stage was loaded with data from a
bus cycle that was terminated abnormally. Stages of the pipe are only filled in response to
specific prefetch requests issued by the sequencer.
Words are loaded into the instruction pipe from the cache holding register. Although the
individual stages of the pipe are only 16 bits wide, the cache holding register is 32 bits
wide and contains the entire long word. This long word is obtained from the instruction
cache or the external bus in response to a prefetch request from the sequencer. When the
sequencer requests an even-word (long-word-aligned) prefetch, the entire long word is
accessed from the instruction cache or the external bus and loaded into the cache holding
register, and the high-order word is also loaded into stage B of the pipe. The instruction
word for the next sequential prefetch can then be accessed directly from the cache
holding register, and no external bus cycle or instruction cache access is required. The
cache holding register provides instruction words to the pipe regardless of whether the
instruction cache is enabled or disabled.
MOTOROLA M68020 USER’S MANUAL 1-13
SEQUENCER
CACHE
HOLDING
REGISTER
CONTROL
UNIT
EXECUTION
UNIT
STAGE
D
INSTRUCTION PIPE
STAGE
CSTAGE
B
INSTRUCTION
FLOW FROM
CACHE AND
MEMORY
Figure 1-5. Instruction Pipe
The sequencer is either executing microinstructions or awaiting completion of accesses
that are necessary to continue executing microcode. The bus controller is responsible for
all bus activity. The sequencer controls the bus controller, instruction execution, and
internal processor operations such as the calculation of effective addresses and the
setting of condition codes. The sequencer initiates instruction word prefetches and
controls the validation of instruction words in the instruction pipe.
Prefetch requests are simultaneously submitted to the cache holding register, the
instruction cache, and the bus controller. Thus, even if the instruction cache is disabled,
an instruction prefetch may hit in the cache holding register and cause an external bus
cycle to be aborted.
1.7 CACHE MEMORY
Due to locality of reference, instructions that are used in a program have a high probability
of being reused within a short time. Additionally, instructions that reside in proximity to the
instructions currently in use also have a high probability of being utilized within a short
period. To exploit these locality characteristics, the MC68020/EC020 contains an on-chip
instruction cache.
The cache improves the overall performance of the system by reducing the number of bus
cycles required by the processor to fetch information from memory and by increasing the
bus bandwidth available for other bus masters in the system.
MOTOROLA M68020 USER’S MANUAL 2-1
SECTION 2
PROCESSING STATES
This section describes the processing states of the MC68020/EC020. It describes the
functions of the bits in the supervisor portion of the SR and the actions taken by the
processor in response to exception conditions.
Unless the processor has halted, it is always in either the normal or the exception
processing state. Whenever the processor is executing instructions or fetching instructions
or operands, it is in the normal processing state. The processor is also in the normal
processing state while it is storing instruction results or communicating with a
coprocessor.
NOTE
Exception processing refers specifically to the transition from
normal processing of a program to normal processing of
system routines, interrupt routines, and other exception
handlers. Exception processing includes all stacking
operations, the fetch of the exception vector, and the filling of
the instruction pipe caused by an exception. Exception
processing has completed when execution of the first
instruction of the exception handler routine begins.
The processor enters the exception processing state when an interrupt is acknowledged,
when an instruction is traced or results in a trap, or when some other exception condition
arises. Execution of certain instructions or unusual conditions occurring during the
execution of any instruction can cause exceptions. External conditions, such as interrupts,
bus errors, and some coprocessor responses, also cause exceptions. Exception
processing provides an efficient transfer of control to handlers and routines that process
the exceptions.
A catastrophic system failure occurs whenever the processor receives a bus error or
generates an address error while in the exception processing state. This type of failure
halts the processor. For example, if during the exception processing of one bus error
another bus error occurs, the MC68020/EC020 has not completed the transition to normal
processing and has not completed saving the internal state of the machine; therefore, the
processor assumes that the system is not operational and halts. Only an external reset
can restart a halted processor. (When the processor executes a STOP instruction, it is in a
special type of normal processing state—one without bus cycles. It is stopped, not halted.)
2-2 M68020 USER’S MANUAL MOTOROLA
2.1 PRIVILEGE LEVELS
The processor operates at one of two privilege levels: the user level or the supervisor
level . The supervisor level has higher privileges than the user level. Not all processor or
coprocessor instructions are permitted to execute at the lower privileged user level, but all
are available at the supervisor level. This arrangement allows a separation of supervisor
and user so the supervisor can protect system resources from uncontrolled access. The
S-bit in the SR is used to select either the user or supervisor privilege level and either the
USP or an SSP for stack operations. The processor identifies a bus access (supervisor or
user mode) via the function codes so that differentiation between supervisor level and
user level can be maintained.
In many systems, the majority of programs execute at the user level. User programs can
access only their own code and data areas and can be restricted from accessing other
information. The operating system typically executes at the supervisor privilege level. It
has access to all resources, performs the overhead tasks for the user-level programs, and
coordinates user-level program activities.
2.1.1 Supervisor Privilege Level
The supervisor level is the higher privilege level. The privilege level is determined by the
S-bit of the SR; if the S-bit is set, the supervisor privilege level applies, and all instructions
are executable. The bus cycles for instructions executed at the supervisor level are
normally classified as supervisor references, and the values of the FC2–FC0 signals refer
to supervisor address spaces.
In a multitasking operating system, it is more efficient to have a supervisor stack space
associated with each user task and a separate stack space for interrupt-associated tasks.
The MC68020/EC020 provides two supervisor stacks, master and interrupt; the M bit of
the SR selects which of the two is active. When the M-bit is set, references to the SSP
implicitly or to address register seven (A7) explicitly, access the MSP. The operating
system sets the MSP for each task to point to a task-related area of supervisor data
space. This arrangement separates task-related supervisor activity from asynchronous,
I/O-related supervisor tasks that may be only coincidental to the currently executing task.
The MSP can separately maintain task control information for each currently executing
user task, and the software updates the MSP when a task switch is performed, providing
an efficient means for transferring task-related stack items. The other supervisor stack
pointer, the ISP, can be used for interrupt control information and workspace area as
interrupt handling routines require.
When the M-bit is clear, the MC68020/EC020 is in the interrupt mode of the supervisor
privilege level, and operation is the same as supervisor mode in the MC68000,
MC68HC001, MC68008, and MC68010. (The processor is in this mode after a reset
operation.) All SSP references access the ISP in this mode.
MOTOROLA M68020 USER’S MANUAL 2-3
The value of the M-bit in the SR does not affect execution of privileged instructions; both
master and interrupt modes are at the supervisor privilege level. Instructions that affect the
M-bit are MOVE to SR, ANDI to SR, EORI to SR, ORI to SR, and RTE. Also, the
processor automatically saves the M-bit value and clears it in the SR as part of exception
processing for interrupts.
All exception processing is performed at the supervisor privilege level. All bus cycles
generated during exception processing are supervisor references, and all stack accesses
use the active SSP.
2.1.2 User Privilege Level
The user level is the lower privilege level. The privilege level is determined by the S-bit of
the SR; if the S-bit is clear, the processor executes instructions at the user privilege level.
Most instructions execute at either privilege level, but some instructions that have
important system effects are privileged and can only be executed at the supervisor level.
For instance, user programs are not allowed to execute the STOP instruction or the
RESET instruction. To prevent a user program from entering the supervisor privilege level
except in a controlled manner, instructions that can alter the S-bit in the SR are privileged.
The TRAP #n instruction provides controlled access to operating system services for user
programs.
The bus cycles for an instruction executed at the user privilege level are classified as user
references, and the values of the FC2–FC0 signals specify user address spaces. While
the processor is at the user level, references to the system stack pointer implicitly, or to
address register seven (A7) explicitly, refer to the USP.
2.1.3 Changing Privilege Level
To change from the user to the supervisor privilege level, one of the conditions that
causes the processor to perform exception processing must occur. This causes a change
from the user level to the supervisor level and can cause a change from the master mode
to the interrupt mode. Exception processing saves the current values of the S and M bits
of the SR (along with the rest of the SR) on the active supervisor stack, and then sets the
S-bit, forcing the processor into the supervisor privilege level. When the exception being
processed is an interrupt and the M-bit is set, the M-bit is cleared, putting the processor
into the interrupt mode. Execution of instructions continues at the supervisor level to
process the exception condition.
To return to the user privilege level, a system routine must execute one of the following
instructions: MOVE to SR, ANDI to SR, EORI to SR, ORI to SR, or RTE. These
instructions execute at the supervisor privilege level and can modify the S-bit of the SR.
After these instructions execute, the instruction pipeline is flushed and is refilled from the
appropriate address space.
The RTE instruction returns to the program that was executing when the exception
occurred. It restores the exception stack frame saved on the supervisor stack. If the frame
2-4 M68020 USER’S MANUAL MOTOROLA
on top of the stack was generated by an interrupt, trap, or instruction exception, the RTE
instruction restores the SR and PC to the values saved on the supervisor stack. The
processor then continues execution at the restored PC address and at the privilege level
determined by the S-bit of the restored SR. If the frame on top of the stack was generated
by a bus fault (bus error or address error exception), the RTE instruction restores the
entire saved processor state from the stack.
2.2 ADDRESS SPACE TYPES
The processor specifies a target address space for every bus cycle with the FC2–FC0
signals according to the type of access required. In addition to distinguishing between
supervisor/user and program/data, the processor can identify special processor cycles,
such as the interrupt acknowledge cycle, and the memory management unit can control
accesses and translate addresses appropriately. Table 2-1 lists the types of accesses
defined for the MC68020/EC020 and the corresponding values of the FC2–FC0 signals.
Table 2-1. Address Space Encodings
FC2 FC1 FC0 Address Space
0 0 0 (Undefined, Reserved)*
0 0 1 User Data Space
0 1 0 User Program Space
0 1 1 (Undefined, Reserved)*
1 0 0 (Undefined, Reserved)*
1 0 1 Supervisor Data Space
1 1 0 Supervisor Program Space
1 1 1 CPU Space
* Address space 3 is reserved for user definition; 0 and 4 are reserved
for future use by Motorola.
The memory locations of user program and data accesses are not predefined; neither are
the locations of supervisor data space. During reset, the first two long words beginning at
memory location zero in the supervisor program space are used for processor
initialization. No other memory locations are explicitly defined by the MC68020/EC020.
A function code of $7 selects the CPU address space. This is a special address space
that does not contain instructions or operands but is reserved for special processor
functions. The processor uses accesses in this space to communicate with external
devices for special purposes. For example, all M68000 processors use the CPU space for
interrupt acknowledge cycles. The MC68020/EC020 also generate CPU space accesses
for breakpoint acknowledge and coprocessor operations.
Supervisor programs can use the MOVES instruction to access all address spaces,
including the user spaces and the CPU address space. Although the MOVES instruction
can be used to generate CPU space cycles, this may interfere with proper system
operation. Thus, the use of MOVES to access the CPU space should be done with
caution.
MOTOROLA M68020 USER’S MANUAL 2-5
2.3 EXCEPTION PROCESSING
An exception is defined as a special condition that preempts normal processing. Both
internal and external conditions can cause exceptions. External conditions that cause
exceptions are interrupts from external devices, bus errors, coprocessor-detected errors,
and reset. Instructions, address errors, tracing, and breakpoints are internal conditions
that cause exceptions. The TRAP, TRAPcc, TRAPV, cpTRAPcc, CHK, CHK2, RTE,
BKPT, CALLM, RTM, cp RESTORE, DIVS and DIVU instructions can generate exceptions
as part of their normal execution. In addition, illegal instructions, privilege violations, and
coprocessor protocol violations cause exceptions.
Exception processing, which is the transition from the normal processing of a program to
the processing required for the exception condition, involves the exception vector table
and an exception stack frame. The following paragraphs describe the exception vectors
and a generalized exception stack frame. Exception processing is discussed in detail in
Section 6 Exception Processing. Coprocessor-detected exceptions are discussed in
detail in Section 7 Coprocessor Interface Description.
2.3.1 Exception Vectors
The VBR contains the base address of the 1024-byte exception vector table, which
consists of 256 exception vectors. Exception vectors contain the memory addresses of
routines that begin execution at the completion of exception processing. These routines
perform a series of operations appropriate for the corresponding exceptions. Because the
exception vectors contain memory addresses, each consists of one long word, except for
the reset vector. The reset vector consists of two long words: the address used to initialize
the ISP and the address used to initialize the PC.
The address of an exception vector is derived from an 8-bit vector number and the VBR.
The vector numbers for some exceptions are obtained from an external device; others are
supplied automatically by the processor. The processor multiplies the vector number by
four to calculate the vector offset, which it adds to the VBR. The sum is the memory
address of the vector. All exception vectors are located in supervisor data space, except
the reset vector, which is located in supervisor program space. Only the initial reset vector
is fixed in the processor's memory map; once initialization is complete, there are no fixed
assignments. Since the VBR provides the base address of the vector table, the vector
table can be located anywhere in memory; it can even be dynamically relocated for each
task that is executed by an operating system. Details of exception processing are provided
in Section 6 Exception Processing, and Table 6-1 lists the exception vector
assignments.
2-6 M68020 USER’S MANUAL MOTOROLA
2.3.2 Exception Stack Frame
Exception processing saves the most volatile portion of the current processor context on
the top of the supervisor stack. This context is organized in a format called the exception
stack frame. This information always includes a copy of the SR, the PC, the vector offset
of the vector, and the frame format field. The frame format field identifies the type of stack
frame. The RTE instruction uses the value in the format field to properly restore the
information stored in the stack frame and to deallocate the stack space. The general form
of the exception stack frame is illustrated in Figure 2-1. Refer to Section 6 Exception
Processing for a complete list of exception stack frames.
0
15
SS
P
12
FORMAT
S
TATUS REGISTE
R
PROGRAM COUNTER
V
ECTOR OFFSE
T
A
DDITIONAL PROCESSOR STATE INFORMATIO
N
(2, 6, 12, OR 42 WORDS, IF NEEDED)
Figure 2-1. General Exception Stack Frame
MOTOROLA M68020 USER’S MANUAL 3-1
SECTION 3
SIGNAL DESCRIPTION
This section contains brief descriptions of the input and output signals in their functional
groups, as shown in Figure 3-1. Each signal is explained in a brief paragraph with
reference to other sections that contain more detail about the signal and the related
operations.
NOTE
In this section and in the remainder of the manual,
assert
and
negate
are used to specify forcing a signal to a particular state.
In particular,
assertion
and
assert
refer to a signal that is active
or true;
negation
and
negate
indicate a signal that is inactive or
false. These terms are used independently of the voltage level
(high or low) that they represent.
F
C2–FC
0
A
31–A
0
D
31–D
0
FUNCTION CODE
S
ADDRESS BU
S
D
ATA BU
S
T
RANSFER SIZ
E
ASYNCHRONOU
S
BUS CONTRO
L
EMULATOR SUPPOR
T
B
US A
RB
C
ONT
RO
I
NTER
RU
C
ONT
RO
B
US E
XC
C
ONT
RO
MC68020
S
IZ
0
S
IZ
1
O
C
S
E
C
S
R
/W
R
M
C
AS
DS
D
BE
N
D
SACK
0
D
SACK
1
C
DI
S
G
N
D
V
C
L
K
B
ER
R
H
AL
T
R
ESE
T
B
GAC
K
BG
BR
A
VE
C
I
PEN
D
I
PL
2
I
PL
1
I
PL
0
CC
*
*
*
**
*
*
Figure 3-1. Functional Signal Groups
3-2 M68020 USER’S MANUAL MOTOROLA
3.1 SIGNAL INDEX
The input and output signals for the MC68020/EC020 are listed in Table 3-1. Both the
names and mnemonics are shown along with brief signal descriptions. Signals that are
implemented in the MC68020, but not in the MC68EC020, have an asterisk (*) preceding
the signal name in Table 3-1. Also, note that the address bus is 32 bits wide for the
MC68020 and 24 bits wide for the MC68EC020. For more detail on each signal, refer to
the paragraph in this section named for the signal and the reference in that paragraph to a
description of the related operations.
Timing specifications for the signals listed in Table 3-1 can be found in Section 10
Electrical Characteristics.
3.2 FUNCTION CODE SIGNALS (FC2–FC0)
These three-state outputs identify the address space of the current bus cycle. Table 2-1
shows the relationships of the function code signals to the privilege levels and the address
spaces. Refer to Section 2 Processing States for more information.
3.3 ADDRESS BUS (A31–A0, MC68020)(A23–A0, MC68EC020)
These three-state outputs provide the address for the current bus cycle, except in the
CPU address space. Refer to Section 2 Processing States for more information on the
CPU address space. A31 is the most significant address signal for the MC68020; A23 is
the most significant address signal for the MC68EC020. The upper eight bits (A31–A24)
are used internally by the MC68EC020 to access the internal instruction cache address
tag. Refer to Section 5 Bus Operation for information on the address bus and its
relationship to bus operation.
3.4 DATA BUS (D31–D0)
These three-state bidirectional signals provide the general-purpose data path between the
MC68020/EC020 and all other devices. The data bus can transfer 8, 16, 24, or 32 bits of
data per bus cycle. D31 is the most significant bit of the data bus. Refer to Section 5 Bus
Operation for more information on the data bus and its relationship to bus operation.
3.5 TRANSFER SIZE SIGNALS (SIZ1, SIZ0)
These three-state outputs indicate the number of bytes remaining to be transferred for the
current bus cycle. Signals A1, A0, DSACK1, DSACK0, SIZ1, and SIZ0 define the number
of bits transferred on the data bus. Refer to Section 5 Bus Operation for more
information on SIZ1 and SIZ0 and their use in dynamic bus sizing.
MOTOROLA M68020 USER’S MANUAL 3-3
Table 3-1. Signal Index
Signal Name Mnemonic Function
Function Codes FC2–FC0 3-bit function code used to identify the address space of each bus cycle.
Address Bus
MC68020
MC68EC020 A31–A0
A23–A0 32-bit address bus
24-bit address bus
Data Bus D31–D0 32-bit data bus used to transfer 8, 16, 24, or 32 bits of data per bus
cycle.
Size SIZ1, SIZ0 Indicates the number of bytes remaining to be transferred for this cycle.
These signals, together with A1 and A0, define the active sections of the
data bus.
*External Cycle Start ECS Provides an indication that a bus cycle is beginning.
*Operand Cycle Start OCS Identical operation to that of ECS except that OCS is asserted only during
the first bus cycle of an operand transfer.
Read/Write R/WDefines the bus transfer as a processor read or write.
Read-Modify-Write Cycle RMC Provides an indicator that the current bus cycle is part of an indivisible
read-modify-write operation.
Address Strobe AS Indicates that a valid address is on the bus.
Data Strobe DS Indicates that valid data is to be placed on the data bus by an external
device or has been placed on the data bus by the MC68020/EC020.
*Data Buffer Enable DBEN Provides an enable signal for external data buffers.
Data Transfer and Size
Acknowledge DSACK1,
DSACK0 Bus response signals that indicate the requested data transfer operation
has completed. In addition, these two lines indicate the size of the
external bus port on a cycle-by-cycle basis and are used for
asynchronous transfers.
Interrupt Priority Level IPL2–IPL0 Provides an encoded interrupt level to the processor.
*Interrupt Pending IPEND Indicates that an interrupt is pending.
Autovector AVEC Requests an autovector during an interrupt acknowledge cycle.
Bus Request BR Indicates that an external device requires bus mastership.
Bus Grant BG Indicates that an external device may assume bus mastership.
*Bus Grant Acknowledge BGACK Indicates that an external device has assumed bus mastership.
Reset RESET System reset.
Halt HALT Indicates that the processor should suspend bus activity or that the
processor has halted due to a double bus fault.
Bus Error BERR Indicates that an erroneous bus operation is being attempted.
Cache Disable CDIS Statically disables the on-chip cache to assist emulator support.
Clock CLK Clock input to the processor.
Power Supply VCC Power supply.
Ground GND Ground connection.
*This signal is implemented in the MC68020 and not implemented in the MC68EC020.
3-4 M68020 USER’S MANUAL MOTOROLA
3.6 ASYNCHRONOUS BUS CONTROL SIGNALS
The following signals control synchronous bus transfer operations for the
MC68020/EC020. Note that OCS, ECS, and DBEN are implemented in MC68020 and not
implemented in the MC68EC020.
Operand Cycle Start (OCS, MC68020 only)
This output signal indicates the beginning of the first external bus cycle for an instruction
prefetch or a data operand transfer. OCS is not asserted for subsequent cycles that are
performed due to dynamic bus sizing or operand misalignment. Refer to Section 5 Bus
Operation for information about the relationship of OCS to bus operation.
OCS is not implemented in the MC68EC020.
External Cycle Start (ECS, MC68020 only)
This output signal indicates the beginning of a bus cycle of any type. Refer to Section 5
Bus Operation for information about the relationship of ECS to bus operation.
ECS is not implemented in the MC68EC020.
Read/Write (R/W)
This three-state output signal defines the type of bus cycle. A high level indicates a read
cycle; a low level indicates a write cycle. Refer to Section 5 Bus Operation for
information about the relationship of R/W to bus operation.
Read-Modify-Write Cycle (RMC)
This three-state output signal identifies the current bus cycle as part of an indivisible
read-modify-write operation; it remains asserted during all bus cycles of the read-
modify-write operation. Refer to Section 5 Bus Operation for information about the
relationship of RMC to bus operation.
Address Strobe (AS)
This three-state output signal indicates that a valid address is on the address bus. The
FC2–FC0, SIZ1, SIZ0, and R/W signals are also valid when AS is asserted. Refer to
Section 5 Bus Operation for information about the relationship of AS to bus operation.
Data Strobe (DS)
During a read cycle, this three-state output signal indicates that an external device
should place valid data on the data bus. During a write cycle, DS indicates that the
MC68020/EC020 has placed valid data on the bus. During two-clock synchronous write
cycles, the MC68020/EC020 does not assert DS. Refer to Section 5 Bus Operation for
more information about the relationship of DS to bus operation.
MOTOROLA M68020 USER’S MANUAL 3-5
Data Buffer Enable (DBEN, MC68020 only)
This output signal is an enable signal for external data buffers. This signal may not be
required in all systems. Refer to Section 5 Bus Operation for more information about
the relationship of DBEN to bus operation.
DBEN is not implemented in the MC68EC020.
Data Transfer and Size Acknowledge (DSACK1, DSACK0)
These input signals indicate the completion of a requested data transfer operation. In
addition, they indicate the size of the external bus port at the completion of each cycle.
These signals apply only to asynchronous bus cycles. Refer to Section 5 Bus
Operation for more information on these signals and their relationship to dynamic bus
sizing.
3.7 INTERRUPT CONTROL SIGNALS
The following signals are the interrupt control signals for the MC68020/EC020. Note that
IPEND is implemented in the MC68020 and not implemented in the MC68EC020.
Interrupt Priority Level Signals (IPL2–IPL0)
These input signals provide an indication of an interrupt condition and the encoding of
the interrupt level from a peripheral or external prioritizing circuitry. IPL2 is the most
significant bit of the level number. For example, since the IPL2–IPL0 signals are active
low, IPL2–IPL0 equal to $5 corresponds to an interrupt request at interrupt level 2.
Refer to Section 6 Exception Processing for information on MC68020/EC020
interrupts.
Interrupt Pending (IPEND, MC68020 only)
This output signal indicates that an interrupt request exceeding the current interrupt
priority mask in the SR has been recognized internally. This output is for use by external
devices (coprocessors and other bus masters, for example) to predict processor
operation on the following instruction boundaries. Refer to Section 6 Exception
Processing for interrupt information. Also, refer to Section 5 Bus Operation for bus
information related to interrupts.
IPEND is not implemented in the MC68EC020.
Autovector (AVEC)
This input signal indicates that the MC68020/EC020 should generate an automatic
vector during an interrupt acknowledge cycle. Refer to Section 5 Bus Operation for
more information about automatic vectors.
3-6 M68020 USER’S MANUAL MOTOROLA
3.8 BUS ARBITRATION CONTROL SIGNALS
The following signals are the bus arbitration control signals used to determine which
device in a system is the bus master. Note that BGACK is implemented in the MC68020
and not implemented in the MC68EC020.
Bus Request (BR)
This input signal indicates that an external device needs to become the bus master. BR
is typically a “wire-ORed” input (but does not need to be constructed from open-collector
devices). Refer to Section 5 Bus Operation for more information on MC68020 bus
arbitration. Refer to Section 5 Bus Operation and Appendix A Interfacing an
MC68EC020 to a DMA Device That Supports a Three-Wire Bus Arbitration
Protocol for more information on MC68EC020 bus arbitration.
Bus Grant (BG)
This output signal indicates that the MC68020/EC020 will release ownership of the bus
when the current processor bus cycle completes. Refer to Section 5 Bus Operation for
more information on MC68020 bus arbitration. Refer to Section 5 Bus Operation and
Appendix A Interfacing an MC68EC020 to a DMA Device That Supports a Three-
Wire Bus Arbitration Protocol for more information on MC68EC020 bus arbitration.
Bus Grant Acknowledge (BGACK, MC68020 only)
This input signal indicates that an external device has become the bus master. Refer to
Section 5 Bus Operation for more information on MC68020 bus arbitration. Refer to
Section 5 Bus Operation and Appendix A Interfacing an MC68EC020 to a DMA
Device That Supports a Three-Wire Bus Arbitration Protocol for more information
on MC68EC020 bus arbitration.
BGACK is not implemented in the MC68EC020.
3.9 BUS EXCEPTION CONTROL SIGNALS
The following signals are the bus exception control signals for the MC68020/EC020.
Reset (RESET)
This bidirectional open-drain signal is used to initiate a system reset. An external reset
signal resets the MC68020/EC020 as well as all external devices. A reset signal from
the processor (asserted as part of the RESET instruction) resets external devices only;
the internal state of the processor is not altered. Refer to Section 5 Bus Operation for
a description of reset bus operation and Section 6 Exception Processing for
information about the reset exception.
MOTOROLA M68020 USER’S MANUAL 3-7
Halt (HALT)
The assertion of this bidirectional open-drain signal indicates that the processor should
suspend bus activity or, when used with BERR, that the processor should retry the
current cycle. Refer to Section 5 Bus Operation for a description of the effects of
HALT on bus operations. When the processor has stopped executing instructions due
to a double bus fault condition, the HALT line is asserted by the processor to indicate to
external devices that the processor has stopped.
Bus Error (BERR)
This input signal indicates that an invalid bus operation is being attempted or, when
used with HALT, that the processor should retry the current cycle. Refer to Section 5
Bus Operation for a description of the effects of BERR on bus operations.
3.10 EMULATOR SUPPORT SIGNAL
The following signal supports emulation by providing a means for an emulator to disable
the on-chip cache by supplying internal status information to an emulator. Refer to
Section 7 Coprocessor Interface Description for more detailed information on
emulation support.
Cache Disable (CDIS)
This input signal statically disables the on-chip cache to assist emulator support. Refer
to Section 4 On-Chip Cache Memory for information about the cache; refer to Section
9 Applications Information for a description of the use of this signal by an emulator.
CDIS does not flush the instruction cache; entries remain unaltered and become
available again when CDIS is negated.
3.11 CLOCK (CLK)
The CLK signal is the clock input to the MC68020/EC020. This TTL-compatible signal
should not be gated off at any time while power is applied to the processor. Refer to
Section 9 Applications Information for suggestions on clock generation. Refer to
Section 10 Electrical Characteristics for electrical characteristics.
3.12 POWER SUPPLY CONNECTIONS
The MC68020/EC020 requires connection to a VCC power supply, positive with respect to
ground. The VCC connections are grouped to supply adequate current for the various
sections of the processor. The ground connections are similarly grouped. Section 11
Ordering Information and Mechanical Data describes the groupings of VCC and ground
connections, and Section 9 Applications Information describes a typical power supply
interface.
3-8 M68020 USER’S MANUAL MOTOROLA
3.13 SIGNAL SUMMARY
Table 3-2 provides a summary of the characteristics of the signals discussed in this
section. Signal names preceded by an asterisk (*) are implemented in the MC68020 and
not implemented in the MC68EC020.
Table 3-2. Signal Summary
Signal Function Signal Name Input/Output Active State Three-State
Function Codes FC2–FC0 Output High Yes
Address Bus
MC68020
MC68EC020 A31–A0
A23–A0
Output High Yes
Data Bus D31–D0 Input/Output High Yes
Transfer Size SIZ1, SIZ0 Output High Yes
*Operand Cycle Start OCS Output Low No
*External Cycle Start ECS Output Low No
Read/Write R/WOutput High/Low Yes
Read-Modify-Write Cycle RMC Output Low Yes
Address Strobe AS Output Low Yes
Data Strobe DS Output Low Yes
*Data Buffer Enable DBEN Output Low Yes
Data Transfer and Size Acknowledge DSACK1, DSACK0 Input Low —
Interrupt Priority Level IPL2–IPL0 Input Low —
*Interrupt Pending IPEND Output Low No
Autovector AVEC Input Low —
Bus Request BR Input Low —
Bus Grant BG Output Low No
*Bus Grant Acknowledge BGACK Input Low —
Reset RESET Input/Output Low No**
Halt HALT Input/Output Low No**
Bus Error BERR Input Low —
Cache Disable CDIS Input Low —
Clock CLK Input — —
Power Supply VCC Input — —
Ground GND Input — —
*This signal is implemented in the MC68020 and not implemented in the MC68EC020.
**Open-drain
MOTOROLA M68020 USER’S MANUAL 4-1
SECTION 4
ON-CHIP CACHE MEMORY
The MC68020/EC020 incorporates an on-chip cache memory as a means of improving
performance. The cache is implemented as a CPU instruction cache and is used to store
the instruction stream prefetch accesses from the main memory.
An increase in instruction throughput results when instruction words required by a
program are available in the on-chip cache and the time required to access them on the
external bus is eliminated. In systems with more than one bus master (e.g., a processor
and a DMA device), reduced external bus activity increases overall performance by
increasing the availability of the bus for use by external devices without degrading the
performance of the MC68020/EC020.
4.1 ON-CHIP CACHE ORGANIZATION AND OPERATION
The MC68020/EC020 on-chip instruction cache is a direct-mapped cache of 64 long-word
entries. Each cache entry consists of a tag field (A31–A8 and FC2), one valid bit, and 32
bits (two words) of instruction data. Figure 4-1 shows a block diagram of the on-chip
cache organization.
Externally, the MC68EC020 does not use the upper eight bits of the address (A31–A24),
and addresses $FF000000 and $00000000 from the MC68EC020 appear the same.
However, the MC68EC020 does use A31–A24 internally in the instruction cache address
tag, and addresses $FF000000 and $00000000 appear different in the MC68EC020
instruction cache. The MC68020, MC68030/EC030, and MC68040/EC040 use all 32 bits
of the address externally. To maintain object-code upgrade compatibility when designing
with the MC68EC020, the upper eight bits should be considered part of the address when
assigning address spaces in hardware.
When enabled, the MC68020/EC020 instruction cache is used to store instruction
prefetches (instruction words and extension words) as they are requested by the CPU.
Instruction prefetches are normally requested from sequential memory addresses except
when a change of program flow occurs (e.g., a branch taken) or when an instruction is
executed that can modify the SR. In these cases, the instruction pipe is automatically
flushed and refilled.
4–2 M68020 USER’S MANUAL MOTOROLA
F
C
2
F
C
1
F
C
0
A
3
1
A
2
3
A
2
2
A
2
1
A
2
0
A
1
9
A
1
8
A
1
7
A
1
6
A
1
5
A
1
4
A
1
3
A
1
2
A
1
1
A
1
0
A
9
A
8
A
7
A
6
A
5
A
4
A
3
A
2
A
1
A
0
TAG
INDEX
TAG
V
WORD
WORD
WORD
SELECT
1 OF
6
4 SELEC
T
TAG REPLACE
COMPARATOR
REPLACEMENT
D
ATA
TO
I
NSTRUCTIO
N
P
ATH
CACHE
C
ONTRO
L
ENTRY HIT
M
C68020/EC020 PREFETCH ADDRES
S
LINE
HIT
VALID
Figure 4-1. MC68020/EC020 On-Chip Cache Organization
When an instruction fetch occurs, the cache (if enabled) is first checked to determine if the
word required is in the cache. This check is achieved by first using the index field (A7–A2)
of the access address as an index into the on-chip cache. This index selects one of the 64
entries in the cache. Next, A31–A8 and FC2 are compared to the tag of the selected entry.
(Note that in the MC68EC020, A31–A24 are used for internal on-chip cache tag
comparison.) If there is a match and the valid bit is set, a cache hit occurs. A1 is then used
to select the proper word from the cache entry, and the cycle ends. If there is no match or
if the valid bit is clear, a cache miss occurs, and the instruction is fetched from external
memory. This new instruction is automatically written into the cache entry, and the valid bit
is set unless the F-bit in the CACR is set. Since the processor always prefetches
instructions externally with long-word-aligned bus cycles, both words of the entry will be
updated, regardless of which word caused the miss.
NOTE
Data accesses are not cached, regardless of their associated
address space.
MOTOROLA M68020 USER’S MANUAL 4-3
4.2 CACHE RESET
During processor reset, the cache is cleared by resetting all of the valid bits. The E and F
bits in the CACR are also cleared.
4.3 CACHE CONTROL
Only the MC68020/EC020 cache control circuitry can directly access the cache array, but
a supervisor program can set bits in the CACR to exercise control over cache operations.
The supervisor level also has access to the CAAR, which contains the address for a
cache entry to be cleared.
System hardware can assert the CDIS signal to disable the cache. The assertion of CDIS
disables the cache, regardless of the state of the E-bit in the CACR. CDIS is primarily
intended for use by in-circuit emulators.
4.3.1 Cache Control Register (CACR)
The CACR, shown in Figure 4-2, is a 32-bit register than can be written or read by the
MOVEC instruction or indirectly modified by a reset. Four of the bits (3–0) control the
instruction cache. Bits 31–4 are reserved for Motorola definition. They are read as zeros
and are ignored when written. For future compatibility, writes should not set these bits.
0
31
E
1
F
2
CE
3
C
4
0
5
0
6
0
7
0
8
0
Figure 4-2. Cache Control Register
C—Clear Cache
The C-bit is set to clear all entries in the instruction cache. Operating systems and other
software set this bit to clear instructions from the cache prior to a context switch. The
processor clears all valid bits in the instruction cache when a MOVEC instruction sets
the C-bit. The C-bit is always read as a zero.
CE—Clear Entry In Cache
The CE bit is set to clear an entry in the instruction cache. The index field of the CAAR
(see Figure 4-3), corresponding to the index and long-word select portion of an address,
specifies the entry to be cleared. The processor clears only the specified long word by
clearing the valid bit for the entry when a MOVEC instruction sets the CE bit, regardless
of the states of the E and F bits. The CE bit is always read as a zero.
4–4 M68020 USER’S MANUAL MOTOROLA
F—Freeze Cache
The F-bit is set to freeze the instruction cache. When the F-bit is set and a cache miss
occurs, the entry (or line) is not replaced. When the F-bit is clear, a cache miss causes
the entry (or line) to be filled. A reset operation clears the F-bit.
E—Enable Cache
The E-bit is set to enable the instruction cache. When it is clear, the instruction cache is
disabled. A reset operation clears the E-bit. The supervisor normally enables the
instruction cache, but it can clear the E-bit for system debugging or emulation, as
required. Disabling the instruction cache does not flush the entries. If the cache is
reenabled, the previously valid entries remain valid and may be used.
4.3.2 Cache Address Register (CAAR)
The format of the 32-bit CAAR is shown in Figure 4-3.
0
31
RESERVED
1
2
INDEX
7
8
R
ESERVE
D
Figure 4-3. Cache Address Register
Bits 31–8, 1, and 0—Reserved
These bits are reserved for use by Motorola.
Index Field
The index field contains the address for the “clear cache entry” operations. The bits of
this field, which correspond to A7–A2, specify the index and a long word of a cache line.
MOTOROLA M68020 USER’S MANUAL 5-1
SECTION 5
BUS OPERATION
This section provides a functional description of the bus, the signals that control it, and the
bus cycles provided for data transfer operations. It also describes the error and halt
conditions, bus arbitration, and reset operation. Operation of the bus is the same whether
the processor or an external device is the bus master; the names and descriptions of bus
cycles are from the point of view of the bus master. For exact timing specifications, refer to
Section 10 Electrical Characteristics.
The MC68020/EC020 architecture supports byte, word, and long-word operands, allowing
access to 8-, 16-, and 32-bit data ports through the use of asynchronous cycles controlled
by the DSACK1 and DSACK0 input signals.
The MC68020/EC020 allows byte, word, and long-word operands to be located in memory
on any byte boundary. For a misaligned transfer, more than one bus cycle may be
required to complete the transfer, regardless of port size. For a port less than 32 bits wide,
multiple bus cycles may be required for an operand transfer due to either misalignment or
a port width smaller than the operand size. Instruction words and their associated
extension words must be aligned on word boundaries. The user should be aware that
misalignment of word or long-word operands can cause the MC68020/EC020 to perform
multiple bus cycles for the operand transfer; therefore, processor performance is
optimized if word and long-word memory operands are aligned on word or long-word
boundaries, respectively.
5.1 BUS TRANSFER SIGNALS
The bus transfers information between the MC68020/EC020 and an external memory,
coprocessor, or peripheral device. External devices can accept or provide 8 bits, 16 bits,
or 32 bits in parallel and must follow the handshake protocol described in this section. The
maximum number of bits accepted or provided during a bus transfer is defined as the port
width. The MC68020/EC020 contains an address bus that specifies the address for the
transfer and a data bus that transfers the data. Control signals indicate the beginning of
the cycle, the address space and size of the transfer, and the type of cycle. The selected
device then controls the length of the cycle with the signal(s) used to terminate the cycle.
Strobe signals, one for the address bus and another for the data bus, indicate the validity
of the address and provide timing information for the data.
The bus operates in an asynchronous mode for any port width. The bus and control input
signals are internally synchronized to the MC68020/EC020 clock, introducing a delay. This
delay is the time period required for the MC68020/EC020 to sample an input signal,
synchronize the input to the internal clocks of the processor, and determine whether the
5-2 M68020 USER’S MANUAL MOTOROLA
input is high or low. Figure 5-1 shows the relationship between the clock signal, a typical
input, and its associated internal signal.
Furthermore, for all inputs, the processor latches the level of the input during a sample
window around the falling edge of the clock signal. This window is illustrated in Figure 5-2.
To ensure that an input signal is recognized on a specific falling edge of the clock, that
input must be stable during the sample window. If an input transitions during the window,
the level recognized by the processor is not predictable; however, the processor always
resolves the latched level to either a logic high or logic low before using it. In addition to
meeting input setup and hold times for deterministic operation, all input signals must obey
the protocols described in this section.
SYNC DELAY
CLK
EXT
INT
Figure 5-1. Relationship between External and Internal Signals
t
su
t
h
SAMPLE
W
INDOW
CLK
EXT
Figure 5-2. Input Sample Window
5.1.1 Bus Control Signals
The MC68020/EC020 initiates a bus cycle by driving the A1–A0, SIZ1, SIZ0, FC2–FC0,
and R/W outputs. However, if the MC68020/EC020 finds the required instruction in the on-
chip cache, the processor aborts the cycle before asserting the AS.The assertion of AS
ensures that the cycle has not been aborted by these internal conditions.
MOTOROLA M68020 USER’S MANUAL 5-3
When initiating a bus cycle, the MC68020 asserts ECS in addition to A1–A0, SIZ1, SIZ0,
FC2–FC0, and R/W. ECS can be used to initiate various timing sequences that are
eventually qualified with AS. Qualification with AS may be required since, in the case of an
internal cache hit, a bus cycle may be aborted after ECS has been asserted. During the
first MC68020 external bus cycle of an operand transfer, OCS is asserted with ECS. When
several bus cycles are required to transfer the entire operand, OCS is asserted only at the
beginning of the first external bus cycle. With respect to OCS , an “operand” is any entity
required by the execution unit, whether a program or data item. Note that ECS and OCS
are not implemented in the MC68EC020.
The FC2–FC0 signals select one of eight address spaces (see Table 2-1) to which the
address applies. Five address spaces are presently defined. Of the remaining three, one
is reserved for user definition, and two are reserved by Motorola for future use. FC2–FC0
are valid while AS is asserted.
The SIZ1 and SIZ0 signals indicate the number of bytes remaining to be transferred
during an operand cycle (consisting of one or more bus cycles) or during a cache fill
operation from a device with a port size that is less than 32 bits. Table 5-2 lists the
encoding of SIZ1 and SIZ0. SIZ1 and SIZ0 are valid while AS is asserted.
The R/W signal determines the direction of the transfer during a bus cycle. When required,
this signal changes state at the beginning of a bus cycle and is valid while AS is asserted.
R/W only transitions when a write cycle is preceded by a read cycle or vice versa. This
signal may remain low for two consecutive write cycles.
The RMC signal is asserted at the beginning of the first bus cycle of a read-modify-write
operation and remains asserted until completion of the final bus cycle of the operation.
The RMC signal is guaranteed to be negated before the end of state 0 for a bus cycle
following a read-modify-write operation.
5.1.2 Address Bus
A31–A0 (for the MC68020) or A23–A0 (for the MC68EC020) define the address of the
byte (or the most significant byte) to be transferred during a bus cycle. The processor
places the address on the bus at the beginning of a bus cycle. The address is valid while
AS is asserted. In the MC68EC020, A31–A24 are used internally, but not externally.
5.1.3 Address Strobe
AS is a timing signal that indicates the validity of an address on the address bus and of
many control signals. It is asserted one-half clock after the beginning of a bus cycle.
5.1.4 Data Bus
D31–D0 comprise a bidirectional, nonmultiplexed parallel bus that contains the data being
transferred to or from the processor. A read or write operation may transfer 8, 16, 24, or
32 bits of data (one, two, three, or four bytes) in one bus cycle. During a read cycle, the
data is latched by the processor on the last falling edge of the clock for that bus cycle. For
5-4 M68020 USER’S MANUAL MOTOROLA
a write cycle, all 32 bits of the data bus are driven, regardless of the port width or operand
size. The processor places the data on the data bus one-half clock cycle after AS is
asserted in a write cycle.
5.1.5 Data Strobe
DS is a timing signal that applies to the data bus. For a read cycle, the processor asserts
DS to signal the external device to place data on the bus. DS is asserted at the same time
as AS during a read cycle. For a write cycle, DS notifies the external device that the data
to be written is valid. The processor asserts DS one full clock cycle after the assertion of
AS during a write cycle.
5.1.6 Data Buffer Enable
The MC68020 DBEN signal is used to enable external data buffers while data is present
on the data bus. During a read operation, DBEN is asserted one clock cycle after the
beginning of the bus cycle and is negated as DS is negated. In a write operation, DBEN is
asserted at the time AS is asserted and is held active for the duration of the cycle. Note
that DBEN is implemented in the MC68020 and is not implemented in the MC68EC020.
5.1.7 Bus Cycle Termination Signals
During bus cycles, external devices assert DSACK1/DSACK0 as part of the bus protocol.
During a read cycle, DSACK1/DSACK0 assertion signals the processor to terminate the
bus cycle and to latch the data. During a write cycle, the assertion of DSACK1/DSACK0
indicates that the external device has successfully stored the data and that the cycle may
terminate. DSACK1/DSACK0 also indicate to the processor the size of the port for the bus
cycle just completed, as shown in Table 5-1. Refer to 5.3.1 Read Cycle for timing
relationships of DSACK1/DSACK0.
The BERR signal is also a bus cycle termination indicator and can be used in the absence
of DSACK1/DSACK0 to indicate a bus error condition. It can also be asserted in
conjunction with DSACK1/DSACK0 to indicate a bus error condition, provided it meets the
appropriate timing described in this section and in Section 10 Electrical Characteristics.
Additionally, the BERR and HALT signals can be asserted together to indicate a retry
termination. Again, the BERR and HALT signals can be simultaneously asserted in lieu of,
or in conjunction with, the DSACK1/DSACK0 signals.
Finally, the AVEC signal can be used to terminate interrupt acknowledge cycles, indicating
that the MC68020/EC020 should generate a vector number to locate an interrupt handler
routine. AVEC is ignored during all other bus cycles.
MOTOROLA M68020 USER’S MANUAL 5-5
5.2 DATA TRANSFER MECHANISM
The MC68020/EC020 architecture supports byte, word, and long-word operands allowing
access to 8-, 16-, and 32-bit data ports through the use of asynchronous cycles controlled
by DSACK1/DSACK0. Byte, word, and long-word operands can be located on any byte
boundary, but misaligned transfers may require additional bus cycles, regardless of port
size.
5.2.1 Dynamic Bus Sizing
The MC68020/EC020 dynamically interprets the port size of the addressed device during
each bus cycle, allowing operand transfers to or from 8-, 16-, and 32-bit ports. During an
operand transfer cycle, the slave device signals its port size (byte, word, or long word) and
indicates completion of the bus cycle to the processor with the DSACK1/DSACK0 signals.
Refer to Table 5-1 for DSACK1/DSACK0 encodings and assertion results.
Table 5-1.
DSACK1/DSACK0
Encodings and Results
DSACK1 DSACK0
Result
Negated Negated Insert Wait States in Current Bus Cycle
Negated Asserted Complete Cycle—Data Bus Port Size is 8 Bits
Asserted Negated Complete Cycle—Data Bus Port Size is 16 Bits
Asserted Asserted Complete Cycle—Data Bus Port Size is 32 Bits
For example, if the processor is executing an instruction that reads a long-word operand
from a long-word-aligned address, it attempts to read 32 bits during the first bus cycle.
(Refer to 5.2.2 Misaligned Operands for the case of a word or byte address.) If the port
responds that it is 32 bits wide, the MC68020/EC020 latches all 32 bits of data and
continues with the next operation. If the port responds that it is 16 bits wide, the
MC68020/EC020 latches the 16 bits of valid data and runs another bus cycle to obtain the
other 16 bits. The operation for an 8-bit port is similar, but requires four read cycles. The
addressed device uses the DSACK1/DSACK0 signals to indicate the port width. For
instance, a 32-bit device always returns DSACK1/DSACK0 for a 32-bit port, regardless of
whether the bus cycle is a byte, word, or long-word operation.
Dynamic bus sizing requires that the portion of the data bus used for a transfer to or from
a particular port size be fixed. A 32-bit port must reside on D31–D0, a 16-bit port must
reside on D32–D16, and an 8-bit port must reside on D31–D24. This requirement
minimizes the number of bus cycles needed to transfer data to 8- and 16-bit ports and
ensures that the MC68020/EC020 correctly transfers valid data. The MC68020/EC020
always attempts to transfer the maximum amount of data on all bus cycles; for a long-
word operation, it always assumes that the port is 32 bits wide when beginning the bus
cycle.
The bytes of operands are designated as shown in Figure 5-3. The most significant byte of
a long-word operand is OP0; the least significant byte is OP3. The two bytes of a word -
length operand are OP2 (most significant) and OP3. The single byte of a byte-length
operand is OP3. These designations are used in the figures and descriptions that follow.
5-6 M68020 USER’S MANUAL MOTOROLA
OP0
OP1
OP2
OP3
31
0
15
0
OP2
OP3
7
0
LONG-WORD OPERAND
WORD OPERAND
BYTE OPERAND
OP3
Figure 5-3. Internal Operand Representation
Figure 5-4 shows the required organization of data ports on the MC68020/EC020 bus for
8-, 16-, and 32-bit devices. The four bytes shown in Figure 5-4 are connected through the
internal data bus and data multiplexer to the external data bus. This path is the means
through which the MC68020/EC020 supports dynamic bus sizing and operand
misalignment. Refer to 5.2.2 Misaligned Operands for the definition of misaligned
operand. The data multiplexer establishes the necessary connections for different
combinations of address and data sizes.
0
1
2
3
ROUTING AND DUPLICATION
BYTE 0
BYTE 2
BYTE 1
BYTE 3
16-BIT PORT
REGISTER
MULTIPLEXER
EXTERNAL DATA BUS
ADDRESS
xxxxxxx0
xxxxxxx0
2
INCREASING
MEMORY
A
DDRESSE
S
D31– D24
D23–D16
D15–D8
D7–D0
BYTE 0
BYTE 1
BYTE 2
BYTE 3
BYTE 0
BYTE 1
BYTE 2
BYTE 3
8-BIT PORT
2
3
1
xxxxxxx0
EXTERNAL BUS
INTERNAL TO
THE MC68020/EC02
0
32-BIT PORT
OP0
OP1
OP2
OP3
Figure 5-4. MC68020/EC020 Interface to Various Port Sizes
MOTOROLA M68020 USER’S MANUAL 5-7
The multiplexer takes the four bytes of the 32-bit bus and routes them to their required
positions. For example, OP0 can be routed to D31–D24, as would be the normal case, or
it can be routed to any other byte position to support a misaligned transfer. The same is
true for any of the operand bytes. The positioning of bytes is determined by the SIZ1,
SIZ0, A1, and A0 outputs.
The SIZ1 and SIZ0 outputs indicate the remaining number of bytes to be transferred
during the current bus cycle, as listed in Table 5-2.
Table 5-2. SIZ1, SIZ0 Signal Encoding
SIZ1 SIZ0 Size
Negated Asserted Byte
Asserted Negated Word
Asserted Asserted 3 Bytes
Negated Negated Long Word
The number of bytes transferred during a write or read bus cycle is equal to or less than
the size indicated by the SIZ1 and SIZ0 outputs, depending on port width and operand
alignment. For example, during the first bus cycle of a long-word transfer to a word port,
the SIZ1 and SIZ0 outputs indicate that four bytes are to be transferred, although only two
bytes are moved on that bus cycle.
A1–A0 also affect operation of the data multiplexer. During an operand transfer, A31–A2
(for the MC68020) or A23–A2 (for the MC68EC020) indicate the long-word base address
of that portion of the operand to be accessed; A1 and A0 indicate the byte offset from the
base. Table 5-3 lists the encodings of A1 and A0 and the corresponding byte offsets from
the long-word base.
Table 5-3. Address Offset Encodings
A1 A0 Offset
Negated Negated +0 Bytes
Negated Asserted +1 Byte
Asserted Negated +2 Bytes
Asserted Asserted +3 Bytes
5-8 M68020 USER’S MANUAL MOTOROLA
Table 5-4 lists the bytes required on the data bus for read cycle s. The entries shown as
OP3, OP2, OP1, and OP0 are portions of the requested operand that are read or written
during that bus cycle and are defined by SIZ1, SIZ0, A1, and A0 for the bus cycle.
Table 5-4. Data Bus Requirements for Read Cycles
Byte Port
External
Data Bytes
Required
Word Port
External Data Bytes
Required
Long-Word Port
External Data Bytes
Required
AddressSize
Transfer
Size
SIZ1 SIZ0 A1 A0
OP3 OP3
OP3
OP3
OP3 OP3
OP3
OP3
OP3
D31–D24D23–D16D31–D24D23–D16D31–D24 D7–D0D15–D8
OP3
OP3
OP3
Byte 0
1
1
1
100
0
0
0
1
1
01
0
1
OP2 OP2
OP2
OP2
OP2 OP2
OP2
OP2
OP2
OP2
OP2
OP2
Word 1
0
1
1
000
1
1
1
0
0
01
0
1
OP1 OP1
OP1
OP1
OP1 OP1
OP1
OP1
OP1
OP1
OP1
OP1
3 Bytes 1
1
1
1
100
1
1
1
1
1
01
0
1
OP0 OP0
OP0
OP0
OP0 OP0
OP0
OP0
OP0
OP0
OP0
OP0
Long Word 0
0
1
1
000
0
0
0
0
0
01
0
1
OP3
OP3
OP2
OP2
OP1
OP1
OP1
OP1
OP2
OP1
OP2
OP3
OP2
OP3OP2
OP3OP2
OP3
OP3
OP3
MOTOROLA M68020 USER’S MANUAL 5-9
Table 5-5 lists the combinations of SIZ1, SIZ0, A1, and A0 and the corresponding pattern
of the data transfer for write cycles from the internal multiplexer of the MC68020/EC020 to
the external data bus.
Table 5-5. MC68020/EC020 Internal to External Data Bus
Multiplexer—Write Cycles
OP0*
External Data Bus
Connection
AddressSize
Transfer
Size
SIZ1 SIZ0 A1 A0
OP3
D23–D16
D31–D24 D7–D0D15–D8
Byte 0 1 x x
OP2
OP2
Word 1
0
0x0
1x1
OP1
OP1
OP1
OP1
3 Bytes 1
1
1
1
100
1
1
1
1
1
01
0
1
OP0
OP0
OP0
OP0
Long Word 0
0
1
1
000
0
0
0
0
0
01
0
1
OP1
OP1
OP2
OP1
OP2
OP3
OP2
OP3OP2
OP3OP2
OP3
OP3
OP3
OP2
OP3
OP2
OP3
OP3
OP1
OP1
OP1
OP2
OP1
OP0
OP0
OP0
OP1
OP0 OP1*
OP2
OP2*
*Due to the current implementation, this byte is output but never used.
x = Don't care
NOTE: The OP tables on the external data bus refer to a particular byte of the operand that
is written on that section of the data bus.
5-10 M68020 USER’S MANUAL MOTOROLA
Figure 5-5 shows the transfer (write) of a long-word operand to a word port. In the first bus
cycle, the MC68020/EC020 places the four operand bytes on the external bus. Since the
address is long-word aligned in this example, the multiplexer follows the pattern in the
entry of Table 5-5 corresponding to SIZ0, SIZ1, A0, A1 = 0000. The port latches the data
on D31–D16, asserts DSACK1 (DSACK0 remains negated), and the processor terminates
the bus cycle. It then starts a new bus cycle with SIZ1, SIZ0, A1, A0 = 1010 to transfer the
remaining 16 bits. SIZ1 and SIZ0 indicate that a word remains to be transferred; A1 and
A0 indicate that the word corresponds to an offset of two from the base address. The
multiplexer follows the pattern corresponding to this configuration of SIZ1, SIZ0, A1, and
A0 and places the two least significant bytes of the long word on the word portion of the
bus (D31–D16). The bus cycle transfers the remaining bytes to the word-sized port. Figure
5-6 shows the timing of the bus transfer signals for this operation.
DATA BUS
D31
D16
LONG-WORD OPERAND
OP0
OP1
OP2
OP3
31
0
WORD MEMORY
MSB
LSB
OP0
OP1
OP2
OP3
MC68020/EC020
SIZ1
SIZ0
A1
A0
0
0
0
0
1
0
1
0
MEMORY CONTROL
DSACK1
DSACK0
L
H
L
H
Figure 5-5. Long-Word Operand Write to Word Port Example
MOTOROLA M68020 USER’S MANUAL 5-11
WORD WRITE
LONG-WORD OPERAND WRITE TO 16-BIT PORT
S0
S2
S4
S0
S2
S4
CLK
A31–A2
A1
A0
FC2–FC0
SIZ1
SIZ0
R/W
ECS
OCS
AS
DS
DSACK1
DSACK0
DBEN
D31–D24
D23–D16
WORD WRITE
OP0
OP1
OP2
OP3
**
**
**
*
*
For the MC68EC020, A23–A2.
This signal does not apply to the MC68EC020.
**
Figure 5-6. Long-Word Operand Write to Word Port Timing
5-12 M68020 USER’S MANUAL MOTOROLA
Figure 5-7 shows a word write to an 8-bit bus port. Like the preceding example, this
example requires two bus cycles. Each bus cycle transfers a single byte. SIZ1 and SIZ0
for the first cycle specify two bytes; for the second cycle, one byte. Figure 5-8 shows the
associated bus transfer signal timing.
OP2
OP3
15
0
WORD OPERAND
D31
DATA BUS
D24
BYTE MEMORY
OP2
OP3
MC68020/EC020
SIZ1
SIZ0
A1
A0
1 0 0 0
0
1 0 1
MEMORY CONTROL
DSACK1
DSACK0
H
L
H
L
Figure 5-7. Word Operand Write to Byte Port Example
MOTOROLA M68020 USER’S MANUAL 5-13
BYTE WRITE
WORD OPERAND WRITE
S0
S2
S4
S0
S2
S4
CLK
A31–A2
A1
A0
FC2–FC0
SIZ1
SIZ0
R/W
ECS
OCS
AS
DS
DSACK1
DSACK0
DBEN
D31–D24
D23–D16
BYTE WRITE
D15–D8
D7–D0
**
**
**
*
*
For the MC68EC020, A23–A2.
This signal does not apply to the MC68EC020.
**
Figure 5-8. Word Operand Write to Byte Port Timing
5-14 M68020 USER’S MANUAL MOTOROLA
5.2.2 Misaligned Operands
Since operands may reside at any byte boundary, they may be misaligned. A byte
operand is properly aligned at any address; a word operand is misaligned at an odd
address; a long word is misaligned at an address that is not evenly divisible by four. The
MC68000, MC68008, and MC68010 implementations allow long-word transfers on odd-
word boundaries but force exceptions if word or long-word operand transfers are
attempted at odd-byte addresses. Although the MC68020/EC020 does not enforce any
alignment restrictions for data operands (including PC relative data addresses), some
performance degradation occurs when additional bus cycles are required for long-word or
word operands that are misaligned. For maximum performance, data items should be
aligned on their natural boundaries. All instruction words and extension words must reside
on word boundaries. Attempting to prefetch an instruction word at an odd address causes
an address error exception.
Figure 5-9 shows the transfer (write) of a long-word operand to an odd address in word-
organized memory, which requires three bus cycles. For the first cycle, SIZ1 and SIZ0
specify a long-word transfer, and A2–A0 = 001. Since the port width is 16 bits, only the
first byte of the long word is transferred. The slave device latches the byte and
acknowledges the data transfer, indicating that the port is 16 bits wide. When the
processor starts the second cycle, SIZ1 and SIZ0 specify that three bytes remain to be
transferred with A2–A0 = 010. The next two bytes are transferred during this cycle. The
processor then initiates the third cycle, with SIZ1 and SIZ0 indicating one byte remaining
to be transferred with A2–A0 = 100. The port latches the final byte, and the operation is
complete. Figure 5-10 shows the associated bus transfer signal timing. Figure 5-11 shows
the equivalent operation for a data read cycle.
DATA BUS
D31
D16
LONG-WORD OPERAND
OP0
OP1
OP2
OP3
31
0
WORD MEMORY
MSB
LSB
XXX
OP0
OP1
OP2
MC68020/EC020
SIZ1
SIZ0
A2
A1
0
0
0
0
1
1
0
1
MEMORY CONTROL
DSACK1
DSACK0
L
H
L
H
OP3
XXX
A0
1
0
0
1
1
0
0
L
H
Figure 5-9. Misaligned Long-Word Operand Write to Word Port Example
MOTOROLA M68020 USER’S MANUAL 5-15
BYTE WRITE
LONG-WORD OPERAND WRITE
S0
S2
S4
S0
S2
S4
CLK
A31–A2
A1
A0
FC2–FC0
SIZ1
SIZ0
R/W
ECS
OCS
AS
DS
DSACK1
DSACK0
DBEN
D31–D24
D23–D16
WORD WRITE
D15–D8
D7–D0
S0
S2
S4
OP0
OP0
OP1
OP2
OP1
OP2
OP1
OP2
OP3
OP3
OP3
OP3
BYTE WRITE
**
**
**
*
*
For the MC68EC020, A23–A2.
This signal does not apply to the MC68EC020.
Figure 5-10. Misaligned Long-Word Operand Write to Word Port Timing
5-16 M68020 USER’S MANUAL MOTOROLA
OP0
OP1
OP2
OP3
31
0
LONG-WORD OPERAND (REGISTER)
DATA BUS
D31
D16
WORD MEMORY
MSB
LSB
XXX
OP0
OP1
OP2
OP3
XXX
MC68020/EC020
SIZ1
SIZ0
A2
A1
0 0 0 0 1
1
1 0 1 0
0
1 1 0 0
A0
MEMORY CONTROL
DSACK1
DSACK0
L
H
L
H
L
H
Figure 5-11. Misaligned Long-Word Operand Read
from Word Port Example
Figures 5-12 and 5-13 show a word transfer (write) to an odd address in word-organized
memory. This example is similar to the one shown in Figures 5-9 and 5-10 except that the
operand is word sized and the transfer requires only two bus cycles. Figure 5-14 shows
the equivalent operation for a data read cycle.
MC68020/EC020
SIZ1
SIZ0
A2
A1
1 0 0 0 1
0
1 0 1 0
A0
MEMORY CONTROL
DSACK1
DSACK0
L
H
L
H
OP2
OP3
15
0
WORD OPERAND
DATA BUS
D31
D16
WORD MEMORY
MSB
LSB
XXX
OP3
OP2
XXX
Figure 5-12. Misaligned Word Operand Write to Word Port Example
MOTOROLA M68020 USER’S MANUAL 5-17
WORD OPERAND WRITE TO A1, A0 = 01
S0
S2
S4
S0
S2
S4
CLK
A31–A2
A1
A0
FC2–FC0
SIZ1
SIZ0
R/W
ECS
OCS
AS
DS
DSACK1
DSACK0
DBEN
D31–D24
D23–D16
WORD WRITE
D15–D8
D7–D0
OP2
OP2
OP3
OP2
OP3
OP3
OP3
OP3
BYTE WRITE
**
**
**
*
*
For the MC68EC020, A23–A2.
This signal does not apply to the MC68EC020.
**
Figure 5-13. Misaligned Word Operand Write to Word Port Timing
5-18 M68020 USER’S MANUAL MOTOROLA
MC68020/EC020
SIZ1
SIZ0
A2
A1
1 0 0 0 1
0
1 0 1 0
A0
MEMORY CONTROL
DSACK1
DSACK0
L
H
L
H
OP2
OP3
15
0
WORD OPERAND (REGISTER)
DATA BUS
D31
D16
WORD MEMORY
MSB
LSB
XXX
OP3
OP2
XXX
Figure 5-14. Misaligned Word Operand Read from Word Bus Example
Figures 5-15 and 5-16 show an example of a long-word transfer (write) to an odd address
in long-word-organized memory. In this example, a long-word access is attempted
beginning at the least significant byte of a long-word-organized memory. Only one byte
can be transferred in the first bus cycle. The second bus cycle then consists of a three -
byte access to a long-word boundary. Since the memory is long word organized, no
further bus cycles are necessary. Figure 5-17 shows the equivalent operation for a data
read cycle.
MC68020/EC020
SIZ1
SIZ0
A2
A1
0 0 0 1 1
1
1 1 0 0
A0
MEMORY CONTROL
DSACK1
DSACK0
L
L
L
OP0
OP1
31
0
LONG-WORD OPERAND
DATA BUS
D31
D0
LONG-WORD MEMORY
MSB
UMB
XXX
OP1
OP2
XXX
OP2
OP3
XXX
OP3
OP0
XXX
LMB
LSB
L
Figure 5-15. Misaligned Long-Word Operand Write
to Long-Word Port Example
MOTOROLA M68020 USER’S MANUAL 5-19
LONG-WORD OPERAND WRITE
S0
S2
S4
S0
S2
S4
CLK
A31–A2
A1
A0
FC2–FC0
SIZ1
SIZ0
R/W
ECS
OCS
AS
DS
DSACK1
DSACK0
DBEN
D31–D24
D23–D16
BYTE WRITE
D15–D8
D7–D0
OP0
OP0
OP1
OP0
OP1
OP2
OP3
OP1
3-BYTE WRITE
**
**
**
*
*
For the MC68EC020, A23–A2.
This signal does not apply to the MC68EC020.
**
Figure 5-16. Misaligned Long-Word Operand Write
to Long-Word Port Timing
5-20 M68020 USER’S MANUAL MOTOROLA
MC68020/EC020
SIZ1
SIZ0
A2
A1
0 0 0 1 1
1
1 1 0 0
A0
MEMORY CONTROL
DSACK1
DSACK0
L
L
L
OP0
OP1
31
0
LONG-WORD OPERAND (REGISTER)
DATA BUS
D31
D0
LONG-WORD MEMORY
MSB
UMB
XXX
OP1
OP2
XXX
OP2
OP3
XXX
OP3
OP0
XXX
LMB
LSB
L
Figure 5-17. Misaligned Long-Word Operand Read
from Long-Word Port Example
5.2.3 Effects of Dynamic Bus Sizing and Operand Misalignment
The combination of operand size, operand alignment, and port size determine the number
of bus cycles required to perform a particular memory access. Table 5-6 lists the number
of bus cycles required for different operand sizes to different port sizes with all possible
alignment conditions for read/write cycles.
Table 5-6. Memory Alignment and Port Size
Influence on Read/Write Bus Cycles
Number of Bus Cycles
(Data Port Size = 32 Bits:16 Bits:8 Bits)
A1, A0
Operand Size 00 01 10 11
Instruction*1:2:4 N/A N/A N/A
Byte Operand 1:1:1 1:1:1 1:1:1 1:1:1
Word Operand 1:1:2 1:2:2 1:1:2 2:2:2
Long-Word Operand 1:2:4 2:3:4 2:2:4 2:3:4
*Instruction prefetches are always two words from a long-word boundary
Table 5-6 reveals that bus cycle throughput is significantly affected by port size and
alignment. The MC68020/EC020 system designer and programmer should be aware of
and account for these effects, particularly in time-critical applications.
MOTOROLA M68020 USER’S MANUAL 5-21
Table 5-6 demonstrates that the processor always prefetches instructions by reading a
long word from a long-word address (A1, A0 = 00), regardless of port size or alignment.
When the required instruction begins at an odd-word boundary, the processor attempts to
fetch the entire 32 bits and loads both words into the instruction cache, if possible,
although the second one is the required word. Even if the instruction access is not cached,
the entire 32 bits are latched into an internal cache holding register from which the two
instructions words can subsequently be referenced. Refer to Section 8 Instruction
Execution Timing for a complete description of the cache holding register and pipeline
operation.
5.2.4 Address, Size, and Data Bus Relationships
The data transfer examples show how the MC68020/EC020 drives data onto or receives
data from the correct byte sections of the data bus. Table 5-7 shows the combinations of
the SIZ1, SIZ0, A1, and A0 signals that can be used to generate byte enable signals for
each of the four sections of the data bus for read and write cycles if the addressed device
requires them. The port size also affects the generation of these enable signals as shown
in the table. The four columns on the right correspond to the four byte enable signals.
Letters B, W, and L refer to port sizes: B for 8-bit ports, W for 16-bit ports, and L for 32-bit
ports. The letters B, W, and L imply that the byte enable signal should be true for that port
size. A dash (—) implies that the byte enable signal does not apply.
The MC68020/EC020 always drives all sections of the data bus because, at the beginning
of a write cycle, the bus controller does not know the port size.
Table 5-7 reveals that the MC68020/EC020 transfers the number of bytes specified by
SIZ1, SIZ0 to or from the specified address unless the operand is misaligned or unless the
number of bytes is greater than the port width. In these cases, the device transfers the
greatest number of bytes possible for the port. For example, if the size is four and A1, A0
= 01, a 32-bit slave can only receive three bytes in the current bus cycle. A 16- or 8-bit
slave can only receive one byte. The table defines the byte enables for all port sizes. Byte
data strobes can be obtained by combining the enable signals with the DS signal. Devices
residing on 8-bit ports can use the data strobe by itself since there is only one valid byte
for every transfer. These enable or strobe signals select only the bytes required for write
or read cycles. The other bytes are not selected, which prevents incorrect accesses in
sensitive areas such as I/O.
5-22 M68020 USER’S MANUAL MOTOROLA
Table 5-7. Data Bus Byte Enable Signals for Byte, Word, and Long-Word Ports
Data Bus Active Sections
Byte (B), Word (W) , Long-Word (L) Ports
Transfer Size SIZ1 SIZ0 A1 A0 D31–D24 D23–D16 D15–D8 D7–D0
Byte 0
0
0
0
1
1
1
1
0
0
1
1
0
1
0
1
B W L
B
B W
B
—
W L
—
W
—
—
L
—
—
—
—
L
Word 1
1
1
1
0
0
0
0
0
0
1
1
0
1
0
1
B W L
B
B W
B
W L
W L
W
W
—
L
L
—
—
—
L
L
3 Bytes 1
1
1
1
1
1
1
1
0
0
1
1
0
1
0
1
B W L
B
B W
B
W L
W L
W
W
L
L
L
—
—
L
L
L
Long Word 0
0
0
0
0
0
0
0
0
0
1
1
0
1
0
1
B W L
B
B W
B
W L
W L
W
W
L
L
L
—
L
L
L
L
Figure 5-18 shows a logic diagram of one method for generating byte enable signals for
16- and 32-bit ports from the SIZ1, SIZ0, A1, and A0 encodings and the R/W signal.
5.2.5 Cache Interactions
The organization and requirements of the on-chip instruction cache affect the
interpretation of DSACK1 and DSACK0. Since the MC68020/EC020 attempts to load all
instructions into the on-chip cache, the bus may operate differently when caching is
enabled. Specifically, on read cycles that terminate normally, the A1, A0, SIZ1, and SIZ0
signals do not apply.
The cache can also affect the assertion of AS and the operation of a read cycle. The
search of the cache by the processor begins when the sequencer requires an instruction.
At this time, the bus controller may also initiate an external bus cycle in case the
requested item is not resident in the instruction cache. If an internal cache hit occurs, the
external cycle aborts, and AS is not asserted.
For the MC68020, if the bus is not occupied with another read or write cycle, the bus
controller asserts the ECS signal (and the OCS signal, if appropriate). It is possible to have
ECS asserted on multiple consecutive clock cycles. Note that there is a minimum time
specified from the negation of ECS to the next assertion of ECS (refer to Section 10
Electrical Characteristics). Instruction prefetches can occur every other clock so that if,
after an aborted cycle due to an instruction cache hit, the bus controller asserts ECS on
the next clock, this second cycle is for a data fetch. Note that, if the bus controller is
executing other cycles, these aborted cycles due to cache hits may not be seen externally.
MOTOROLA M68020 USER’S MANUAL 5-23
A1
SIZ0
SIZ1
R/W
LD
UD
LLD
LMD
UMD
UUD
A0
UUD
U
MD
L
MD
L
LD
U
D
L
D
UPPER UPPER DATA (32-BIT PORT)
U
PPER MIDDLE DATA (32-BIT PORT)
L
OWER MIDDLE DATA (32-BIT PORT)
L
OWER LOWER DATA (32-BIT PORT)
U
PPER DATA (16-BIT PORT)
L
OWER DATA (16-BIT PORT)
=
=
=
=
=
=
Figure 5-18. Byte Enable Signal Generation for 16- and 32-Bit Ports
5-24 M68020 USER’S MANUAL MOTOROLA
5.2.6 Bus Operation
The MC68020/EC020 bus is used in an asynchronous manner allowing external devices
to operate at clock frequencies different from the MC68020/EC020 clock. Bus operation
uses the handshake lines (AS, DS , DSACK0, DSACK1, BERR, and HALT) to control data
transfers. AS signals the start of a bus cycle, and DS is used as a condition for valid data
on a write cycle. Decoding SIZ1, SIZ0, A1, and A0 provides byte enable signals that select
the active portion of the data bus. The slave device (memory or peripheral) then responds
by placing the requested data on the correct portion of the data bus for a read cycle or
latching the data on a write cycle and by asserting the DSACK0/DSACK1 combination that
corresponds to the port size to terminate the cycle. If no slave responds or the access is
invalid, external control logic asserts BERR to abort or BERR and HALT to retry the bus
cycle.
DSACK1/DSACK0 can be asserted before the data from a slave device is valid on a read
cycle. The length of time that DSACK1/DSACK0 may precede data is given by parameter
#31, and it must be met in any asynchronous system to ensure that valid data is latched
into the processor. (Refer to Section 10 Electrical Characteristics for timing
parameters.) Note that no maximum time is specified from the assertion of AS to the
assertion of DSACK1/DSACK0. Although the processor can transfer data in a minimum of
three clock cycles when the cycle is terminated with DSACK1/DSACK0, the processor
inserts wait cycles in clock period increments until DSACK1/DSACK0 is recognized.
The BERR and/or HALT signals can be asserted after DSACK1/DSACK0 is asserted.
BERR and/or HALT must be asserted within the time given (parameter #48), after
DSACK1/DSACK0 is asserted in any asynchronous system. If this maximum delay time is
violated, the processor may exhibit erratic behavior.
5.2.7 Synchronous Operation with
DSACK1
/
DSACK0
Although cycles terminated with DSACK1/DSACK0 are classified as asynchronous, cycles
terminated with DSACK1/DSACK0 can also operate synchronously in that signals are
interpreted relative to clock edges. The devices that use these synchronous cycles must
synchronize the responses to the MC68020/EC020 clock. Since these devices terminate
bus cycles with DSACK1/DSACK0, the dynamic bus sizing capabilities of the
MC68020/EC020 are available. In addition, the minimum cycle time for these synchronous
cycles is three clocks.
To support systems that use the system clock to generate DSACK1/DSACK0 and other
asynchronous inputs, the asynchronous input setup time (parameter #47A) and the
asynchronous input hold time (parameter #47B) are provided in Section 10 Electrical
Characteristics. (Note: although a misnomer, these “asynchronous” parameters are the
setup and hold times for synchronous operation.) If the setup and hold times are met for
the assertion or negation of a signal, such as DSACK1/DSACK0, the processor can be
guaranteed to recognize that signal level on that specific falling edge of the system clock.
If the assertion of DSACK1/DSACK0 is recognized on a particular falling edge of the clock,
valid data is latched into the processor (for a read cycle) on the next falling clock edge
provided the data meets the data setup time (parameter #27). In this case, parameter #31
MOTOROLA M68020 USER’S MANUAL 5-25
for asynchronous operation can be ignored. All timing parameters referred to are
described in Section 10 Electrical Characteristics. If a system asserts
DSACK1/DSACK0 for the required window around the falling edge of state 2 and obeys
the proper bus protocol by maintaining DSACK1/DSACK0 (and/or BERR/HALT) until and
throughout the clock edge that negates AS (with the appropriate asynchronous input hold
time specified by parameter #47B), no wait states are inserted. The bus cycle runs at its
maximum speed of three clocks per cycle for bus cycles terminated with
DSACK1/DSACK0.
To ensure proper operation in a synchronous system when BERR or BERR/HALT is
asserted after DSACK1/DSACK0, BERR (and HALT) must meet the appropriate setup time
(parameter #27A) prior to the falling clock edge one clock cycle after DSACK1/DSACK0 is
recognized. This setup time is critical, and the MC68020/EC020 may exhibit erratic
behavior if it is violated.
When operating synchronously, the data-in setup (parameter #27) and hold (parameter
#30) times for synchronous cycles may be used instead of the timing requirements for
data relative to the DS signal.
5.3 DATA TRANSFER CYCLES
The transfer of data between the processor and other devices involves the following
signals:
• Address Bus (A31–A0 for the MC68020) (A23–A0 for the MC68EC020)
• Data Bus (D31–D0)
• Control Signals
The address and data buses are both parallel, nonmultiplexed buses. The bus master
moves data on the bus by issuing control signals, and the bus uses a handshake protocol
to ensure correct movement of the data. In all bus cycles, the bus master is responsible
for de-skewing all signals it issues at both the start and end of the cycle. In addition, the
bus master is responsible for de-skewing DSACK1/DSACK0, D31–D0, BERR, HALT, and,
for the MC68020, DBEN from the slave devices. The following paragraphs define read,
write, and read-modify-write cycle operations.
Each of the bus cycles is defined as a succession of states. These states apply to the bus
operation and are different from the processor states described in Section 2 Processing
States. The clock cycles used in the descriptions and timing diagrams of data transfer
cycles are independent of the clock frequency. Bus operations are described in terms of
external bus states.
5-26 M68020 USER’S MANUAL MOTOROLA
5.3.1 Read Cycle
During a read cycle, the processor receives data from a memory, coprocessor, or
peripheral device. If the instruction specifies a long-word operation, the MC68020/EC020
attempts to read four bytes at once. For a word operation, it attempts to read two bytes at
once and for a byte operation, one byte. For some operations, the processor requests a
three-byte transfer. The processor properly positions each byte internally. The section of
the data bus from which each byte is read depends on the operand size, A1–A0, and the
port size. Refer to 5.2.1 Dynamic Bus Sizing and 5.2.2 Misaligned Operands for more
information on dynamic bus sizing and misaligned operands.
Figure 5-19 is a flowchart of a long-word read cycle. Figure 5-20 is a flowchart of a byte
read cycle. Figures 5-21–5-23 are read cycle timing diagrams in terms of clock periods.
Figure 5-21 corresponds to byte and word read cycles from a 32-bit port. Figure 5-22
corresponds to a long-word read cycle from an 8-bit port. Figure 5-23 also applies to a
long-word read cycle, but from 16- and 32-bit ports.
PROCESSOR
ADDRESS DEVICE
1) ASSERT ECS/OCS FOR ONE-HALF CLOCK
2
) SET R/W TO READ
3
) DRIVE ADDRESS ON A31–A0
4
) DRIVE FUNCTION CODE ON FC2–FC0
5
) DRIVE SIZ1, SIZ0 (FOUR BYTES)
6
) ASSERT AS
7
) ASSERT DS
8
) ASSERT DBEN
ACQUIRE DATA
1) LATCH DATA
2
) NEGATE AS AND D
S
3
) NEGATE DBEN
START NEXT CYCLE
PRESENT DATA
1) DECODE ADDRESS
2
) PLACE DATA ON D31–D0
3
) ASSERT DSACK1/DSACK0
TERMINATE CYCLE
1) REMOVE DATA FROM D31–D0
2
) NEGATE DSACK1/DSACK0
EXTERNAL DEVICE
*
*
*
This step does not apply to the MC68EC020.
For the MC68EC020, A23–A0.
*
**
Figure 5-19. Long-Word Read Cycle Flowchart
MOTOROLA M68020 USER’S MANUAL 5-27
ACQUIRE DATA
1) LATCH DATA
2
) NEGATE AS AND D
S
3
) NEGATE DBEN
START NEXT CYCLE
PRESENT DATA
1) DECODE ADDRESS
2
) PLACE DATA ON D31–D24 OR
D23–D16 OR
D15–D8 OR
D7–D0
(BASED ON A1, A0, AND BUS WIDTH
)
3
) ASSERT DSACK1/DSACK0
TERMINATE CYCLE
1) REMOVE DATA FROM D31–D0
2
) NEGATE DSACK1/DSACK0
EXTERNAL DEVICE
PROCESSOR
ADDRESS DEVICE
1) ASSERT ECS/OCS FOR ONE-HALF CLOCK
2
) SET R/W TO READ
3
) DRIVE ADDRESS ON A31–A0
4
) DRIVE FUNCTION CODE ON FC2–FC0
5
) DRIVE SIZ1, SIZ0 (FOUR BYTES)
6
) ASSERT AS
7
) ASSERT DS
8
) ASSERT DBEN
*
*
This step does not apply to the MC68EC020.
For the MC68EC020, A23–A0.
**
*
**
*
Figure 5-20. Byte Read Cycle Flowchart
5-28 M68020 USER’S MANUAL MOTOROLA
WORD READ
S0
S2
S4
S0
S2
S4
CLK
A31–A2
A1
A0
FC2–FC0
SIZ1
SIZ0
R/W
ECS
OCS
AS
DS
DSACK1
DSACK0
DBEN
D31–D24
D23–D16
BYTE READ
D15–D8
D7–D0
S0
S2
S4
OP2
OP3
OP3
OP3
WORD
BYTE
BYTE READ
**
**
**
*
*
Figure 5-21. Byte and Word Read Cycles—32-Bit Port
MOTOROLA M68020 USER’S MANUAL 5-29
BYTE READ
A31–A2
A1
A0
FC2–FC0
SIZ1
SIZ0
R/W
ECS
OCS
AS
DS
DSACK1
DSACK0
DBEN
D31–D24
D23–D16
D15–D8
D7–D0
OP0
OP1
OP3
LONG WORD
3-BYTE
BYTE READ
CLK
WORD
BYTE
OP2
BYTE READ
BYTE READ
LONG-WORD OPERAND READ FROM 8-BIT PORT
S0
S2
S4
S0
S2
S4
S0
S2
S4
S0
S2
S4
**
**
**
*
*
For the MC68EC020, A23–A2.
This signal does not apply to the MC68EC020.
Figure 5-22. Long-Word Read—8-Bit Port
5-30 M68020 USER’S MANUAL MOTOROLA
WORD READ
S0
S2
S4
S0
S2
S4
CLK
A31–A2
A1
A0
FC2–FC0
SIZ1
SIZ0
R/W
ECS
OCS
AS
DS
DSACK1
DSACK0
DBEN
D31–D24
D23–D16
WORD READ
D15–D8
D7–D0
S0
S2
S4
OP0
OP1
OP3
OP3
LONG WORD
WORD
LONG-WORD READ
FROM 32-BIT POR
T
OP2
OP1
OP0
OP2
LONG WORD
LONG-WORD OPERAND READ FROM 16-BIT PORT
**
**
**
*
*
For the MC68EC020, A23–A2.
This signal does not apply to the MC68EC020.
**
Figure 5-23. Long-Word Read—16- and 32-Bit Ports
MOTOROLA M68020 USER’S MANUAL 5-31
State 0
MC68020—The read cycle starts in state 0 (S0). The processor asserts ECS, indicating
the beginning of an external cycle. If the cycle is the first external cycle of a read
operation, OCS is asserted simultaneously. During S0, the processor places a valid
address on A31–A0 and valid function codes on FC2–FC0. The function codes select
the address space for the cycle. The processor drives R/ W high for a read cycle and
negates DBEN to disable the data buffers. SIZ0 and SIZ1 become valid, indicating the
number of bytes requested to be transferred.
MC68EC020—The read cycle starts in S0. During S0, the processor places a valid
address on A23–A0 and valid function codes on FC2–FC0. The function codes select
the address space for the cycle. The processor drives R/W high for a read cycle. SIZ0
and SIZ1 become valid, indicating the number of bytes requested to be transferred.
State 1
MC68020—One-half clock later in state 1 (S1), the processor asserts AS, indicating that
the address on the address bus is valid. The processor also asserts DS during S1. In
addition, the ECS (and OCS, if asserted) signal is negated during S1.
MC68EC020—One-half clock later in S1, the processor asserts AS, indicating that the
address on the address bus is valid. The processor also asserts DS during S1.
State 2
MC68020—During state 2 (S2), the processor asserts DBEN to enable external data
buffers. The selected device uses R/W, SIZ1–SIZ0, A1–A0, and DS to place its
information on the data bus. Any or all of the bytes (D31–D24, D23–D16, D15–D8, and
D7–D0) are selected by SIZ1–SIZ0 and A1–A0. Concurrently, the selected device
asserts DSACK1/DSACK0.
MC68EC020—During S2, the selected device uses R/W, SIZ1–SIZ0, A1–A0, and DS to
place its information on the data bus. Any or all of the bytes (D31–D24, D23–D16,
D15–D8, and D7–D0) are selected by SIZ1–SIZ0 and A1–A0. Concurrently, the
selected device asserts DSACK1/DSACK0.
State 3
MC68020/EC020—As long as at least one of the DSACK1/DSACK0 signals is
recognized by the end of S2 (meeting the asynchronous input setup time requirement),
data is latched on the next falling edge of the clock, and the cycle terminates. If
DSACK1/DSACK0 is not recognized by the start of state 3 (S3), the processor inserts
wait states instead of proceeding to states 4 and 5. To ensure that wait states are
inserted, both DSACK1 and DSACK0 must remain negated throughout the
asynchronous input setup and hold times around the end of S2. If wait states are
added, the processor continues to sample the DSACK1/DSACK0 signals on the falling
edges of the clock until an assertion is recognized.
5-32 M68020 USER’S MANUAL MOTOROLA
State 4
MC68020/EC020—At the end of state 4 (S4), the processor latches the incoming data.
State 5
MC68020—The processor negates AS, DS, and DBEN during state 5 (S5). It holds the
address valid during S5 to provide address hold time for memory systems. R/W, SIZ1–
SIZ0, and FC2–FC0 also remain valid throughout S5.
The external device keeps its data and DSACK1/DSACK0 signals asserted until it
detects the negation of AS or DS (whichever it detects first). The device must remove
its data and negate DSACK1/DSACK0 within approximately one clock period after
sensing the negation of AS or DS . DSACK1/DSACK0 signals that remain asserted
beyond this limit may be prematurely detected for the next bus cycle.
MC68EC020—The processor negates AS and DS during state S5. It holds the address
valid during S5 to provide address hold time for memory systems. R/W, SIZ1, SIZ0,
and FC2–FC0 also remain valid throughout S5.
The external device keeps its data and DSACK1/DSACK0 signals asserted until it
detects the negation of AS or DS (whichever it detects first). The device must remove
its data and negate DSACK1/DSACK0 within approximately one clock period after
sensing the negation of AS or DS . DSACK1/DSACK0 signals that remain asserted
beyond this limit may be prematurely detected for the next bus cycle.
MOTOROLA M68020 USER’S MANUAL 5-33
5.3.2 Write Cycle
During a write cycle, the processor transfers data to memory or a peripheral device.
Figure 5-24 is a flowchart of a write cycle operation for a long-word transfer. Figures 5-25–
5-28 are write cycle timing diagrams in terms of clock periods. Figure 5-25 shows two
write cycles (between two read cycles with no idle time in between) for a 32-bit port.
Figure 5-26 shows byte and word write cycles to a 32-bit port. Figure 5-27 shows a long-
word write cycle to an 8-bit port. Figure 5-28 shows a long-word write cycle to a 16-bit
port.
1) ASSERT ECS/OCS FOR ONE-HALF CLOCK
2
) DRIVE ADDRESS ON A31–A0
3
) DRIVE FUNCTION CODES ON FC2–FC0
4
) DRIVE SIZ1, SIZ0 (FOUR BYTES)
5
) SET R/W TO WRITE
6
) ASSERT AS
7
) ASSERT DBEN
8
) DRIVE DATA LINES D31–D0
9
) ASSERT DS
1) NEGATE AS AND DS
2
) REMOVE DATA FROM D31–D0
3
) NEGATE DBEN
EXTERNAL DEVICE
PROCESSOR
1) NEGATE DSACK1/DSACK0
TERMINATE CYCLE
ACCEPT DATA
1) DECODE ADDRESS
2
) STORE DATA FROM D31–D0
3
) ASSERT DSACK1/DSACK0
ADDRESS DEVICE
TERMINATE OUTPUT TRANSFER
START NEXT CYCLE
*
*
This step does not apply to the MC68EC020.
For the MC68EC020, A23–A0.
**
*
**
*
Figure 5-24. Write Cycle Flowchart
5-34 M68020 USER’S MANUAL MOTOROLA
WRITE
A31–A2
A1
A0
FC2–FC0
SIZ1
SIZ0
R/W
ECS
OCS
AS
DS
DSACK1
DSACK0
DBEN
D31–D0
LONG WORD
CLK
WRITE
BYTE READ
S0
S2
S4
S0
S2
S4
S0
S2
S4
S0
S2
Sw
Sw
S4
READ WITH WAIT STATES
**
**
**
*
*
For the MC68EC020, A23–A2.
This signal does not apply to the MC68EC020.
**
Figure 5-25. Read-Write-Read Cycles—32-Bit Port
MOTOROLA M68020 USER’S MANUAL 5-35
WORD WRITE
S0
S2
S4
S0
S2
S4
CLK
A31-A2
A1
A0
FC2–FC0
SIZ1
SIZ0
R/W
ECS
OCS
AS
DS
DSACK1
DSACK0
DBEN
D31–D24
D23–D16
BYTE WRITE
D15–D8
D7–D0
S0
S2
S4
OP2
OP3
OP3
OP3
WORD
OP3
OP3
OP3
OP3
BYTE
OP2
OP3
OP3
OP3
BYTE WRITE
**
**
**
*
*
For the MC68EC020, A23–A2.
This signal does not apply to the MC68EC020.
Figure 5-26. Byte and Word Write Cycles—32-Bit Port
5-36 M68020 USER’S MANUAL MOTOROLA
BYTE WRITE
A31–A2
A1
A0
FC2–FC0
SIZ1
SIZ0
R/W
ECS
OCS
AS
DS
DSACK1
DSACK0
DBEN
D31–D24
D23–D16
D15–D8
D7–D0
LONG WORD
3-BYTE
BYTE WRITE
CLK
WORD
BYTE
BYTE WRITE
BYTE WRITE
LONG-WORD OPERAND WRITE TO 8-BIT PORT
S0
S2
S4
S0
S2
S4
S0
S2
S4
S0
S2
S4
OP0
OP3
OP2
OP1
OP1
OP3
OP3
OP1
OP2
OP3
OP2
OP2
OP3
OP3
OP3
OP3
**
**
**
*
*
For the MC68EC020, A23–A2.
This signal does not apply to the MC68EC020.
Figure 5-27. Long-Word Operand Write—8-Bit Port
MOTOROLA M68020 USER’S MANUAL 5-37
WORD WRITE
S0
S2
S4
S0
S2
S4
CLK
A31–A2
A1
A0
FC2–FC0
SIZ1
SIZ0
R/W
ECS
OCS
AS
DS
DSACK1
DSACK0
DBEN
D31–D24
D23–D16
WORD WRITE
D15–D8
D7–D0
S0
S2
S4
OP0
OP1
OP3
OP3
LONG WORD
OP2
OP1
OP0
OP2
WORD
OP2
OP3
OP3
OP2
LONG-WORD WRITE
TO 32-BIT PORT
LONG-WORD OPERAND WRITE TO 16-BIT PORT
**
**
**
*
*
For the MC68EC020, A23–A2.
This signal does not apply to the MC68EC020.
**
LONG WORD
Figure 5-28. Long-Word Operand Write—16-Bit Port
5-38 M68020 USER’S MANUAL MOTOROLA
State 0
MC68020—The write cycle starts in S0. The processor negates ECS, indicating the
beginning of an external cycle. If the cycle is the first external cycle of a write
operation, OCS is asserted simultaneously. During S0, the processor places a valid
address on A31–A0 and valid function codes on FC2–FC0. The function codes select
the address space for the cycle. The processor drives R/W low for a write cycle. SIZ1–
SIZ0 become valid, indicating the number of bytes to be transferred.
MC68EC020—The write cycle starts in S0. During S0, the processor places a valid
address on A23–A0 and valid function codes on FC2–FC0. The function codes select
the address space for the cycle. The processor drives R/W low for a write cycle. SIZ1,
SIZ0 become valid, indicating the number of bytes to be transferred.
State 1
MC68020—One-half clock later in S1, the processor asserts AS, indicating that the
address on the address bus is valid. The processor also asserts DBEN during S1,
which can enable external data buffers. In addition, the ECS (and OCS, if asserted)
signal is negated during S1.
MC68EC020—One-half clock later in S1, the processor asserts AS, indicating that the
address on the address bus is valid.
State 2
MC68020/EC020—During S2, the processor places the data to be written onto D31–D0.
At the end of S2, the processor samples DSACK1/DSACK0.
State 3
MC68020/EC020—The processor asserts DS during S3, indicating that the data on the
data bus is stable. As long as at least one of the DSACK1/DSACK0 signals is
recognized by the end of S2 (meeting the asynchronous input setup time requirement),
the cycle terminates one clock later. If DSACK1/DSACK0 is not recognized by the start
of S3, the processor inserts wait states instead of proceeding to S4 and S5. To ensure
that wait states are inserted, both DSACK1 and DSACK0 must remain negated
throughout the asynchronous input setup and hold times around the end of S2. If wait
states are added, the processor continues to sample the DSACK1/DSACK0 signals on
the falling edges of the clock until one is recognized.
The external device uses R/W, DS , SIZ1, SIZ0, A1, and A0 to latch data from the
appropriate byte(s) of the data bus (D31–D24, D23–D16, D15–D8, and D7–D0). SIZ1,
SIZ0, A1, and A0 select the bytes of the data bus. If it has not already done so, the
device asserts DSACK1/DSACK0 to signal that it has successfully stored the data.
MOTOROLA M68020 USER’S MANUAL 5-39
State 4
MC68020/EC020—The processor issues no new control signals during S4.
State 5
MC68020—The processor negates AS and DS during S5. It holds the address and data
valid during S5 to provide address hold time for memory systems. R/W, SIZ1, SIZ0,
FC2–FC0, and DBEN also remain valid throughout S5.
The external device must keep DSACK1/DSACK0 asserted until it detects the negation
of AS or DS (whichever it detects first). The device must negate DSACK1/DSACK0
within approximately one clock period after sensing the negation of AS or DS .
DSACK1/DSACK0 signals that remain asserted beyond this limit may be prematurely
detected for the next bus cycle.
MC68EC020—The processor negates AS and DS during S5. It holds the address and
data valid during S5 to provide address hold time for memory systems. R/W, SIZ1,
SIZ0, and FC2–FC0 also remain valid throughout S5.
The external device must keep DSACK1/DSACK0 asserted until it detects the negation
of AS or DS (whichever it detects first). The device must negate DSACK1/DSACK0
within approximately one clock period after sensing the negation of AS or DS .
DSACK1/DSACK0 signals that remain asserted beyond this limit may be prematurely
detected for the next bus cycle.
5.3.3 Read-Modify-Write Cycle
The read-modify-write cycle performs a read, conditionally modifies the data in the
arithmetic logic unit, and may write the data out to memory. In the MC68020/EC020, this
operation is indivisible, providing semaphore capabilities for multiprocessor systems.
During the entire read-modify-write sequence, the MC68020/EC020 asserts RMC to
indicate that an indivisible operation is occurring. The MC68020/EC020 does not issue a
BG signal in response to a BR signal during this operation.
The TAS, CAS, and CAS2 instructions are the only MC68020/EC020 instructions that
utilize read-modify-write operations. Depending on the compare results of the CAS and
CAS2 instructions, the write cycle(s) may not occur.
Figure 5-29 is a flowchart of the read-modify-write cycle operation. Figure 5-30 is an
example timing diagram of a TAS instruction specified in terms of clock periods.
5-40 M68020 USER’S MANUAL MOTOROLA
LOCK BUS
1) ASSERT RMC
ADDRESS DEVICE
1) ASSERT ECS/OCS FOR ONE-HALF CLOCK
2
) SET R/W TO READ
3
) DRIVE ADDRESS ON A31–A0
4
) DRIVE FUNCTION CODES ON FC2–FC0
5
) DRIVE SIZ1, SIZ0
6
) ASSERT AS
7
) ASSERT DS
8
) ASSERT DBEN
ACQUIRE DATA
1) LATCH DATA
2
) NEGATE AS AND DS
3
) NEGATE DBEN
4
) START DATA MODIFICATIO
N
START OUTPUT TRANSFER
1) ASSERT ECS/OCS FOR ONE-HALF CLOCK
2
) DRIVE ADDRESS ON A31–A0 (IF DIFFERENT
)
3
) DRIVE SIZ1, SIZ0
4
) SET R/W TO WRITE
5
) ASSERT AS
6
) ASSERT DBEN
7
) PLACE DATA ON D31–D0
8
) ASSERT DS
TERMINATE OUTPUT TRANSFER
1) NEGATE AS AND DS
2
) REMOVE DATA FROM D31–D0
3
) NEGATE DBEN
UNLOCK BUS
1) NEGATE RMC
START NEXT CYCLE
PRESENT DATA
1) DECODE ADDRESS
2
) PLACE DATA ON D31–D0
3
) ASSERT DSACK1/DSACK0
TERMINATE CYCLE
1) REMOVE DATA FROM D31–D0
2
) NEGATE DSACK1/DSACK0
ACCEPT DATA
1) DECODE ADDRESS
2
) STORE DATA FROM D31–D0
3
) ASSERT DSACK1/DSACK0
TERMINATE CYCLE
A
IF CAS2 INSTRUCTION
A
ND ONLY ONE OPERAN
D
READ, THEN GO TO A
;
IF OPERANDS DO NOT
MATCH, THEN GO TO
C ; ELSE GO TO B
C
B
1) NEGATE DSACK1/DSACK0
IF CAS2 INSTRUCTION
A
ND ONLY ONE OPERAND
WRITTEN, THEN GO TO
D ; ELSE GO TO E
E
D
PROCESSOR
EXTERNAL DEVICE
*
This step does not apply to the MC68EC020.
For the MC68EC020, A23–A0.
**
*
**
*
*
*
*
**
*
Figure 5-29. Read-Modify-Write Cycle Flowchart
MOTOROLA M68020 USER’S MANUAL 5-41
INDIVISIBLE CYCLE
NEXT CYCLE
CLK
A31–A2
A1
A0
FC2–FC0
SIZ1
R/W
AS
DS
DSACK0
DBEN
D31–D24
SIZ0
DSACK1
S0
S2
S4
Si
S6
S8
S10
S0
D7–D0
D23–D16
RMC
ECS
OP3
OP3
OP3
OP3
OP3
BERR
HALT
BG
D15–8
S11
**
**
**
*
*
For the MC68EC020, A23–A2.
This signal does not apply to the MC68EC020.
**
Si
OCS
Figure 5-30. Byte Read-Modify-Write Cycle—32-Bit Port (TAS Instruction)
5-42 M68020 USER’S MANUAL MOTOROLA
State 0
MC68020—The processor asserts ECS and OCS in S0 to indicate the beginning of an
external operand cycle. The processor also asserts RMC in S0 to identify a read-
modify-write cycle. The processor places a valid address on A31–A0 and valid function
codes on FC2–FC0. The function codes select the address space for the operation.
SIZ1, SIZ0 become valid in S0 to indicate the operand size. The processor drives R/W
high for the read cycle.
MC68EC020—The processor asserts RMC in S0 to identify a read-modify-write cycle.
The processor places a valid address on A23–A0 and valid function codes on FC2–
FC0. The function codes select the address space for the operation. SIZ1–SIZ0
become valid in S0 to indicate the operand size. The processor drives R/W high for the
read cycle.
State 1
MC68020—One-half clock later in S1, the processor asserts AS to indicate that the
address on the address bus is valid. The processor also asserts DS during S1. In
addition, the ECS (and OCS, if asserted) signal is negated during S1.
MC68EC020—One-half clock later in S1, the processor asserts AS to indicate that the
address on the address bus is valid. The processor also asserts DS during S1.
State 2
MC68020—During S2, the processor asserts DBEN to enable external data buffers. The
selected device uses R/W, SIZ1, SIZ0, A1, A0, and DS to place information on the
data bus. Any or all of the bytes (D31–D24, D23–D16, D15–D8, and D7–D0) are
selected by SIZ1, SIZ0, A1, and A0. Concurrently, the selected device may assert the
DSACK1/DSACK0 signals.
MC68EC020—During S2, the selected device uses R/W, SIZ1, SIZ0, A1, A0, and DS to
place information on the data bus. Any or all of the bytes (D31–D24, D23–D16, D15–
D8, and D7–D0) are selected by SIZ1, SIZ0, A1, and A0. Concurrently, the selected
device may assert the DSACK1/DSACK0 signals.
State 3
MC68020/EC020—As long as at least one of the DSACK1/DSACK0 signals is
recognized by the end of S2 (meeting the asynchronous input setup time requirement),
data is latched on the next falling edge of the clock, and the cycle terminates. If
DSACK1/DSACK0 is not recognized by the start of S3, the processor inserts wait
states instead of proceeding to S4 and S5. To ensure that wait states are inserted,
both DSACK0 and DSACK1 must remain negated throughout the asynchronous input
setup and hold times around the end of S2. If wait states are added, the processor
continues to sample the DSACK1/DSACK0 signals on the falling edges of the clock
until one is recognized.
State 4
MC68020/EC020—At the end of S4, the processor latches the incoming data.
MOTOROLA M68020 USER’S MANUAL 5-43
State 5
MC68020—The processor negates AS, DS , and DBEN during S5. If more than one read
cycle is required to read in the operand(s), S0–S5 are repeated for each read cycle.
When the read cycle(s) are complete, the processor holds the address, R/W, and
FC2–FC0 valid in preparation for the write portion of the cycle.
The external device keeps its data and DSACK1/DSACK0 signals asserted until it
detects the negation of AS or DS (whichever it detects first). The device must remove
the data and negate DSACK1/DSACK0 within approximately one clock period after
sensing the negation of AS or DS . DSACK1/DSACK0 signals that remain asserted
beyond this limit may be prematurely detected for the next portion of the operation.
MC68EC020—The processor negates AS, DS , and DBEN during S5. If more than one
read cycle is required to read in the operand(s), S0–S5 are repeated for each read
cycle. When the read cycle(s) is complete, the processor holds the address, R/W, and
FC2–FC0 valid in preparation for the write portion of the cycle.
The external device keeps its data and DSACK1/DSACK0 signals asserted until it
detects the negation of AS or DS (whichever it detects first). The device must remove
the data and negate DSACK1/DSACK0 within approximately one clock period after
sensing the negation of AS or DS . DSACK1/DSACK0 signals that remain asserted
beyond this limit may be prematurely detected for the next portion of the operation.
Idle States
MC68020/EC020—The processor does not assert any new control signals during the
idle states, but it may internally begin the modify portion of the cycle at this time. S6–
S11 are omitted if no write cycle is required. If a write cycle is required, the R/W signal
remains in the read mode until S6 to prevent bus conflicts with the preceding read
portion of the cycle; the data bus is not driven until S8.
State 6
MC68020—The processor asserts ECS and OCS in S6 to indicate that another external
cycle is beginning. The processor drives R/W low for a write cycle. Depending on the
write operation to be performed, the address lines may change during S6.
MC68EC020—During S6, the processor drives R/W low for a write cycle. Depending on
the write operation to be performed, the address lines may change during S6.
State 7
MC68020—During S7, the processor asserts AS, indicating that the address on the
address bus is valid. The processor also asserts DBEN, which can be used to enable
data buffers. In addition, ECS (and OCS, if asserted) is negated during S7.
MC68EC020—During S7, the processor asserts AS, indicating that the address on the
address bus is valid.
State 8
MC68020/EC020—During S8, the processor places the data to be written onto the data
bus.
5-44 M68020 USER’S MANUAL MOTOROLA
State 9
MC68020/EC020—The processor asserts DS during S9, indicating that the data on the
data bus is stable. As long as at least one of the DSACK1/DSACK0 signals is
recognized by the end of S8 (meeting the asynchronous input setup time requirement),
the cycle terminates one clock later. If DSACK1/DSACK0 is not recognized by the start
of S9, the processor inserts wait states instead of proceeding to S10 and S11. To
ensure that wait states are inserted, both DSACK1 and DSACK0 must remain negated
throughout the asynchronous input setup and hold times around the end of S8. If wait
states are added, the processor continues to sample DSACK1/DSACK0 signals on the
falling edges of the clock until one is recognized.
The external device uses R/W, DS , SIZ1, SIZ0, A1, and A0 to latch data from the
appropriate section(s) of the data bus (D31–D24, D23–D16, D15–D8, and D7–D0).
SIZ1, SIZ0, A1, and A0 select the data bus sections. If it has not already done so, the
device asserts DSACK1/DSACK0 when it has successfully stored the data.
State 10
MC68020/EC020—The processor issues no new control signals during S10.
State 11
MC68020/EC020—The processor negates AS and DS during S11. It holds the address
and data valid during S11 to provide address hold time for memory systems. R/W and
FC2–FC0 also remain valid throughout S11.
If more than one write cycle is required, S6–S11 are repeated for each write cycle.
The external device keeps DSACK1/DSACK0 asserted until it detects the negation of
AS or DS (whichever it detects first). The device must remove its data and negate
DSACK1/DSACK0 within approximately one clock period after sensing the negation of
AS or DS .
5.4 CPU SPACE CYCLES
FC2–FC0 select user and supervisor program and data areas as listed in Table 2-1. The
area selected by FC2–FC0 = 111 is classified as the CPU space. The interrupt
acknowledge, breakpoint acknowledge, module operations, and coprocessor
communication cycles described in the following paragraphs utilize CPU space.
The CPU space type is encoded on A19–A16 during a CPU space operation and indicates
the function that the processor is performing. On the MC68020/EC020, four of the
encodings are implemented as shown in Figure 5-31. All unused values are reserved by
Motorola for future use.
MOTOROLA M68020 USER’S MANUAL 5-45
1 1 1
1 1 1
1 1 1
1 1 1
BREAKPOINT
A
CKNOWLEDG
E
ACCESS LEVEL
CONTRO
L
COPROCESSOR
COMMUNICATIO
N
INTERRUPT
A
CKNOWLEDG
E
1 1 1
1 1 1
1 1 1
1 1 1
1 1 1
1 1 1
1 1 1
1 1 1
1 1 1
1
LEVEL
1
0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0
CpID
0 0 0 0 0 0 0 0
CP REG
15
13
4
0
3
1
0
31
31
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1
0 0 0 0 0 0 0 0 0
6
0
MMU REG
BKPT #
0 0
31
31
4
2
0
0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
19
16
23
2
0
FUNCTION
CODE
ADDRESS BUS
CPU SPACE
T
YPE FIELD
24
20
15
5
1
7
19
16
20
15
12
16
19
20
19
20
5
4
15
16
Figure 5-31. MC68020/EC020 CPU Space Address Encoding
5.4.1 Interrupt Acknowledge Bus Cycles
When a peripheral device signals the processor (with the IPL2–IPL0 signals) that the
device requires service and when the internally synchronized value on these signals
indicates a higher priority than the interrupt mask in the status register (or that a transition
has occurred in the case of a level 7 interrupt), the processor makes the interrupt a
pending interrupt. Refer to Section 6 Exception Processing for details on the recognition
of interrupts.
The MC68020/EC020 takes an interrupt exception for a pending interrupt within one
instruction boundary (after processing any other pending exception with a higher priority).
The following paragraphs describe the various kinds of interrupt acknowledge bus cycles
that can be executed as part of interrupt exception processing.
5.4.1.1 INTERRUPT ACKNOWLEDGE CYCLE—TERMINATED NORMALLY. When the
MC68020/EC020 processes an interrupt exception, it performs an interrupt acknowledge
cycle to obtain the number of the vector that contains the starting location of the interrupt
service routine.
Some interrupting devices have programmable vector registers that contain the interrupt
vectors for the routines they use. The following paragraphs describe the interrupt
acknowledge cycle for these devices. Other interrupting conditions or devices cannot
supply a vector number and use the autovector cycle described in 5.4.1.2 Autovector
Interrupt Acknowledge Cycle.
5-46 M68020 USER’S MANUAL MOTOROLA
The interrupt acknowledge cycle is a read cycle. It differs from the read cycle described in
5.3.1 Read Cycle in that it accesses the CPU address space. Specifically, the differences
are:
1. FC2–FC0 are set 111 for CPU address space.
2. A3, A2, and A1 are set to the interrupt request level (the inverted values of IPL2,
IPL1, and IPL0, respectively).
3. The CPU space type field (A19–A16) is set to 1111, the interrupt acknowledge code.
4. Other address signals (A31–A20, A15–A4, and A0 for the MC68020; A23–A20,
A15–A4, and A0 for the MC68EC020) are set to one.
The responding device places the vector number on the data bus during the interrupt
acknowledge cycle. Beyond this, the cycle is terminated normally with DSACK1/DSACK0.
Figure 5-32 is the flowchart of the interrupt acknowledge cycle.
Figure 5-33 shows the timing for an interrupt acknowledge cycle terminated with
DSACK1/DSACK0.
REQUEST INTERRUPT
INTERRUPTING DEVICE
PROCESSOR
1) PLACE VECTOR NUMBER ON LEAST
SIGNIFICANT BYTE OF DATA PORT
(DEPENDS ON PORT SIZE)
2
) ASSERT DSACK1/DSACK0
OR
ASSERT AVEC FOR AUTOMATIC GENERA-
TION OF VECTOR NUMBER
PROVIDE VECTOR INFORMATION
ACKNOWLEDGE INTERRUPT
1) INTERRUPT PENDING CONDITION (IPEND FOR
M
C68020) RECOGNIZED BY CURRENT INSTRUC-
T
ION—WAIT FOR INSTRUCTION BOUNDARY.
2
) SET R/W TO READ
3
) SET FUNCTION CODE TO CPU SPACE
4
) PLACE INTERRUPT LEVEL ON A1, A2, AND A3.
TYPE FIELD = IACK
5
) SET SIZE TO BYTE
6
) NEGATE IPEND
7
) ASSERT AS AND DS
ACQUIRE VECTOR NUMBER
1) LATCH VECTOR NUMBER
2
) NEGATE AS AND DS
CONTINUE INTERRUPT EXCEPTION PROCESSING
RELEASE
1) REMOVE VECTOR NUMBER FROM DATA BUS
2
) NEGATE DSACK1/DSACK0
*
This step does not apply to the MC68EC020.
*
Figure 5-32. Interrupt Acknowledge Cycle Flowchart
MOTOROLA M68020 USER’S MANUAL 5-47
READ CYCLE
INTERRUPT
ACKNOWLEDGE
WRITE STACK
CLK
A31–A4
A3–A1
A0
FC2–FC0
SIZ1
R/W
ECS
OCS
AS
DS
DSACK0
DBEN
D24–D31
IPL2–IPL0
SIZ0
DSACK1
S0
S2
S4
S0
S2
S4
S0
S2
INTERRUPT LEVEL
IPEND
D7–D0
D23–D16
VECTOR # FROM 8-BIT PORT
VECTOR # FROM 16-BIT PORT
VECTOR # FROM 32-BIT PORT
**
**
**
*
*
For the MC68EC020, A23–A4.
This signal does not apply to the MC68EC020.
**
**
Figure 5-33. Interrupt Acknowledge Cycle Timing
5-48 M68020 USER’S MANUAL MOTOROLA
5.4.1.2 AUTOVECTOR INTERRUPT ACKNOWLEDGE CYCLE. When the interrupting
device cannot supply a vector number, it requests an automatically generated vector or
autovector. Instead of placing a vector number on the data bus and asserting
DSACK1/DSACK0, the device asserts AVEC to terminate the cycle. The DSACK1/DSACK0
signals may not be asserted during an interrupt acknowledge cycle terminated by AVEC.
The vector number supplied in an autovector operation is derived from the interrupt level
of the current interrupt. When AVEC is asserted instead of DSACK1/DSACK0 during an
interrupt acknowledge cycle, the MC68020/EC020 ignores the state of the data bus and
internally generates the vector number, the sum of the interrupt level plus 24 ($18). Seven
distinct autovectors, which correspond to the seven levels of interrupt available with IPL2–
IPL0, can be used. Figure 5-34 shows the timing for an autovector operation.
5.4.1.3 SPURIOUS INTERRUPT CYCLE. When a device does not respond to an interrupt
acknowledge cycle with AVEC or DSACK1/DSACK0, the external logic typically returns
BERR. In this case, the MC68020/EC020 automatically generates 24, the spurious
interrupt vector number. If HALT is also asserted, the processor retries the cycle.
MOTOROLA M68020 USER’S MANUAL 5-49
READ CYCLE
INTERRUPT
ACKNOWLEDGE
AUTOVECTORED
WRITE STACK
CLK
A31–A4
A1–A3
A0
FC2–FC0
SIZ1
R/W
ECS
OCS
AS
DS
DSACK0
DBEN
D31–D0
IPL2–IPL0
AVEC
SIZ0
DSACK1
S0
S2
S4
S0
S2
S4
S0
S2
INTERRUPT LEVEL
**
**
**
*
*
For the MC68EC020, A23–A4.
This signal does not apply to the MC68EC020.
**
Figure 5-34. Autovector Operation Timing
5-50 M68020 USER’S MANUAL MOTOROLA
5.4.2 Breakpoint Acknowledge Cycle
The breakpoint acknowledge cycle is generated by the execution of a BKPT instruction.
The breakpoint acknowledge cycle allows the external hardware to provide an instruction
word directly into the instruction pipeline as the program executes. This cycle accesses
the CPU space with a type field of zero and provides the breakpoint number specified by
the instruction on address lines A4–A2. If the external hardware terminates the cycle with
DSACK1/DSACK0, the data on the bus (an instruction word) is inserted into the instruction
pipe, replacing the breakpoint opcode, and is executed after the breakpoint acknowledge
cycle completes. The BKPT instruction requires a word to be transferred so that if the first
bus cycle accesses an 8-bit port, a second cycle is required. If the external logic
terminates the breakpoint acknowledge cycle with BERR (i.e., no instruction word
available), the processor takes an illegal instruction exception. Figure 5-35 is a flowchart
of the breakpoint acknowledge cycle. Figure 5-36 shows the timing for a breakpoint
acknowledge cycle that returns an instruction word. Figure 5-37 shows the timing for a
breakpoint acknowledge cycle that signals an exception.
1) PLACE REPLACEMENT OPCODE ON DATA
BUS
2
) ASSERT DSACK1/DTACK0
OR
1
) ASSERT BERR TO INITIATE EXCEPTION
P
ROCESSING
PROCESSOR
1) SET R/W TO READ
2
) SET FUNCTION CODE TO CPU SPACE
3
) PLACE CPU SPACE TYPE 0 ON A19–A16
4
) PLACE BREAKPOINT NUMBER ON A4–A2
5
) SET SIZE TO WORD
6
) ASSERT AS AND DS
BREAKPOINT ACKNOWLEDGE
1) PLACE LATCHED DATA IN INSTRUCTION
PIPELINE
2
) CONTINUE PROCESSING
1) INITIATE ILLEGAL INSTRUCTION PROCESSING
SLAVE NEGATES DSACK1/DSACK0 OR BERR
EXTERNAL DEVICE
IF DSACK1/DSACK0 ASSERTED:
1) LATCH DATA
2) NEGATE AS AND DS
3) GO TO A
I
F BERR ASSERTED:
1) NEGATE AS AND DS
2) GO TO B A B
Figure 5-35. Breakpoint Acknowledge Cycle Flowchart
MOTOROLA M68020 USER’S MANUAL 5-51
BREAKPOINT
ACKNOWLEDGE
INSTRUCTION WORD
FETCH
READ CYCLE
CLK
A31–A20
A19–A16
A15–A2
FC2–FC0
SIZ1
R/W
ECS
OCS
AS
DS
DSACK0
DBEN
D23–D16
SIZ0
DSACK1
S0
S2
S4
S0
S2
S4
S0
S2
D7–D0
D15–D8
BREAKPOINT NUMBER
WORD
FETCHED
I
NSTRUCTION
E
XECUTION
(0000)
B
REAKPOINT ENCODIN
G
A1–A0
HALT
BERR
**
**
**
*
Figure 5-36. Breakpoint Acknowledge Cycle Timing
5-52 M68020 USER’S MANUAL MOTOROLA
CLK
A31–A0
FC2–FC0
R/W
ECS
OCS
AS
DS
DSACK0
DBEN
SIZ1–SIZ0
DSACK1
S0
S2
S4
S0
S2
HALT
Sw
Sw
Sw
S4
D31–D0
BERR
READ WITH BERR ASSERTED
INTERNAL
PROCESSING
STACK WRITE
**
**
**
*
*
For the MC68EC020, A23–A0.
This signal does not apply to the MC68EC020.
**
Figure 5-37. Breakpoint Acknowledge Cycle Timing (Exception Signaled)
MOTOROLA M68020 USER’S MANUAL 5-53
5.4.3 Coprocessor Communication Cycles
The MC68020/EC020 coprocessor interface provides instruction-oriented communication
between the processor and as many as eight coprocessors. Coprocessor accesses use
the MC68020/EC020 bus protocol except that the address bus supplies access
information rather than a 32-bit address. The CPU space type field (A19–A16) for a
coprocessor operation is 0010. A15–A13 contain the coprocessor identification number
(CpID), and A5–A0 specify the coprocessor interface register to be accessed. The
memory management unit of an MC68020/EC020 system is always identified by a CpID of
zero and has an extended register select field (A7–A0) in CPU space 0001 for use by the
CALLM and RTM access level checking mechanism. Refer to Section 9 Applications
Information for more details.
5.5 BUS EXCEPTION CONTROL CYCLES
The MC68020/EC020 bus architecture requires assertion of DSACK1/DSACK0 from an
external device to signal that a bus cycle is complete. DSACK1/DSACK0 or AVEC is not
asserted if:
• The external device does not respond,
• No interrupt vector is provided, or
• Various other application-dependent errors occur.
External circuitry can assert BERR when no device responds by asserting
DSACK1/DSACK0 or AVEC within an appropriate period of time after the processor
asserts AS. Assertion of BERR allows the cycle to terminate and the processor to enter
exception processing for the error condition.
HALT is also used for bus exception control. HALT can be asserted by an external device
for debugging purposes to cause single bus cycle operation or can be asserted in
combination with BERR to cause a retry of a bus cycle in error.
To properly control termination of a bus cycle for a retry or a bus error condition,
DSACK1/DSACK0, BERR, and HALT can be asserted and negated with the rising edge of
the MC68020/EC020 clock. This procedure ensures that when two signals are asserted
simultaneously, the required setup time (#47A) and hold time (#47B) for both of them is
met for the same falling edge of the processor clock. (Refer to Section 10 Electrical
Characteristics for timing requirements.) This or some equivalent precaution should be
designed into the external circuitry that provides these signals.
5-54 M68020 USER’S MANUAL MOTOROLA
The acceptable bus cycle terminations for asynchronous cycles are summarized in
relation to DSACK1/DSACK0 assertion as follows (case numbers refer to Table 5-8):
Normal Termination:
DSACK1/DSACK0 is asserted; BERR and HALT remain negated (case 1).
Halt Termination:
HALT is asserted at same time or before DSACK1/DSACK0, and BERR remains
negated (case 2).
Bus Error Termination:
BERR is asserted in lieu of, at the same time, or before DSACK1/DSACK0 (case 3) or
after DSACK1/DSACK0 (case 4), and HALT remains negated; BERR is negated at the
same time or after DSACK1/DSACK0.
Retry Termination:
HALT and BERR are asserted in lieu of, at the same time, or before DSACK1/DSACK0
(case 5) or after DSACK1/DSACK0 (case 6); BERR is negated at the same time or after
DSACK1/DSACK0; HALT may be negated at the same time or after BERR.
Table 5-8.
DSACK1/DSACK0
,
BERR
,
HALT
Assertion Results
Asserted on Rising
Edge of State
Case No. Control Signal n n+2 Result
1DSACK1/DSACK0
BERR
HALT
A
N
N
S
N
X
Normal cycle terminate and continue.
2DSACK1/DSACK0
BERR
HALT
A
N
A/S
S
N
S
Normal cycle terminate and halt. Continue when
HALT negated.
3DSACK1/DSACK0
BERR
HALT
N/A
A
N
X
S
N
Terminate and take bus error exception, possibly
deferred.
4DSACK1/DSACK0
BERR
HALT
A
N
N
X
A
N
Terminate and take bus error exception, possibly
deferred.
5DSACK1/DSACK0
BERR
HALT
N/A
A
A/S
X
S
S
Terminate and retry when HALT negated.
6DSACK1/DSACK0
BERR
HALT
A
N
N
X
A
A
Terminate and retry when HALT negated.
Legend:
n—The number of current even bus state (e.g., S2, S4, etc.)
A—Signal is asserted in this bus state
N—Signal is not asserted and/or remains negated in this bus state
X—Don’t care
S—Signal was asserted in previous state and remains asserted in this state
MOTOROLA M68020 USER’S MANUAL 5-55
Table 5-8 lists various combinations of control signal sequences and the resulting bus
cycle terminations. To ensure predictable operation, BERR and HALT should be negated
according to parameters #28 and #57 in Section 10 Electrical Characteristics.
DSACK1/DSACK0, BERR, and HALT may be negated after AS. If DSACK1/DSACK0 or
BERR remain asserted into S2 of the next bus cycle, that cycle may be terminated
prematurely.
Example A:
A system uses a watchdog timer to terminate accesses to an unpopulated address
space. The timer asserts BERR after timeout (case 3).
Example B:
A system uses error detection and correction on RAM contents. The designer may:
1. Delay DSACK1/DSACK0 assertion until data is verified and assert BERR and
HALT simultaneously to indicate to the processor to automatically retry the error
cycle (case 5) or, if data is valid, assert DSACK1/DSACK0 (case 1).
2. Delay DSACK1/DSACK0 assertion until data is verified and assert BERR with or
without DSACK1/DSACK0 if data is in error (case 3). This configuration initiates
exception processing for software handling of the condition.
3. Assert DSACK1/DSACK0 prior to data verification. If data is invalid, BERR is
asserted on the next clock cycle (case 4). This configuration initiates exception
processing for software handling of the condition.
4. Assert DSACK1/DSACK0 prior to data verification; if data is invalid, assert BERR
and HALT on the next clock cycle (case 6). The memory controller can then
correct the RAM prior to or during the automatic retry.
5.5.1 Bus Errors
The BERR signal can be used to abort the bus cycle and the instruction being executed.
BERR takes precedence over DSACK1/DSACK0, provided it meets the timing constraints
described in Section 10 Electrical Characteristics. If BERR does not meet these
constraints, it may cause unpredictable operation of the MC68020/EC020. If BERR
remains asserted into the next bus cycle, it may cause incorrect operation of that cycle.
When BERR is issued to terminate a bus cycle, the MC68020/EC020 may enter exception
processing immediately following the bus cycle, or it may defer processing the exception.
The instruction prefetch mechanism requests instruction words from the bus controller and
the instruction cache before it is ready to execute them. If a bus error occurs on an
instruction fetch, the processor does not take the exception until it attempts to use that
instruction word. Should an intervening instruction cause a branch or should a task switch
occur, the bus error exception does not occur.
5-56 M68020 USER’S MANUAL MOTOROLA
BERR is recognized during a bus cycle in any of the following cases:
1. DSACK1/DSACK0 and HALT are negated and BERR is asserted.
2. HALT and BERR are negated and DSACK1/DSACK0 is asserted. BERR is then
asserted within one clock cycle (HALT remains negated).
3. BERR and HALT are asserted (see 5.5.2 Retry Operation).
When the processor recognizes a bus error condition, it terminates the current bus cycle
in the normal way. Figure 5-38 shows the timing of a bus error for the case in which
DSACK1/DSACK0 is not asserted. Figure 5-39 shows the timing for a bus error for the
case in which BERR is asserted after DSACK1/DSACK0. Exceptions are taken in both
cases. (Refer to Section 6 Exception Processing for details of bus error exception
processing.) When BERR is asserted during a read cycle that supplies an instruction to
the on-chip cache, the instruction in the cache is marked invalid.
When BERR is asserted after DSACK1/DSACK0, BERR must be asserted within
parameter #48 (refer to Section 10 Electrical Characteristics) for purely asynchronous
operation, or it must be asserted and remain stable during the sample window, defined by
parameters #27A and #47B, around the next falling edge of the clock after
DSACK1/DSACK0 is recognized. If BERR is not stable at this time, the processor may
exhibit erratic behavior. In this case, data may be present on the bus, but may not be
valid. This sequence may be used by systems that have memory error detection and
correction logic and by external cache memories.
5.5.2 Retry Operation
When BERR and HALT are asserted simultaneously by an external device during a bus
cycle, the processor enters the retry sequence. A delayed retry similar to the delayed
BERR signal described previously can also occur.
The processor terminates the bus cycle, negates the control signals (AS, DS, R/W, SIZ1,
SIZ0, RMC, and, for the MC68020 only, ECS and OCS), and does not begin another bus
cycle until the BERR and HALT signals have been negated by external logic. After a
synchronization delay, the processor retries the previous cycle using the same access
information (address, function code, size, etc.) The BERR signal should be negated before
S2 of the read cycle to ensure correct operation of the retried cycle. Figure 5-40 shows a
late retry operation of a cycle.
The processor retries any read or write cycle of a read-modify-write operation separately;
RMC remains asserted during the entire retry sequence.
Asserting BR along with BERR and HALT provides a relinquish and retry operation. The
MC68020/EC020 does not relinquish the bus during a read-modify-write operation. Any
device that requires the processor to give up the bus and retry a bus cycle during a read-
modify-write cycle must assert BERR and BR only (HALT must not be included). The bus
error handler software should examine the read-modify-write bit in the special status word
(refer to Section 6 Exception Processing) and take the appropriate action to resolve this
type of fault when it occurs.
MOTOROLA M68020 USER’S MANUAL 5-57
BREAKPOINT
ACKNOWLEDGE
BUS ERROR
FETCH
READ CYCLE
CLK
A31–A20
A19–A16
A15–A2
FC2–FC0
SIZ1
R/W
ECS
OCS
AS
DS
DSACK0
DBEN
D23–D16
SIZ0
DSACK1
S0
S2
S4
S0
S2
S4
S0
S2
D7–D0
D15–D8
BREAKPOINT NUMBER
WORD
EXCEPTION
S
TACKING
(0000)
B
REAKPOINT ENCODING
A1–A0
HALT
BERR
CPU SPACE
D31–D24
**
**
**
*
*
For the MC68EC020, A23–A20.
This signal does not apply to the MC68EC020.
**
Figure 5-38. Bus Error without
DSACK1/DSACK0
5-58 M68020 USER’S MANUAL MOTOROLA
CLK
A31–A0
FC2–FC0
R/W
ECS
OCS
AS
DS
DSACK0
DBEN
D31–D0
IPL2–IPL0
DSACK1
S0
S2
Sw
S4
S0
S2
Sw
S4
SIZ1–SIZ0
BERR
HALT
WRITE WITH BERR ASSERTED
INTERNAL
P
ROCESSIN
G
STACK WRITE
**
**
**
*
*
For the MC68EC020, A23–A0.
This signal does not apply to the MC68EC020.
**
Figure 5-39. Late Bus Error with
DSACK1/DSACK0
MOTOROLA M68020 USER’S MANUAL 5-59
A31–A0
FC2–FC0
ECS
OCS
AS
DS
DSACK1
CLK
S0
S4
S0
SIZ1–SIZ0
R/W
DSACK0
D31–D0
DATA BUS NOT DRIVEN
BERR
HALT
WRITE CYCLE RETRY SIGNALED
HALT
RETRY CYCLE
S2
Sw
S2
S4
**
**
*
*
For the MC68EC020, A23–A0.
This signal does not apply to the MC68EC020.
**
Figure 5-40. Late Retry
5-60 M68020 USER’S MANUAL MOTOROLA
5.5.3 Halt Operation
When HALT is asserted and BERR is not asserted, the MC68020/EC020 halts external
bus activity at the next bus cycle boundary. HALT by itself does not terminate a bus cycle.
Negating and reasserting HALT in accordance with the correct timing requirements
provides a single-step (bus cycle to bus cycle) operation. The HALT signal affects external
bus cycles only; thus, a program that resides in the instruction cache and does not require
use of the external bus may continue executing unaffected by HALT.
The single-cycle mode allows the user to proceed through (and debug) external processor
operations, one bus cycle at a time. Figure 5-41 shows the timing requirements for a
single-cycle operation. Since the occurrence of a bus error while HALT is asserted causes
a retry operation, the user must anticipate retry cycles while debugging in the single-cycle
mode. The single-step operation and the software trace capability allow the system
debugger to trace single bus cycles, single instructions, or changes in program flow.
These processor capabilities, along with a software debugging package, give complete
debugging flexibility.
When the processor completes a bus cycle with the HALT signal asserted, the data bus is
placed in the high-impedance state, and the bus control signals (AS, DS , and, for the
MC68020 only, ECS and OCS) are negated (not placed in the high-impedance state);
A31–A0 for the MC68020 or A23–A0 for the MC68EC020, FC2–FC0, SIZ1, SIZ0, and
R/W remain in the same state. The halt operation has no effect on bus arbitration (refer to
5.7 Bus Arbitration). When bus arbitration occurs while the MC68020/EC020 is halted,
the address and control signals (A31–A0, FC2–FC0, SIZ1, SIZ0, R/W, AS, DS, and, for
the MC68020 only, ECS and OCS ) are also placed in the high-impedance state. Once bus
mastership is returned to the MC68020/EC020, if HALT is still asserted, A31–A0 for the
MC68020 or A23–A0 for the MC68EC020, FC2–FC0, SIZ1, SIZ0, and R/W are again
driven to their previous states. The MC68020/EC020 does not service interrupt requests
while it is halted (although the MC68020 may assert the IPEND signal as appropriate).
5.5.4 Double Bus Fault
When a bus error or an address error occurs during the exception processing sequence
for a previous bus error, a previous address error, or a reset exception, a double bus fault
occurs. For example, the processor attempts to stack several words containing
information about the state of the machine while processing a bus error exception. If a bus
error exception occurs during the stacking operation, the second error is considered a
double bus fault. When a double bus fault occurs, the processor halts and asserts HALT.
Only an external reset operation can restart a halted processor. However, bus arbitration
can still occur (refer to 5.7 Bus Arbitration).
A second bus error or address error that occurs after exception processing has completed
(during the execution of the exception handler routine or later) does not cause a double
bus fault. A bus cycle that is retried does not constitute a bus error or contribute to a
double bus fault. The processor continues to retry the same bus cycle as long as the
external hardware requests it.
MOTOROLA M68020 USER’S MANUAL 5-61
CLK
A31–A0
FC2–FC0
R/W
ECS
OCS
AS
DS
DSACK0
DBEN
DSACK1
S0
S2
S0
BERR
HALT
S4
S2
SIZ1–SIZ0
S4
D31–D0
**
**
**
*
READ
HALT
(BUS ARBITRATION
PERMITTED
WHILE THE PROCESSOR
IS HALTED)
READ
*
For the MC68EC020, A23–A0.
This signal does not apply to the MC68EC020.
**
Figure 5-41. Halt Operation Timing
5-62 M68020 USER’S MANUAL MOTOROLA
5.6 BUS SYNCHRONIZATION
The MC68020/EC020 overlaps instruction execution —that is, during bus activity for one
instruction, instructions that do not use the external bus can be executed. Due to the
independent operation of the on-chip cache relative to the operation of the bus controller,
many subsequent instructions can be executed, resulting in seemingly nonsequential
instruction execution. When this is not desired and the system depends on sequential
execution following bus activity, the NOP instruction can be used. The NOP instruction
forces instruction and bus synchronization by freezing instruction execution until all
pending bus cycles have completed.
An example of the use of the NOP instruction for this purpose is the case of a write
operation of control information to an external register in which the external hardware
attempts to control program execution based on the data that is written with the
conditional assertion of BERR. Since the MC68020/EC020 cannot process the bus error
until the end of the bus cycle, the external hardware has not successfully interrupted
program execution. To prevent a subsequent instruction from executing until the external
cycle completes, the NOP instruction can be inserted after the instruction causing the
write. In this case, bus error exception processing proceeds immediately after the write
and before subsequent instructions are executed. This is an irregular situation, and the
use of the NOP instruction for this purpose is not required by most systems.
5.7 BUS ARBITRATION
The bus design of the MC68020/EC020 provides for a single bus master at any one time:
either the processor or an external device. One or more of the external devices on the bus
can have the capability of becoming bus master. Bus arbitration is the protocol by which
an external device becomes bus master; the bus controller in the MC68020/EC020
manages the bus arbitration signals so that the processor has the lowest priority.
Bus arbitration differs in the MC68020 and MC68EC020 due to the absence of BGACK in
the MC68EC020. Because of this difference, bus arbitration of the MC68020 and
MC68EC020 is discussed separately.
External devices that need to obtain the bus must assert the bus arbitration signals in the
sequences described in 5.7.1 MC68020 Bus Arbitration or 5.7.2 MC68EC020 Bus
Arbitration. Systems having several devices that can become bus master require
external circuitry to assign priorities to the devices, so that when two or more external
devices attempt to become bus master at the same time, the one having the highest
priority becomes bus master first.
MOTOROLA M68020 USER’S MANUAL 5-63
5.7.1 MC68020 Bus Arbitration
The sequence of the MC68020 bus arbitration protocol is as follows:
1. An external device asserts the BR signal.
2. The processor asserts the BG signal to indicate that the bus will become available at
the end of the current bus cycle.
3. The external device asserts the BGACK signal to indicate that it has assumed bus
mastership.
BR may be issued any time during a bus cycle or between cycles. BG is asserted in
response to BR; it is usually asserted as soon as BR has been synchronized and
recognized, except when the MC68020 has made an internal decision to execute a bus
cycle. Then, the assertion of BG is deferred until the bus cycle has begun. Additionally, BG
is not asserted until the end of a read-modify-write operation (when RMC is negated) in
response to a BR signal. When the requesting device receives BG and more than one
external device can be bus master, the requesting device should begin whatever
arbitration is required. The external device asserts BGACK when it assumes bus
mastership, and maintains BGACK during the entire bus cycle (or cycles) for which it is
bus master. The following conditions must be met for an external device to assume
mastership of the bus through the normal bus arbitration procedure:
• The external device must have received BG through the arbitration process.
•AS must be negated, indicating that no bus cycle is in progress, and the external
device must ensure that all appropriate processor signals have been placed in the
high-impedance state (by observing specification #7 in Section 10 Electrical
Specifications).
• The termination signal (DSACK1/DSACK0) for the most recent cycle must have been
negated, indicating that external devices are off the bus (optional, refer to 5.7.1.3 Bus
Grant Acknowledge (MC68020)).
•BGACK must be inactive, indicating that no other bus master has claimed ownership
of the bus.
Figure 5-42 is a flowchart of MC68020 bus arbitration for a single device. Figure 5-43 is a
timing diagram for the same operation. This technique allows processing of bus requests
during data transfer cycles.
5-64 M68020 USER’S MANUAL MOTOROLA
1) ASSERT BG
GRANT BUS ARBITRATION
TERMINATE ARBITRATION
1) NEGATE BG AND WAIT FOR BGACK TO
BE NEGATED
RE-ARBITRATE OR RESUME
PROCESSOR OPERATION
REQUEST THE BUS
1) ASSERT BR
REQUESTING DEVICE
PROCESSOR
ACKNOWLEDGE BUS MASTERSHIP
1) EXTERNAL ARBITRATION DETERMINES
NEXT BUS MASTER
2
) NEXT BUS MASTER WAITS FOR
CURRENT CYCLE TO COMPLETE
3
) NEXT BUS MASTER ASSERTS BGACK
T
O BECOME NEW MASTER
4
) BUS MASTER NEGATES BR
OPERATE AS BUS MASTER
RELEASE BUS MASTERSHIP
1) PERFORM DATA TRANSFERS
(READ AND WRITE CYCLES)
1) NEGATE BGACK
Figure 5-42. MC68020 Bus Arbitration Flowchart for Single Request
The timing diagram (see Figure 5-43) shows that BR is negated at the time that BGACK is
asserted. This type of operation applies to a system consisting of the processor and one
device capable of bus mastership. In a system having a number of devices capable of bus
mastership, the BR line from each device can be wire-ORed to the processor. In such a
system, more than one bus request can be asserted simultaneously.
The timing diagram in Figure 5-43 shows that BG is negated a few clock cycles after the
transition of BGACK. However, if bus requests are still pending after the negation of BG,
the processor asserts another BG within a few clock cycles after it was negated. This
additional assertion of BG allows external arbitration circuitry to select the next bus master
before the current bus master has finished with the bus. The following paragraphs provide
additional information about the three steps in the arbitration process.
Bus arbitration requests are recognized during normal processing, RESET assertion,
HALT assertion, and when the processor has halted due to a double bus fault.
MOTOROLA M68020 USER’S MANUAL 5-65
A31–A0
FC2–FC0
ECS
OCS
AS
DS
DSACK1
CLK
S0
S4
S0
SIZ1–SIZ0
R/W
DSACK0
DBEN
S2
S2
BGACK
BG
BR
D31–D0
PROCESSOR
DMA DEVICE
PROCESSOR
Figure 5-43. MC68020 Bus Arbitration Operation Timing for Single Request
5-66 M68020 USER’S MANUAL MOTOROLA
5.7.1.1 BUS REQUEST (MC68020). External devices capable of becoming bus masters
request the bus by asserting BR . BR can be a wire-ORed signal (although it need not be
constructed from open-collector devices) that indicates to the processor that some
external device requires control of the bus. The processor is at a lower bus priority level
than the external device and relinquishes the bus after it has completed the current bus
cycle (if one has started).
If no BGACK is received while BR is asserted, the processor remains bus master once BR
is negated. This prevents unnecessary interference with ordinary processing if the
arbitration circuitry inadvertently responds to noise or if an external device determines that
it no longer requires use of the bus before it has been granted mastership.
5.7.1.2 BUS GRANT (MC68020). The processor asserts BG as soon as possible after
receipt of the bus request. BG assertion immediately follows internal synchronization
except during a read-modify-write cycle or follows an internal decision to execute a bus
cycle. During a read-modify-write cycle, the processor does not assert BG until the entire
operation has completed. RMC is asserted to indicate that the bus is locked. In the case of
an internal decision to execute another bus cycle, BG is deferred until the bus cycle has
begun.
BG may be routed through a daisy-chained network or through a specific priority-encoded
network. The processor allows any type of external arbitration that follows the protocol.
5.7.1.3 BUS GRANT ACKNOWLEDGE (MC68020). Upon receiving BG, the requesting
device waits until AS, DSACK1/DSACK0, and BGACK are negated before asserting its
own BGACK. The negation of AS indicates that the previous master releases the bus after
specification #7 (refer to Section 10 Electrical Characteristics). The negation of
DSACK1/DSACK0 indicates that the previous slave has completed its cycle with the
previous master. Note that in some applications, DSACK1/DSACK0 might not be used in
this way.
General-purpose devices are connected to be dependent only on AS. When BGACK is
asserted, the device is the bus master until it negates BGACK. BGACK should not be
negated until all bus cycles required by the alternate bus master have been completed.
Bus mastership terminates at the negation of BGACK. The BR from the granted device
should be negated after BGACK is asserted. If another BR is still pending after the
assertion of BGACK, another BG is asserted within a few clocks of the negation of the the
first BG, as described in 5.7.1.4 Bus Arbitration Control (MC68020). Note that the
processor does not perform any external bus cycles before it reasserts BG in this case.
MOTOROLA M68020 USER’S MANUAL 5-67
5.7.1.4 BUS ARBITRATION CONTROL (MC68020). The bus arbitration control unit in the
MC68020 is implemented with a finite state machine. As discussed previously, all
asynchronous inputs to the MC68020 are internally synchronized in a maximum of two
cycles of the processor clock.
As shown in Figure 5-44, input signals labeled R and A are internally synchronized
versions of the BR and BGACK signals, respectively. The BG output is labeled G, and the
internal high-impedance control signal is labeled T. If T is true, the address, data, and
control buses are placed in the high-impedance state after the next rising edge following
the negation of AS and RMC. All signals are shown in positive logic (active high),
regardless of their true active voltage level.
GT
GT
GT
GT
GT
GT
GT
RA
RA
XX
RA
RA
RA
XX
RX
RA
XA
RA
RX
XA
RA
STATE 1
STATE 0
STATE 4
STATE 5
STATE 6
STATE 2
STATE 3
XX
R—BUS REQUEST
A
—BUS GRANT ACKNOWLEDGE
G
—BUS GRANT
T
—THREE-STATE CONTROL TO BUS CONTROL LOGIC
X
—DON'T CARE
NOTE: The BG output will not be asserted while RMC is asserted.
Figure 5-44. MC68020 Bus Arbitration State Diagram
5-68 M68020 USER’S MANUAL MOTOROLA
State changes occur on the next rising edge of the clock after the internal signal is
recognized as valid. The BG signal transitions on the falling edge of the clock after a state
is reached during which G changes. The bus control signals (controlled by T) are driven
by the processor immediately following a state change when bus mastership is returned to
the MC68020.
State 0, at the top center of the diagram, in which both G and T are negated, is the state
of the bus arbiter while the processor is bus master. Request R and acknowledge A keep
the arbiter in state 0 as long as they are both negated. When a request R is received, both
grant G and signal T are asserted (in state 1 at the top left). The next clock causes a
change to state 2, at the lower left, in which G and T are held. The bus arbiter remains in
that state until acknowledge A is asserted or request R is negated. Once either occurs, the
arbiter changes to the center state, state 3, and negates grant G. The next clock takes the
arbiter to state 4, at the upper right, in which grant G remains negated and signal T
remains asserted. With acknowledge A asserted, the arbiter remains in state 4 until A is
negated or request R is again asserted. When A is negated, the arbiter returns to the
original state, state 0, and negates signal T. This sequence of states follows the normal
sequence of signals for relinquishing the bus to an external bus master. Other states apply
to other possible sequences of combinations of R and A.
The MC68020 does not allow arbitration of the external bus during the read-modify-write
sequence. For the duration of this sequence, the MC68020 ignores the BR input. If
mastership of the MC68020 bus is required during a read-modify-write operation, BERR
must be used to abort the read-modify-write sequence. The bus arbitration sequence
while the bus is inactive (i.e., executing internal operations such as a multiply instruction)
is shown in Figure 5-45.
MOTOROLA M68020 USER’S MANUAL 5-69
A31–A0
FC2–FC0
ECS
OCS
AS
DS
DSACK1
CLK
S4
S0
SIZ1–SIZ0
R/W
DSACK0
DBEN
BGACK
BG
BR
D31–D0
PROCESSOR
PROCESSOR
ALTERNATE MASTER
BUS INACTIVE
(ARBITRATION PERMITTED
WHILE THE PROCESSOR IS
INACTIVE OR HALTED)
Figure 5-45. MC68020 Bus Arbitration Operation Timing—Bus Inactive
5-70 M68020 USER’S MANUAL MOTOROLA
5.7.2 MC68EC020 Bus Arbitration
The sequence of the MC68EC020 bus arbitration protocol is as follows:
1. An external device asserts the BR signal.
2. The processor asserts the BG signal to indicate that the bus will become available at
the end of the current bus cycle.
3. The external device asserts the BR signal throughout its bus mastership.
BR may be issued any time during a bus cycle or between cycles. BG is asserted in
response to BR; it is usually asserted as soon as BR has been synchronized and
recognized, except when the MC68020 has made an internal decision to execute a bus
cycle. Then, the assertion of BG is deferred until the bus cycle has begun. Additionally, BG
is not asserted until the end of a read-modify-write operation (when RMC is negated) in
response to a BR signal. When the requesting device receives BG and more than one
external device can be bus master, the requesting device should begin whatever
arbitration is required. The external device continues to assert BR when it assumes bus
mastership, and maintains BR during the entire bus cycle (or cycles) for which it is bus
master. The following conditions must be met for an external device to assume mastership
of the bus through the normal bus arbitration procedure:
• The external device must have received BG through the arbitration process.
•AS must be negated, indicating that no bus cycle is in progress, and the external
device must ensure that all appropriate processor signals have been placed in the
high-impedance state (by observing specification #7 in Section 10 Electrical
Specifications).
• The termination signal (DSACK1/DSACK0) for the most recent cycle must have been
negated, indicating that external devices are off the bus.
• No other bus master has claimed ownership of the bus.
Figure 5-46 is a flowchart of MC68EC020 bus arbitration for a single device. Figure 5-47 is
a timing diagram for the same operation. This technique allows processing of bus
requests during data transfer cycles.
Bus arbitration requests are recognized during normal processing, RESET assertion,
HALT assertion, and when the processor has halted due to a double bus fault.
MOTOROLA M68020 USER’S MANUAL 5-71
1) ASSERT BG
GRANT BUS ARBITRATION
RE-ARBITRATE OR RESUME
PROCESSOR OPERATION
REQUEST THE BUS
1) ASSERT BR
REQUESTING DEVICE
PROCESSOR
ACKNOWLEDGE BUS MASTERSHIP
1) EXTERNAL ARBITRATION DETERMINES
NEXT BUS MASTER
2
) NEXT BUS MASTER WAITS FOR
CURRENT CYCLE TO COMPLETE
3
) PERFORM DATA TRANSFERS
(READ AND WRITE CYCLES)
OPERATE AS BUS MASTER
RELEASE BUS MASTERSHIP
1) NEGATE BR
Figure 5-46. MC68EC020 Bus Arbitration Flowchart for Single Request
5.7.2.1 BUS REQUEST (MC68EC020). External devices capable of becoming bus
masters request the bus by asserting BR. BR can be a wire-ORed signal (although it need
not be constructed from open-collector devices) that indicates to the processor that some
external device requires control of the bus. The processor is at a lower bus priority level
than the external device and relinquishes the bus after it has completed the current bus
cycle (if one has started). BR remains asserted throughout the external device’s bus
mastership.
5.7.2.2 BUS GRANT (MC68EC020). The processor asserts BG as soon as possible after
receipt of the bus request. BG assertion immediately follows internal synchronization
except during a read-modify-write cycle or follows an internal decision to execute a bus
cycle. During a read-modify-write cycle, the processor does not assert BG until the entire
operation has completed. RMC is asserted to indicate that the bus is locked. In the case of
an internal decision to execute another bus cycle, BG is deferred until the bus cycle has
begun.
BG may be routed through a daisy-chained network or through a specific priority-encoded
network. The processor allows any type of external arbitration that follows the protocol.
5-72 M68020 USER’S MANUAL MOTOROLA
A23–A0
FC2–FC0
AS
DS
DSACK1
CLK
S0
S4
S0
SIZ1–SIZ0
R/W
DSACK0
S2
S2
BG
BR
D31–D0
PROCESSOR
DMA DEVICE
PROCESSOR
Figure 5-47. MC68EC020 Bus Arbitration Operation Timing for Single Request
MOTOROLA M68020 USER’S MANUAL 5-73
5.7.2.3 BUS ARBITRATION CONTROL (MC68EC020). The bus arbitration control unit in
the MC68EC020 is implemented with a finite state machine. As discussed previously, all
asynchronous inputs to the MC68EC020 are internally synchronized in a maximum of two
cycles of the processor clock.
As shown in Figure 5-48, the input signal labeled R is an internally synchronized version
of the BR signal. The BG output is labeled G, and the internal high-impedance control
signal is labeled T. If T is true, the address, data, and control buses are placed in the high-
impedance state after the next rising edge following the negation of AS and RMC. All
signals are shown in positive logic (active high), regardless of their true active voltage
level.
GT
GT
GT
STATE 3
GT
GT
GT
STATE 2
GT
STATE1
STATE 0
STATE 4
STATE 5
STATE 6
R
—BUS REQUEST
G
—BUS GRANT
T
—THREE-STATE CONTROL TO BUS CONTROL LOGIC
X
—DON'T CARE
R
R
R
R
XRX
R
RX
R
Figure 5-48. MC68EC020 Bus Arbitration State Diagram
5-74 M68020 USER’S MANUAL MOTOROLA
State changes occur on the next rising edge of the clock after the internal signal is
recognized as valid. The BG signal transitions on the falling edge of the clock after a state
is reached during which G changes. The bus control signals (controlled by T) are driven
by the processor immediately following a state change when bus mastership is returned to
the MC68EC020.
State 0, at the top center of the diagram, in which both G and T are negated, is the state
of the bus arbiter while the processor is bus master. Request R keeps the arbiter in state 0
as long as it is negated. When a request R is received, both grant G and signal T are
asserted (in state 1 at the top left). The next clock causes a change to state 2, at the lower
left, in which G and T are held. The bus arbiter remains in that state until request R is
negated. Then the arbiter changes to the center state, state 3, and negates grant G. The
next clock takes the arbiter to state 4, at the upper right, in which grant G remains negated
and signal T remains asserted. The arbiter returns to the original state, state 0, and
negates signal T. This sequence of states follows the normal sequence of signals for
relinquishing the bus to an external bus master. Other states apply to other possible
sequences of R.
The MC68EC020 does not allow arbitration of the external bus during the read-modify-
write sequence. For the duration of this sequence, the MC68EC020 ignores the BR input.
If mastership of the MC68EC020 bus is required during a read-modify-write operation,
BERR must be used to abort the read-modify-write sequence. The bus arbitration
sequence while the bus is inactive (i.e., executing internal operations such as a multiply
instruction) is shown in Figure 5-49.
MOTOROLA M68020 USER’S MANUAL 5-75
A23–A0
FC2–FC0
AS
DS
DSACK1
CLK
S4
S0
SIZ1–SIZ0
R/W
DSACK0
BG
BR
D31–D0
PROCESSOR
PROCESSOR
ALTERNATE MASTER
BUS INACTIVE
(ARBITRATION PERMITTED
WHILE THE PROCESSOR IS
INACTIVE OR HALTED)
Figure 5-49. MC68EC020 Bus Arbitration Operation Timing—Bus Inactive
The existing three-wire arbitration design (BR, BG, and BGACK) of some peripherals can
be converted to the MC68EC020 two-wire arbitration with the addition of an AND gate.
Figure 5-50 shows the combination of BR and BGACK for a three-wire arbitration system
to BR of the MC68EC020 or BR and BG from an MC68EC020 to BG for a three-wire
arbitration system. The speed of the AND gate must be faster than the time between the
assertion of BGACK and the negation of BR by the alternate bus master. Figure 5-50
assumes the alternate bus master does not assume bus mastership until the MC68EC020
AS is negated and MC68EC020 BG is asserted.
5-76 M68020 USER’S MANUAL MOTOROLA
An example of MC68EC020 bus arbitration to a DMA device that supports three-wire bus
arbitration is described in Appendix A Interfacing an MC68EC020 to a DMA Device
That Supports a Three-Wire Bus Arbitration Protocol.
AS
B
G
B
R
BGAC
K
ALTERNATE
BUS MASTER
AS
B
G
MC68EC020
BR
Figure 5-50. Interface for Three-Wire to Two-Wire Bus Arbitration
5.8 RESET OPERATION
RESET is a bidirectional signal with which an external device resets the system or the
processor resets external devices. When power is applied to the system, external circuitry
should assert RESET for a minimum of 520 clocks after VCC and clock timing have
stabilized and are within specification limits. Figure 5-51 is a timing diagram of the power-
up reset operation, showing the relationships between RESET, VCC, and bus signals. The
clock signal is required to be stable by the time VCC reaches the minimum operating
specification. During the reset period, the entire bus three-states (except for non-three-
statable signals, which are driven to their inactive state). Once RESET negates, all control
signals are negated, the data bus is in read mode, and the address bus is driven. After
this, the first bus cycle for reset exception processing begins.
The external RESET signal resets the processor and the entire system. Except for the
initial reset, RESET should be asserted for at least 520 clock periods to ensure that the
processor resets. Asserting RESET for 10 clock periods is sufficient for resetting the
processor logic; the additional clock periods prevent a RESET instruction from overlapping
the external RESET signal.
MOTOROLA M68020 USER’S MANUAL 5-77
ISP
R
EAD
S
TART
S
ALL CONTROL SIGNALS
NEGATED, DATA BUS IN
READ MODE, ADDRESS
BUS DRIVEN
ENTIRE BUS
THREE-
STATED
BUS STATE UNKNOWN
t
≥
520 CLOCKS
t < 4 CLOCKS
4 CLOCKS
CLK
+5 V
V
CC
BUS
CYCLE
S
RESET
Figure 5-51. Initial Reset Operation Timing
Resetting the processor causes any bus cycle in progress to terminate as if
DSACK1/DSACK0 or BERR had been asserted. In addition, the processor initializes
registers appropriately for a reset exception. Exception processing for a reset operation is
described in Section 6 Exception Processing.
When a RESET instruction is executed, the processor drives the RESET signal for 512
clock cycles. In this case, the processor resets the external devices of the system, and the
internal registers of the processor are unaffected. The external devices connected to the
RESET signal are reset at the completion of the RESET instruction. An external RESET
signal that is asserted to the processor during execution of a RESET instruction must
extend beyond the reset period of the instruction by at least eight clock cycles to reset the
processor. Figure 5-52 shows the timing information for the RESET instruction.
5-78 M68020 USER’S MANUAL MOTOROLA
CLK
A31–A0
FC2–FC0
R/W
ECS
OCS
AS
DS
DSACK0
DBEN
SIZ1–SIZ0
DSACK1
HALT
S0
S2
S4
D31–D0
S2
S0
RESET
READ
RESET INTERNAL
512 CLOCKS
RESUME NORMAL
O
PERATION
**
**
**
*
*
For the MC68EC020, A23–A0.
This signal does not apply to the MC68EC020.
**
Figure 5-52. RESET Instruction Timing
MOTOROLA M68020 USER’S MANUAL 6-1
SECTION 6
EXCEPTION PROCESSING
Exception processing is defined as the activities performed by the processor in preparing
to execute a handler routine for any condition that causes an exception. In particular,
exception processing does not include execution of the handler routine itself. An
introduction to exception processing, as one of the processing states of the
MC68020/EC020, is given in Section 2 Processing States.
This section describes exception processing in detail, describing the processing for each
type of exception. It describes the return from an exception and bus fault recovery. This
section also describes the formats of the exception stack frames. For more detail on
protocol violation and coprocessor-related exceptions, refer to Section 7 Coprocessor
Interface Description . Also, for more detail on exceptions defined for floating-point
coprocessors, refer to MC68881UM/AD
, MC68881/MC68882 Floating-Point Coprocessor
User's Manual
.
6.1 EXCEPTION PROCESSING SEQUENCE
Exception processing occurs in four functional steps. However, all individual bus cycles
associated with exception processing (vector acquisition, stacking, etc.) are not
guaranteed to occur in the order in which they are described in this section. Nonetheless,
all addresses and offsets from the stack pointer are guaranteed to be as described.
The first step of exception processing involves the SR. The processor makes an internal
copy of the SR, then sets the S-bit in the SR, changing to the supervisor privilege level.
Next, the processor inhibits tracing of the exception handler by clearing the T1 and T0 bits
in the SR. For the reset and interrupt exceptions, the processor also updates the interrupt
priority mask (bits 10–8 of the SR).
In the second step, the processor determines the vector number of the exception. For
interrupts, the processor performs an interrupt acknowledge cycle (a read from the CPU
address space type 1111; see Figures 5-32 and 5-33) to obtain the vector number. For
coprocessor-detected exceptions, the vector number is included in the coprocessor
exception primitive response. (Refer to Section 7 Coprocessor Interface Description for
a complete discussion of coprocessor exceptions.) For all other exceptions, internal logic
provides the vector number. This vector number is used in the last step to calculate the
address of the exception vector. Throughout this section, vector numbers are given in
decimal notation.
6-2 M68020 USER’S MANUAL MOTOROLA
For all exceptions other than reset, the third step is to save the current processor context.
The processor creates an exception stack frame on the active supervisor stack and fills it
with context information appropriate for the type of exception. Other information may also
be stacked, depending on which exception is being processed and the state of the
processor prior to the exception. If the exception is an interrupt and the M-bit in the SR is
set, the processor clears the M-bit and builds a second stack frame on the interrupt stack.
The last step initiates execution of the exception handler. The processor multiplies the
vector number by four to determine the exception vector offset. The processor then adds
the offset to the value stored in the VBR to obtain the memory address of the exception
vector. Next, the processor loads the PC (and the ISP for the reset exception) from the
exception vector table in memory. After prefetching the first three words to fill the
instruction pipe, the processor resumes normal processing at the address in the PC. Table
6-1 contains a description of all the exception vector offsets defined for the
MC68020/EC020.
As shown in Table 6-1, the first 64 vectors are defined by Motorola, and 192 vectors are
reserved for interrupt vectors defined by the user. However, external devices may use
vectors reserved for internal purposes at the discretion of the system designer.
MOTOROLA M68020 USER’S MANUAL 6-3
Table 6-1. Exception Vector Assignments
Vector Offset
Vector Number He x Space Assignment
0
1
2
3
000
004
008
00C
SP
SP
SD
SD
Reset Initial Interrupt Stack Pointer
Reset Initial Program Counter
Bus Error
Address Error
4
5
6
7
010
014
018
01C
SD
SD
SD
SD
Illegal Instruction
Zero Divide
CHK, CHK2 Instruction
cpTRAPcc, TRAPcc, TRAPV Instructions
8
9
10
11
020
024
028
02C
SD
SD
SD
SD
Privilege Violation
Trace
Line 1010 Emulator
Line 1111 Emulator
12
13
14
15
030
034
038
03C
SD
SD
SD
SD
(Unassigned, Reserved)
Coprocessor Protocol Violation
Format Error
Uninitialized Interrupt
16–23 040
05C SD
SD Unassigned, Reserved
24
25
26
27
060
064
068
06C
SD
SD
SD
SD
Spurious Interrupt
Level 1 Interrupt Autovector
Level 2 Interrupt Autovector
Level 3 Interrupt Autovector
28
29
30
31
070
074
078
07C
SD
SD
SD
SD
Level 4 Interrupt Autovector
Level 5 Interrupt Autovector
Level 6 Interrupt Autovector
Level 7 Interrupt Autovector
32–47 080
0BC SD
SD TRAP #0–15 Instruction Vectors
48
49
50
51
0C0
0C4
0C8
0CC
SD
SD
SD
SD
FPCP Branch or Set on Unordered Condition
FPCP Inexact Result
FPCP Divide by Zero
FPCP Underflow
52
53
54
55
0D0
0D4
0D8
0DC
SD
SD
SD
SD
FPCP Operand Error
FPCP Overflow
FPCP Signaling NAN
Unassigned, Reserved
56
57
58
0E0
0E4
0E8
SD
SD
SD
PMMU Configuration
PMMU Illegal Operation
PMMU Access Level Violation
59–63 0EC
0FC SD
SD Unassigned, Reserved
64–255 100
3FC SD
SD User-Defined Vectors (192)
SP—Supervisor Program Space
SD—Supervisor Data Space
6-4 M68020 USER’S MANUAL MOTOROLA
6.1.1 Reset Exception
Assertion of the RESET signal by external hardware causes a reset exception. For details
on the requirements for the assertion of RESET, refer to Section 5 Bus Operation.
The reset exception has the highest priority of any exception; it provides for system
initialization and recovery from catastrophic failure. When a reset exception is recognized,
it aborts any processing in progress and that processing cannot be recovered. Figure 6-1
is a flowchart of the reset exception, which performs the following operations:
1. Clears the T1 and T0 bits in the SR to disable tracing.
2. Places the processor in the interrupt mode of the supervisor privilege level by setting
the S-bit and clearing the M-bit in the SR.
3. Sets the I2–I0 bits in the SR to the highest priority level (level 7).
4. Initializes the VBR to zero ($00000000).
5. Clears the E and F bits in the CACR.
6. Invalidates all entries in the instruction cache.
7. Generates a vector number to reference the reset exception vector (two long words)
at offset zero in the supervisor program address space.
8. Loads the first long word of the reset exception vector into the interrupt stack pointer.
9. Loads the second long word of the reset exception vector into the PC.
After the initial instruction prefetches, program execution begins at the address in the PC.
The reset exception does not save the value of either the PC or the SR.
As described in Section 5 Bus Operation, if a bus error or address error occurs during
the exception processing sequence for a reset, a double bus fault occurs. The processor
halts and asserts the HALT signal to indicate the halted condition.
Execution of the RESET instruction does not cause a reset exception, nor does it affect
any internal registers, but it does cause the MC68020/EC020 to assert the RESET signal,
resetting all external devices.
6.1.2 Bus Error Exception
A bus error exception occurs when external logic aborts a bus cycle by asserting the
BERR signal. If the aborted bus cycle is a data access, the processor immediately begins
exception processing. If the aborted bus cycle is an instruction prefetch, the processor
may delay taking the exception until it attempts to use the prefetched information.
MOTOROLA M68020 USER’S MANUAL 6-5
OTHERWISE
SP (VECTOR #0)
EXIT
FETCH VECTOR #0
(DOUBLE BUS FAULT)
S (SR)
M (SR
)
T1, T0 (SR
)
I2–I0 (SR
)
VB
R
CAC
R
1
0
0
$
7
$
00000000
$
00000000
➧
➧
➧
➧
➧
➧
(DOUBLE BUS FAULT)
(DOUBLE BUS FAULT)
ENTRY
OTHERWISE
BEGIN INSTRUCTION
EXECUTION
OTHERWISE
INSTRUCTION CACHE
E
NTRIES INVALIDATE
D
➧
FETCH VECTOR #1
PC (VECTOR #1)
➧
PREFETCH 3 WORDS
EXIT
EXIT
EXIT
BUS ERROR
BUS ERROR
BUS ERROR OR
ADDRESS ERROR
Figure 6-1. Reset Operation Flowchart
The processor begins exception processing for a bus error by making an internal copy of
the current SR. The processor then enters the supervisor privilege level (by setting the S-
bit in the SR) and clears the T1 and T0 bits in the SR. The processor generates exception
vector number 2 for the bus error vector. It saves the vector offset, PC, and the internal
copy of the SR on the active supervisor stack. The saved PC value is the logical address
of the instruction that was executing at the time the fault was detected. This is not
necessarily the instruction that initiated the bus cycle since the processor overlaps
6-6 M68020 USER’S MANUAL MOTOROLA
execution of instructions. The processor also saves the contents of some of its internal
registers. The information saved on the stack is sufficient to identify the cause of the bus
fault and recover from the error.
For efficiency, the MC68020/EC020 uses two different bus error stack frame formats.
When the bus error exception is taken at an instruction boundary, less information is
required to recover from the error, and the processor builds the short bus fault stack frame
as shown in Table 6-5. When the exception is taken during the execution of an instruction,
the processor must save its entire state for recovery and uses the long bus fault stack
frame shown in Table 6-5. The format code in the stack frame distinguishes the two stack
frame formats. Stack frame formats are described in detail in 6.4 Exception Stack Frame
Formats.
If a bus error occurs during the exception processing for a bus error, address error, or
reset or while the processor is loading internal state information from the stack during the
execution of an RTE instruction, a double bus fault occurs and the processor enters the
halted state. In this case, the processor does not attempt to alter the current state of
memory. Only an external RESET can restart a processor halted by a double bus fault.
6.1.3 Address Error Exception
An address error exception occurs when the processor attempts to prefetch an instruction
from an odd address. This exception is similar to a bus error exception but is internally
initiated. A bus cycle is not executed, and the processor begins exception processing
immediately. After exception processing commences, the sequence is the same as that
for bus error exceptions described in the preceding paragraphs, except that the vector
number is 3 and the vector offset in the stack frame refers to the address error vector.
Either a short or long bus fault stack frame may be generated. If an address error occurs
during the exception processing for a bus error, address error, or reset, a double bus fault
occurs.
6.1.4 Instruction Trap Exception
Certain instructions are used to explicitly cause trap exceptions. The TRAP instruction
always forces an exception and is useful for implementing system calls in user programs.
The TRAPcc, TRAPV, cpTRAPcc, CHK, and CHK2 instructions force exceptions if the
user program detects an error, which may be an arithmetic overflow or a subscript value
that is out of bounds.
The DIVS and DIVU instructions force exceptions if a division operation is attempted with
a divisor of zero.
When a trap exception occurs, the processor copies the SR internally, enters the
supervisor privilege level (by setting the S-bit in the SR), and clears the T1 and T0 bits in
the SR. If tracing is enabled for the instruction that caused the trap, a trace exception is
taken after the RTE instruction from the trap handler is executed, and the trace
corresponds to the trap instruction; the trap handler routine is not traced. The processor
generates a vector number according to the instruction being executed; for the TRAP
MOTOROLA M68020 USER’S MANUAL 6-7
instruction, the vector number is 32 plus n. The stack frame saves the trap vector offset,
the PC, and the internal copy of the SR on the supervisor stack. The saved value of the
PC is the logical address of the instruction following the instruction that caused the trap.
For all instruction traps other than TRAP, a pointer to the instruction that caused the trap
is also saved. Instruction execution resumes at the address in the exception vector after
the required instruction prefetches.
6.1.5 Illegal Instruction and Unimplemented Instruction Exceptions
An illegal instruction is an instruction that contains any bit pattern in its first word that does
not correspond to the bit pattern of the first word of a valid MC68020/EC020 instruction or
a MOVEC instruction with an undefined register specification field in the first extension
word. An illegal instruction exception corresponds to vector number 4 and occurs when
the processor attempts to execute an illegal instruction.
An illegal instruction exception is also taken if a breakpoint acknowledge bus cycle (see
Section 5 Bus Operation) is terminated with the assertion of the BERR signal. This
implies that the external circuitry did not supply an instruction word to replace the BKPT
instruction word in the instruction pipe.
Instruction word patterns with bits 15–12 = 1010 are referred to as unimplemented
instructions with A-line opcodes. When the processor attempts to execute an
unimplemented instruction with an A-line opcode, an exception is generated with vector
number 10, permitting efficient emulation of unimplemented instructions.
Instructions that have word patterns with bits 15–12 = 1111, bits 11–9 = 000, and defined
word patterns for subsequent words, are legal PMMU instructions. Instructions that have
bits 15–12 of the first words = 1111, bits 11–9 = 000, and undefined patterns in the
subsequent words, are treated as unimplemented instructions with F-line opcodes when
execution is attempted in the supervisor privilege level. When execution of the same
instruction is attempted in the user privilege level, a privilege violation exception is taken.
The exception vector number for an unimplemented instruction with an F-line opcode is
11.
The word patterns with bits 15–12 = 1111 and bits 11–9 ≠ 000 are used for coprocessor
instructions. When the processor identifies a coprocessor instruction, it runs a bus cycle
referencing CPU space type $2 (refer to Section 2 Processing States ) and addressing
one of eight coprocessors (0–7, according to bits 11–9). If the addressed coprocessor is
not included in the system and the cycle terminates with the assertion of BERR, the
instruction takes an unimplemented instruction (F-line opcode) exception. The system can
emulate the functions of the coprocessor with an F-line exception handler. Refer to
Section 7 Coprocessor Interface Description for more details.
6-8 M68020 USER’S MANUAL MOTOROLA
Exception processing for illegal and unimplemented instructions is similar to that for
instruction traps. When the processor has identified an illegal or unimplemented
instruction, it initiates exception processing instead of attempting to execute the
instruction. The processor copies the SR, enters the supervisor privilege level (by setting
the S bit in the SR), and clears the T1 and T0 bits in the SR, disabling further tracing. The
processor generates the vector number, either 4, 10, or 11, according to the exception
type. The illegal or unimplemented instruction vector offset, current PC, and copy of the
SR are saved on the supervisor stack, with the saved value of the PC being the address
of the illegal or unimplemented instruction. Instruction execution resumes at the address
contained in the exception vector. It is the responsibility of the handling routine to adjust
the stacked PC if the instruction is emulated in software or is to be skipped on return from
the handler.
6.1.6 Privilege Violation Exception
To provide system security, the following instructions are privileged:
ANDI to SR
EORI to SR
cpRESTORE
cpSAVE
MOVE from SR
MOVE to SR
MOVE USP
MOVEC
MOVES
ORI to SR
RESET
RTE
STOP
An attempt to execute one of the privileged instructions while at the user privilege level
causes a privilege violation exception. Also, a privilege violation exception occurs if a
coprocessor requests a privilege check and the processor is at the user level.
Exception processing for privilege violations is similar to that for illegal instructions. When
the processor identifies a privilege violation, it begins exception processing before
executing the instruction. The processor copies the SR, enters the supervisor privilege
level by setting the S-bit in the SR, and clears the T1 and T0 bits in the SR. The processor
generates vector number 8, the privilege violation exception vector, and saves the
privilege violation vector offset, the current PC value, and the internal copy of the SR on
the supervisor stack. The saved value of the PC is the logical address of the first word of
the instruction that caused the privilege violation. Instruction execution resumes after the
required prefetches from the address in the privilege violation exception vector.
MOTOROLA M68020 USER’S MANUAL 6-9
6.1.7 Trace Exception
To aid in program development, the M68000 processors include an instruction-by-
instruction tracing capability. The MC68020/EC020 can be programmed to trace all
instructions or only instructions that change program flow. In the trace mode, an
instruction generates a trace exception after it completes execution, allowing a debugger
program to monitor execution of a program.
The T1 and T0 bits in the supervisor portion of the SR control tracing. The state of these
bits when an instruction begins execution determines whether the instruction generates a
trace exception after the instruction completes. Clearing both the T1 and T0 bits disables
tracing, and instruction execution proceeds normally. Clearing the T1 bit and setting the
T0 bit causes an instruction that forces a change of flow to take a trace exception.
Instructions that increment the PC normally do not take the trace exception. Instructions
that are traced in this mode include all branches, jumps, instruction traps, returns, and
coprocessor instructions that modify the PC flow. This mode also includes SR
manipulations because the processor must re-prefetch instruction words to fill the pipe
again any time an instruction that can modify the SR is executed. The execution of the
BKPT instruction causes a change of flow if the opcode replacing the BKPT is an
instruction that causes a change of flow (i.e., a jump, branch, etc.). Setting the T1 bit and
clearing the T0 bit causes the execution of all instructions to force trace exceptions. Table
6-2 shows the trace mode selected by each combination of T1 and T0.
Table 6-2. Tracing Control
TI T0 Tracing Function
0 0 No Tracing
0 1 Trace on Change of Flow (BRA, JMP, etc.)
1 0 Trace on Instruction Execution (Any Instruction)
1 1 Undefined, Reserved
In general terms, a trace exception is an extension to the function of any traced
instruction—i.e., the execution of a traced instruction is not complete until completion of
trace exception processing. If an instruction does not complete due to a bus error or
address error exception, trace exception processing is deferred until after the execution of
the suspended instruction is resumed, and the instruction execution completes normally. If
an interrupt is pending at the completion of an instruction, the trace exception processing
occurs before the interrupt exception processing starts. If an instruction forces an
exception as part of its normal execution, the forced exception processing occurs before
the trace exception is processed. See 6.1.11 Multiple Exceptions for a more complete
discussion of exception priorities.
When tracing is enabled and the processor attempts to execute an illegal or
unimplemented instruction, that instruction does not cause a trace exception since it is not
executed. This is of particular importance to an instruction emulation routine that performs
the instruction function, adjusts the stacked PC to skip the unimplemented instruction, and
returns. Before returning, the T1 and T0 bits of the SR on the stack should be checked. If
6-10 M68020 USER’S MANUAL MOTOROLA
tracing is enabled, the trace exception processing should also be emulated for the trace
exception handler to account for the emulated instruction.
The exception processing for a trace starts at the end of normal processing for the traced
instruction and before the start of the next instruction. The processor makes an internal
copy of the SR and enters the supervisor privilege level (by setting the S-bit in the SR). It
also clears the T0 and T1 bits of the SR, disabling further tracing. The processor supplies
vector number 9 for the trace exception and saves the trace exception vector offset, PC
value, and the copy of the SR on the supervisor stack. The saved value of the PC is the
logical address of the next instruction to be executed. Instruction execution resumes after
the required prefetches from the address in the trace exception vector.
The STOP instruction does not perform its function when it is traced. A STOP instruction
that begins execution with T1, T0 = 10 forces a trace exception after it loads the SR. Upon
return from the trace handler routine, execution continues with the instruction following the
STOP instruction, and the processor never enters the stopped condition.
6.1.8 Format Error Exception
Just as the processor checks that prefetched instructions are valid, the processor (with the
aid of a coprocessor, if needed) also performs some checks of data values for control
operations, including the type and option fields of the descriptor for CALLM, the
coprocessor state frame format word for a cpRESTORE instruction, and the stack frame
format for an RTE or an RTM instruction.
The RTE instruction checks the validity of the stack format code. For long bus fault format
frames, the RTE instruction also compares the internal version number of the processor to
that contained in the frame at memory location SP + 54 (SP + $36). This check ensures
that the processor can correctly interpret internal state information from the stack frame.
The CALLM and RTM both check the values in the option and type fields in the module
descriptor and module stack frame, respectively. If these fields do not contain proper
values or if an illegal access rights change request is detected by an external memory
management unit, then an illegal call or return is being requested and is not executed.
Refer to Section 9 Applications Information for more information on the module
call/return mechanism.
The cpRESTORE instruction passes the format word of the coprocessor state frame to the
coprocessor for validation. If the coprocessor does not recognize the format value, it
signals the MC68020/EC020 to take a format error exception. Refer to Section 7
Coprocessor Interface Description for details of coprocessor-related exceptions.
If any of the checks previously described determine that the format of the stacked data is
improper, the instruction generates a format error exception. This exception saves a short
bus fault stack frame, generates exception vector number 14, and continues execution at
the address in the format exception vector. The stacked PC value is the logical address of
the instruction that detected the format error.
MOTOROLA M68020 USER’S MANUAL 6-11
6.1.9 Interrupt Exceptions
When a peripheral device requires the services of the MC68020/EC020 or is ready to
send information that the processor requires, it may signal the processor to take an
interrupt exception. The interrupt exception transfers control to a routine that responds
appropriately.
The peripheral device uses the IPL2–IPL0 signals to signal an interrupt condition to the
processor and to specify the priority of that condition. These three signals encode a value
of zero through seven (IPL0 is the least significant bit). When IPL2–IPL0 are all negated,
the interrupt request level is zero. IPL2–IPL0 values 1–7 specify one of seven levels of
prioritized interrupts; level 7 has the highest priority. External circuitry can chain or
otherwise merge signals from devices at each level, allowing an unlimited number of
devices to interrupt the processor.
The IPL2–IPL0 signals must maintain the interrupt request level until the
MC68020/EC020 acknowledges the interrupt to guarantee that the interrupt is recognized.
The MC68020/EC020 continuously samples the IPL2–IPL0 signals on consecutive falling
edges of the processor clock to synchronize and debounce these signals. An interrupt
request that is the same for two consecutive falling clock edges is considered a valid
input. Although the protocol requires that the request remain until the processor runs an
interrupt acknowledge cycle for that interrupt value, an interrupt request that is held for as
short a period as two clock cycles could be recognized.
The I2–I0 bits in the SR specify the interrupt priority mask. The value in the interrupt mask
is the highest priority level that the processor ignores. When an interrupt request has a
priority higher than the value in the mask, the processor makes the request a pending
interrupt. Figure 6-2 is a flowchart of the procedure for making an interrupt pending.
When several devices are connected to the same interrupt level, each device should hold
its interrupt priority level constant until its corresponding interrupt acknowledge cycle to
ensure that all requests are processed.
Table 6-3 lists the interrupt levels, the states of IPL2–IPL0 that define each level, and the
mask value that allows an interrupt at each level.
6-12 M68020 USER’S MANUAL MOTOROLA
RESET
SAMPLE AND SYNCH
IPL2–IPL0
INTERRUPT PENDING
(MC68020 ASSERTS IPEND
)
(COMPARE INTERRUPT LEVEL
W
ITH STATUS REGISTER MASK
)
INTERRUPT LEVEL I2–I0,
OR TRANSITION ON LEVEL 7
>
*
*
IPEND is not implemented in the MC68EC020.
OTHERWISE
Figure 6-2. Interrupt Pending Procedure
Table 6-3. Interrupt Levels and Mask Values
Control Line Status
Requested
Interrupt Level IPL2 IPL1 IPL0 Interrupt Mask Value
Required for Recognition
0*NNN N/A*
1 NNA 0
2 N A N 1–0
3 N A A 2–0
4 A N N 3–0
5 A N A 4–0
6 AAN 5–0
7 AAA 7–0
*Indicates that no interrupt is requested.
A—Asserted
N—Negated
Priority level 7, the nonmaskable interrupt, is a special case. Level 7 interrupts cannot be
masked by the interrupt priority mask, and they are transition sensitive. The processor
recognizes an interrupt request each time the external interrupt request level changes
from some lower level to level 7, regardless of the value in the mask. Figure 6-3 shows
two examples of interrupt recognitions, one for level 6 and one for level 7. When the
MC68020/EC020 processes a level 6 interrupt, the interrupt priority mask is automatically
updated with a value of 6 before entering the handler routine so that subsequent level 6
interrupts are masked. Provided no instruction that lowers the mask value is executed, the
MOTOROLA M68020 USER’S MANUAL 6-13
external request can be lowered to level 3 and then raised back to level 6, and a second
6-14 M68020 USER’S MANUAL MOTOROLA
level 6 interrupt is not processed. However, if the MC68020/EC020 is handling a level 7
interrupt (I2–I0 in the SR set to 111) and the external request is lowered to level 3 and
then raised back to level 7, a second level 7 interrupt is processed. The second level 7
interrupt is processed because the level 7 interrupt is transition sensitive. A level 7
interrupt is also generated by a level comparison if the request level and mask level are at
7 and the priority mask is then set to a lower level (with the MOVE to SR or RTE
instruction, for example). As shown in Figure 6-3 for level 6 interrupt request level and
mask level, this is the case for all interrupt levels.
Note that a mask value of 6 and a mask value of 7 both inhibit request levels of 1–6 from
being recognized. In addition, neither masks a transition to an interrupt request level of 7.
The only difference between mask values of 6 and 7 occurs when the interrupt request
level is 7 and the mask value is 7. If the mask value is lowered to 6, a second level 7
interrupt is recognized.
EXTERNAL IPL2–IPL0
INTERRUPT PRIORITY MASK (I2–I0 IN SR)
ACTION
LEVEL 6 EXAMPLE
INITIAL CONDITIONS
100 ($3)
101 ($5)
(LEVEL COMPARISON)
IF
001 ($6)
THEN
110 ($6)
AND
LEVEL 6 INTERRUPT
IF
100 ($3)
AND STILL
110 ($6)
THEN
NO ACTION
IF
001 ($6)
AND STILL
110 ($6)
THEN
NO ACTION
IF STILL
001 ($6)
AND RTE SO THAT
101 ($5)
THEN
LEVEL 6 INTERRUPT
(LEVEL COMPARISON)
(TRANSITION)
(TRANSITION)
LEVEL 7 EXAMPLE
INITIAL CONDITIONS
100 ($3)
101 ($5)
IF
000 ($7)
THEN
111 ($7)
AND
LEVEL 7 INTERRUPT
IF
100 ($3)
AND STILL
111 ($7)
THEN
NO ACTION
IF
000 ($7)
AND STILL
111 ($7)
THEN
NO ACTION
IF STILL
000 ($7)
AND RTE SO THAT
101 ($5)
THEN
LEVEL 7 INTERRUPT
(LEVEL COMPARISON)
Figure 6-3. Interrupt Recognition Examples
MOTOROLA M68020 USER’S MANUAL 6-15
The MC68020 asserts IPEND (note that IPEND is not implemented in the MC68EC020)
when it makes an interrupt request pending. Figure 6-4 shows the assertion of IPEND
relative to the assertion of an interrupt level on IPL2–IPL0. IPEND signals to external
devices that an interrupt exception will be taken at an upcoming instruction boundary
(following any higher priority exception). The state of the IPEND signal is internally
checked by the processor once per instruction, independently of bus operation. In
addition, it is checked during the second instruction prefetch associated with exception
processing.
Figure 6-5 is a flowchart of the interrupt recognition and associated exception processing
sequence.
CLK
IPL2–IPL0
IPEND
COMPARE REQUEST
WITH MASK IN S
R
ASSERT IPEND
IPL2–IPL0 RECOGNIZED
IPL2–IPL0 SYNCHRONIZED
Figure 6-4. Assertion of IPEND (MC68020 Only)
6-16 M68020 USER’S MANUAL MOTOROLA
TEMP
S
T1, T
0
SR
1
0
➧
➧
➧
UPDATE I2–I0
➧
➧➧➧
– (SP)
TEMP
– (SP)
PC
– (SP)
FORMAT WORD
– (SP) OTHER EXCEPTION
DEPENDENT INFORMATION
ONCE PER INSTRUCTION
EXECUTE INTERRUPT
ACKNOWLEDGE CYCLE
AT INSTRUCTION
BOUNDARY
EXIT
M = 1
TEMP
SR
M
0
➧
➧
M = 0
PC VECTOR TABLE ENTRY
➧
PREFETCH 3 WORDS
END OF EXCEPTION PROCESSING
FOR THE INTERRUPT
BEGIN EXECUTION OF THE INTERRUPT
H
ANDLER ROUTINE OR PROCESS A
H
IGHER PRIORITY EXCEPTION
THESE
INDIVIDUA
L
BUS CYCLE
S
MAY OCCU
R
IN ANY ORDE
R
N
EGATE IPEN
D
*
OTHERWISE
I
PEND ASSERTE
D
*
Does not apply to the MC68EC020.
*
Figure 6-5. Interrupt Exception Processing Flowchart
MOTOROLA M68020 USER’S MANUAL 6-17
For the MC68020, if no higher priority interrupt has been synchronized, the IPEND signal
is negated during state 0 (S0) of an interrupt acknowledge cycle, and the IPL2–IPL0
signals for the interrupt being acknowledged can be negated at this time. For the
MC68EC020, if no higher priority interrupt has been synchronized, the IPL2–IPL0 signals
for the interrupt being acknowledged can be negated at this time. Refer to Section 5 Bus
Operation for more information on interrupt acknowledge cycles.
When processing an interrupt exception, the MC68020/EC020 first makes an internal copy
of the SR, sets the privilege level to supervisor, suppresses tracing, and sets the
processor interrupt mask level to the level of the interrupt being serviced. The processor
attempts to obtain a vector number from the interrupting device using an interrupt
acknowledge bus cycle with the interrupt level number output on pins A3–A1 of the
address bus. For a device that cannot supply an interrupt vector, the AVEC signal can be
asserted, and the MC68020/EC020 uses an internally generated autovector, which is one
of vector numbers 31–25, that corresponds to the interrupt level number. If external logic
indicates a bus error during the interrupt acknowledge cycle, the interrupt is considered
spurious, and the processor generates the spurious interrupt vector number (24). Refer to
Section 5 Bus Operation for complete interrupt bus cycle information.
Once the vector number is obtained, the processor saves the exception vector offset, PC
value, and the internal copy of the SR on the active supervisor stack. The saved value of
the PC is the logical address of the instruction that would have been executed had the
interrupt not occurred. If the interrupt was acknowledged during the execution of a
coprocessor instruction, further internal information is saved on the stack so that the
MC68020/EC020 can continue executing the coprocessor instruction when the interrupt
handler completes execution.
If the M-bit in the SR is set, the processor clears the M-bit and creates a throwaway
exception stack frame on top of the interrupt stack as part of interrupt exception
processing. This second frame contains the same PC value and vector offset as the frame
created on top of the master stack, but has a format number of 1 instead of 0 or 9. The
copy of the SR saved on the throwaway frame is exactly the same as that placed on the
master stack except that the S-bit is set in the version placed on the interrupt stack. (It
may or may not be set in the copy saved on the master stack.) The resulting SR (after
exception processing) has the S-bit set and the M-bit cleared.
The processor loads the address in the exception vector into the PC, and normal
instruction execution resumes after the required prefetches for the interrupt handler
routine.
Most M68000 family peripherals use programmable interrupt vector numbers as part of
the interrupt request/acknowledge mechanism of the system. If this vector number is not
initialized after reset and the peripheral must acknowledge an interrupt request, the
peripheral usually returns the uninitialized interrupt vector number (15).
6-18 M68020 USER’S MANUAL MOTOROLA
6.1.10 Breakpoint Instruction Exception
To use the MC68020/EC020 in a hardware emulator, it must provide a means of inserting
breakpoints in the emulator code and of performing appropriate operations at each
breakpoint. For the MC68000 and MC68008, this can be done by inserting an illegal
instruction at the breakpoint and detecting the illegal instruction exception from its vector
location. However, since the VBR on M68000 family processors MC68010 and later
allows arbitrary relocation of exception vectors, the exception address cannot reliably
identify a breakpoint. The MC68020/EC020 processor provides a breakpoint capability
with a set of breakpoint instructions, $4848–$484F, for eight unique breakpoints. The
breakpoint facility also allows external hardware to monitor the execution of a program
residing in the on-chip instruction cache without severe performance degradation.
When the MC68020/EC020 executes a breakpoint instruction, it performs a breakpoint
acknowledge cycle (read cycle) from CPU space type $0 with address lines A4–A2
corresponding to the breakpoint number. Refer to Section 5 Bus Operation for a
description of the breakpoint acknowledge cycle. The external hardware can return either
BERR or DSACK1/DSACK0 with an instruction word on the data bus. If the bus cycle
terminates with BERR, the processor performs illegal instruction exception processing. If
the bus cycle terminates with DSACK1/DSACK0, the processor uses the data returned to
replace the breakpoint instruction in the internal instruction pipe and begins execution of
that instruction. The remainder of the pipe remains unaltered. In addition, no stacking or
vector fetching is involved with the execution of the instruction. Figure 6-6 is a flowchart of
the breakpoint instruction execution.
6.1.11 Multiple Exceptions
When several exceptions occur simultaneously, they are processed according to a fixed
priority. Table 6-4 lists the exceptions grouped by characteristics. Each group has a
priority from 4–0. Priority 0 has the highest priority.
As soon as the MC68020/EC020 has completed exception processing for a condition
when another exception is pending, it begins exception processing for the pending
exception instead of executing the exception handler for the original exception condition.
Also, whenever a bus error or address error occurs, its exception processing takes
precedence over lower priority exceptions and occurs immediately. For example, if a bus
error occurs during the exception processing for a trace condition, the system processes
the bus error and executes its handler before completing the trace exception processing.
However, most exceptions cannot occur during exception processing, and very few
combinations of the exceptions shown in Table 6-4 can be pending simultaneously.
MOTOROLA M68020 USER’S MANUAL 6-19
EXIT
ENTRY
INITIATE READ BUS CYCLE
CYCLE TERMINATED WITH
BERR
PIPE STAGE D INSTRUCTION WORD ON DATA BUS
EXECUTE INSTRUCTION WORD
➧
TAKE ILLEGAL INSTRUCTION
EXCEPTION
A19–A16 $0
A4–A2 BREAKPOINT NUMBER
➧
➧
CYCLE TERMINATED WITH
DSACK1/DSACK0
Figure 6-6. Breakpoint Instruction Flowchart
Table 6-4. Exception Priority Groups
Group/
Priority Exception and Relative Priority Characteristic
00.0—Reset Aborts all processing (instruction or exception) and
does not save old context.
11.0—Address Error
1.1—Bus Error Suspends processing (instruction or exception) and
saves internal context.
22.0—BKPT, CALLM, CHK, CHK2,
cp Midinstruction, cp Protocol Violation,
cpTRAPcc, Divide by Zero, RTE, RTM,
TRAP, TRAPcc, TRAPV
Exception processing is part of instruction execution.
3 3.0—Illegal Instruction, Line A, Unimplemented
Line F, Privilege Violation, cp Preinstruction Exception processing begins before instruction is
executed.
44.0—cp Postinstruction
4.1—Trace
4.2—Interrupt
Exception processing begins when current instruction
or previous exception processing has completed.
NOTE: 0.0 is the highest priority; 4.2 is the lowest.
6-20 M68020 USER’S MANUAL MOTOROLA
The priority scheme is very important in determining the order in which exception handlers
execute when several exceptions occur at the same time. As a general rule, the lower the
priority of an exception, the sooner the handler routine for that exception executes. For
example, if simultaneous trap, trace, and interrupt exceptions are pending, the exception
processing for the trap occurs first, followed immediately by exception processing for the
trace, and then for the interrupt. When the processor resumes normal instruction
execution, it is in the interrupt handler, which returns to the trace handler, which returns to
the trap exception handler. This rule does not apply to the reset exception; its handler is
executed first even though it has the highest priority because the reset operation clears all
other exceptions.
6.1.12 Return from Exception
After the MC68020/EC020 has completed exception processing for all pending
exceptions, it resumes normal instruction execution at the address in the vector for the last
exception processed. Once the exception handler has completed execution, the processor
must return to the system context prior to the exception (if possible). The RTE instruction
returns from the handler to the previous system context for any exception.
When the processor executes an RTE instruction, it examines the stack frame on top of
the active supervisor stack to determine if it is a valid frame and what type of context
restoration it requires. The following paragraphs describe the processing for each of the
stack frame types; refer to 6.3 Coprocessor Considerations for a description of the
stack frame type formats.
For a normal four-word frame, the processor updates the SR and PC with the data read
from the stack, increments the stack pointer by eight, and resumes normal instruction
execution.
For the throwaway four-word frame, the processor reads the SR value from the frame,
increments the active stack pointer by eight, updates the SR with the value read from the
stack, and then begins RTE processing again, as shown in Figure 6-7. The processor
reads a new format word from the stack frame on top of the active stack (which may or
may not be the same stack used for the previous operation) and performs the proper
operations corresponding to that format. In most cases, the throwaway frame is on the
interrupt stack and when the SR value is read from the stack, the S and M bits are set. In
that case, there is a normal four-word frame or a ten-word coprocessor midinstruction
frame on the master stack. However, the second frame may be any format (even another
throwaway frame) and may reside on any of the three system stacks.
For the six-word stack frame, the processor restores the SR and PC values from the
stack, increments the active supervisor stack pointer by 12, and resumes normal
instruction execution.
MOTOROLA M68020 USER’S MANUAL 6-21
ENTRY
SR TEMP
S
P SP +
6
➧
➧
TEMP (SP) +
R
EAD FORMAT WOR
D
➧
OTHERWISE
FORMAT CODE = $1
(THROWAWAY FRAME)
OTHERWISE
PC (SP) +
SP SP +
6
SR TEM
P
➧
➧
➧
EXIT
OTHER FORMATS
TAKE FORMAT
E
RROR EXCEPTIO
N
OTHERWISE
FORMAT CODE = $0 (FOUR-WORD FRAME)
INVALID FORMAT WORD
Figure 6-7. RTE Instruction for Throwaway Four-Word Frame
For the coprocessor midinstruction stack frame, the processor reads the SR, PC,
instruction address, internal register values, and the evaluated effective address from the
stack, restores these values to the corresponding internal registers, and increments the
stack pointer by 20. The processor then reads from the response register of the
coprocessor that initiated the exception to determine the next operation to be performed.
Refer to Section 7 Coprocessor Interface Description for details of coprocessor-related
exceptions.
For both the short and long bus fault stack frames, the processor first checks the format
value on the stack for validity. In addition, for the long stack frame, the processor
compares the version number in the stack with its own version number. The version
number is located in the most significant nibble (bits 15–12) of the word at location SP +
$36 in the long stack frame. This validity check is required in a multiprocessor system to
ensure that the data is properly interpreted by the RTE instruction. The RTE instruction
also reads from both ends of the stack frame to make sure it is accessible. If the frame is
invalid or inaccessible, the processor takes a format error or a bus error exception,
respectively. Otherwise, the processor reads the entire frame into the proper internal
registers, deallocates the stack, and resumes normal processing. Once the processor
begins to load the frame to restore its internal state, the assertion of the BERR signal
6-22 M68020 USER’S MANUAL MOTOROLA
causes the processor to enter the halted state. Refer to 6.2 Bus Fault Recovery for a
description of the processing that occurs after the frame is read into the internal registers.
If a format error or bus error exception occurs during the frame validation sequence of the
RTE instruction, either due to any of the errors previously described or due to an illegal
format code, the processor creates a normal four-word or a bus fault stack frame below
the frame that it was attempting to use. In this way, the faulty stack frame remains intact.
The exception handler can examine or repair the faulty frame. In a multiprocessor system,
the faulty frame can be left to be used by another processor of a different type (e.g., an
MC68010 or a future M68000 family processor) when appropriate.
6.2 BUS FAULT RECOVERY
An address error exception or a bus error exception indicates a bus fault . The saving of
the processor state for a bus error or address error is described in 6.1.2 Bus Error
Exception, and the restoring of the processor state by an RTE instruction is described in
6.1.12 Return from Exception.
Processor accesses of either data items or the instruction stream can result in bus errors.
When a bus error exception occurs while accessing a data item, the exception is taken
immediately after the bus cycle terminates. The processor may never access an
instruction that is part of the instruction stream. In this case, the bus error would not be
processed. For instruction faults, when the short bus fault stack frame applies, the
address of the pipe stage B word is the value in the PC plus four, and the address of the
stage C word is the value in the PC plus two. For the long format, the long word at SP +
$24 contains the address of the stage B word; the address of the stage C word is the
address of the stage B word minus two. Address error faults occur only for instruction
stream accesses, and the exceptions are taken before the bus cycles are attempted.
6.2.1 Special Status Word (SSW)
The internal SSW (see Figure 6-8) is one of several registers saved as part of the bus
fault exception stack frame. Both the short bus fault format and the long bus fault format
include this word at offset $A. The bus cycle fault stack frame formats are described in
detail in 6.4 Exception Stack Frame Formats.
The SSW information indicates whether the fault was caused by an access to the
instruction stream, data stream, or both. The high-order half of the SSW contains two
status bits each for the B and C stages of the instruction pipe. If an address error
exception occurs, the fault bits written to the stack frame are not set (they are only set due
to a bus error, as previously described), and the rerun bits alone show the cause of the
exception. Depending on the state of the pipeline, either RB and RC are set, or only RC is
set. To correct the pipeline contents and continue execution of the suspended instruction,
software must place the correct instruction stream data in the stage C and/or stage B
images requested by the rerun bits and must clear the rerun bits. The least significant half
of the SSW applies to data cycles only. Data and instruction stream faults may be pending
simultaneously; the fault handler should be able to recognize any combination of the FC,
FB, RC, RB, and DF bits.
MOTOROLA M68020 USER’S MANUAL 6-23
15
0
FC
FB
RC
14
13
12
RB
11
10
9
8
7
6
5
4
3
2
0
0
0
DF
R
M
R
W SIZE
0
FC2–FC0
Figure 6-8. Special Status Word Format
FC—Fault on Stage C
When the FC bit is set, the processor attempted to use stage C and found it to be
marked invalid due to a bus error on the prefetch for that stage. FC can be used by a
bus error handler to determine the cause(s) of a bus error exception.
FB—Fault on Stage B
When the FB bit is set, the processor attempted to use stage B and found it to be
marked invalid due to a bus error on the prefetch for that stage. FB can be used by a
bus error handler to determine the cause(s) of a bus error exception.
RC—Rerun Flag for Stage C
The RC bit is set to indicate that a fault occurred during a prefetch for stage C. The RC
bit is always set when the FC bit is set. The RC bit indicates that the word in stage C of
the instruction pipe is invalid, and the state of the bit can be used by a handler to repair
the values in the pipe after an address error or a bus error, if necessary. If the RC bit is
set when the processor executes an RTE instruction, the processor may execute a bus
cycle to prefetch the instruction word for stage C of the pipe (if it is required). If the RC
and FC bits are set, the RTE instruction automatically reruns the prefetch cycle for
stage C. The address space for the bus cycle is the program space for the privilege
level indicated in the copy of the SR on the stack. If the RC bit is clear, the words on the
stack for stage C of the pipe are accepted as valid; the processor assumes that there is
no prefetch pending for stage C and that software has repaired or filled the image of
stage C, if necessary.
1 = Rerun faulted bus cycle or run pending prefetch
0 = Do not rerun bus cycle
RB—Rerun Flag for Stage B
The RB bit is set to indicate that a fault occurred during a prefetch for stage B. The RB
bit is always set when the FB bit is set. The RB bit indicates that the word in stage B of
the instruction pipe is invalid, and the state of the bit can be used by a handler to repair
the values in the pipe after an address error or a bus error, if necessary. If the RB bit is
set when the processor executes an RTE instruction, the processor may execute a bus
cycle to prefetch the instruction word for stage B of the pipe (if it is required). If the RB
and FB bits are set, the RTE instruction automatically reruns the prefetch cycle for stage
B. The address space for the bus cycle is the program space for the privilege level
indicated in the copy of the SR on the stack. If the RB bit is clear, the words on the
6-24 M68020 USER’S MANUAL MOTOROLA
stack for stage B of the pipe are accepted as valid; the processor assumes that there is
no prefetch pending for stage B and that software has repaired or filled the image of
stage B, if necessary.
1 = Rerun faulted bus cycle or run pending prefetch
0 = Do not rerun bus cycle
Bits 11–9—Reserved by Motorola
DF—Fault/Rerun Flag
If the DF bit is set, a data fault has occurred and caused the exception. If the DF bit is
set when the processor reads the stack frame, it reruns the faulted data access;
otherwise, it assumes that the data input buffer value on the stack is valid for a read or
that the data has been correctly written to memory for a write (or that no data fault
occurred).
1 = Rerun faulted bus cycle or run pending prefetch
0 = Do not rerun bus cycle
RM—Read-Modify-Write
1 = Read-modify-write operation on data cycle
0 = Not a read-modify-write operation
RW—Read/Write
1 = Read on data cycle
0 = Write on data cycle
SIZE—Size Code
The SIZE field indicates the size of the operand access for the data cycle.
Bit 3—Reserved by Motorola
FC2–FC0—Specifies the address space for data cycle
6.2.2 Using Software to Complete the Bus Cycles
One method of completing a faulted bus cycle is to use a software handler to emulate the
cycle. This is the only method for correcting address errors. The handler should emulate
the faulted bus cycle in a manner that is transparent to the instruction that caused the
fault. For instruction stream faults, the handler may need to run bus cycles for both the B
and C stages of the instruction pipe. The RB and RC bits of the SSW identify the stages
that may require a bus cycle; the FB and FC bits of the SSW indicate that a stage was
invalid when an attempt was made to use its contents. Those stages must be repaired.
For each faulted stage, the software handler should copy the instruction word from the
proper address space as indicated by the S-bit of the copy of the SR saved on the stack to
the image of the appropriate stage in the stack frame. In addition, the handler must clear
the RB or RC bit associated with the stage that it has corrected. The handler should not
change the FB and FC bits.
MOTOROLA M68020 USER’S MANUAL 6-25
To repair data faults (indicated by DF = 1), the software should first examine the RM bit in
the SSW to determine if the fault was generated during a read-modify-write operation. If
RM = 0, the handler should then check the RW bit of the SSW to determine if the fault was
caused by a read or a write cycle. For data write faults, the handler must transfer the
properly sized data from the data output buffer on the stack frame to the location indicated
by the data fault address in the address space defined by the SSW. (Both the data output
buffer and the data fault address are part of the stack frame at SP + $18 and SP + $10,
respectively.) Data read faults only generate the long bus fault frame, and the handler
must transfer properly sized data from the location indicated by the fault address and
address space to the image of the data input buffer at location SP + $2C of the long
format stack frame. Byte, word, and 3-byte operands are right justified in the 4-byte data
buffers. In addition, the software handler must clear the DF bit of the SSW to indicate that
the faulted bus cycle has been corrected.
To emulate a read-modify-write cycle, the exception handler must first read the operation
word at the PC address (SP + 2 of the stack frame). This word identifies the CAS, CAS2,
or TAS instruction that caused the fault. Then the handler must emulate this entire
instruction (which may consist of up to four long-word transfers) and update the CCR
portion of the SR appropriately, because the RTE instruction expects the entire operation
to have been completed if the RM bit is set and the DF bit is cleared. This is true even if
the fault occurred on the first read cycle.
To emulate the entire instruction, the handler must save the data and address registers for
the instruction (with a MOVEM instruction, for example). Next, the handler reads and
modifies (if necessary) the memory location. It clears the DF bit in the SSW of the stack
frame and modifies the condition codes in the SR copy and the copies of any data or
address registers required for the CAS and CAS2 instructions. Last, the handler restores
the registers that it saved at the beginning of the emulation. Except for the data input
buffer, the copy of the SR, and the SSW, the handler should not modify a bus fault stack
frame. The only bits in the SSW that may be modified are DF, RB, and RC; all other bits,
including those defined for internal use, must remain unchanged.
Address error faults must be repaired in software. Address error faults can be
distinguished from bus error faults by the value in the vector offset field of the format word.
6.2.3 Completing the Bus Cycles with RTE
Another method of completing a faulted bus cycle is to allow the processor to rerun the
bus cycles during execution of the RTE instruction that terminates the exception handler.
This method cannot be used to recover from address errors. The RTE instruction is
always executed. Unless the handler routine has corrected the error and cleared the fault
(and cleared the RB/RC and DF bits of the SSW), the RTE instruction cannot complete
the bus cycle(s). If the DF bit is still set at the time of the RTE execution, the faulted data
cycle is rerun by the RTE instruction. If the FB or FC bit is set and the corresponding rerun
bit (RB or RC) was not cleared by the software, the RTE reruns the associated instruction
prefetch. The fault occurs again unless the cause of the fault, such as a nonresident page
in a virtual memory system, has been corrected. If the RB or RC bit is set and the
6-26 M68020 USER’S MANUAL MOTOROLA
corresponding fault bit (FB or FC) is cleared, the associated prefetch cycle may or may
not be run by the RTE instruction (depending on whether the stage is required).
If a fault occurs when the RTE instruction attempts to rerun the bus cycle(s), the processor
creates a new stack frame on the supervisor stack after deallocating the previous frame,
and address error or bus error exception processing starts in the normal manner.
The read-modify-write operations of the MC68020/EC020 can also be completed by the
RTE instruction that terminates the handler routine. The rerun operation, executed by the
RTE instruction with the DF bit of the SSW set, reruns the entire instruction. If the cause of
the error has been corrected, the handler does not need to emulate the instruction but can
leave the DF bit set and execute the RTE instruction.
6.3 COPROCESSOR CONSIDERATIONS
Exception handler programmers should consider carefully whether to save and restore the
context of a coprocessor at the beginning and end of handler routines for exceptions that
can occur during the execution of a coprocessor instruction (i.e., bus errors, interrupts,
and coprocessor-related exceptions). The nature of the coprocessor and the exception
handler routine determines whether or not saving the state of one or more coprocessors
with the cpSAVE and cpRESTORE instructions is required. If the coprocessor allows
multiple coprocessor instructions to be executed concurrently, it may require its state to be
saved and restored for all coprocessor-generated exceptions, regardless of whether or not
the coprocessor is accessed during the handler routine. The MC68882 floating-point
coprocessor is an example of this type of coprocessor. On the other hand, the MC68881
floating-point coprocessor requires FSAVE and FRESTORE instructions within an
exception handler routine only if the exception handler itself uses the coprocessor.
6.4 EXCEPTION STACK FRAME FORMATS
The MC68020/EC020 provides six different stack frames for exception processing. The
set of frames includes the normal four- and six-word stack frames, the four-word
throwaway stack frame, the coprocessor midinstruction stack frame, and the short and
long bus fault stack frames.
When the MC68020/EC020 writes or reads a stack frame, it uses long-word operand
transfers wherever possible. Using a long-word-aligned stack pointer with memory that is
on a 32-bit port greatly enhances exception processing performance. The processor does
not necessarily read or write the stack frame data in sequential order.
The system software should not depend on a particular exception generating a particular
stack frame. For compatibility with future devices, the software should be able to handle
any type of stack frame for any type of exception.
Table 6-5 summarizes the stack frames defined for the MC68020/EC020.
MOTOROLA M68020 USER’S MANUAL 6-27
Table 6-5. Exception Stack Frames
SIX-WORD
STACK FRAME — FORMAT $2
Exception Types (Stacked PC Points to)Stack Frames
Interrupt
Format Error
TRAP #N
Illegal Instruction
A-Line Instruction
F-Line Instruction
Privilege Violation
Coprocessor
Preinstruction
[Next instruction]
[RTE or cpRESTORE instruction
[NEXT instruction]
[Illegal instruction]
[A-line instruction]
[F-line instruction]
[First word of instruction causing
Privilege Violation]
[Opword of instruction that
returned the 'take preinstruction'
primitive]
THROWAWAY FOUR-WORD
STACK FRAME — FORMAT $1
Created on
Interrupt Stack
during interrupt
exception processing
when transition from
master state to
interrupt state occurs
[Next instruction — same as on
master stack]
STATUS REGISTER
15 0
PROGRAM COUNTER
VECTOR OFFSET0010
SP
+$06
+$02
CHK
CHK2
cpTRAPcc
TRAPcc
TRAPPV
Trace
Zero Divide
MMU Configuration
Coprocessor
Postinstruction
[Next instruction for all these
exceptions]
INSTRUCTION ADDRESS
is the address of the instruction
that caused the exception
INSTRUCTION ADDRESS
+$08
STATUS REGISTER
15 0
PROGRAM COUNTER
VECTOR OFFSET1001
SP
+$06
+$02
Coprocessor
Midinstruction
Main-Detected
Protocol Violation
Interrupt Detected
During Coprocessor
Instruction
(supported with 'null
come again with
interrupts allowed'
primitive)
INSTRUCTION ADDRESS+$08
COPROCESSOR MIDINSTRUCTION
STACK FRAME (10 WORDS) — FORMAT $9
INTERNAL REGISTERS,
4 WORDS
+$0C
+$12
[Next word to be fetched from
instruction stream for all these
exceptions]
INSTRUCTION ADDRESS
is the address of the instruction
that caused the exception
FOUR-WORD STACK FRAME — FORMAT $0
STATUS REGISTER
15 0
PROGRAM COUNTER
VECTOR OFFSET0000
SP
+$06
+$02
STATUS REGISTER
15 0
PROGRAM COUNTER
VECTOR OFFSET0001
SP
+$06
+$02
6-28 M68020 USER’S MANUAL MOTOROLA
Table 6-5. Exception Stack Frames (Continued)
Exception Types (Stacked PC Points to)Stack Frames
Address Error or
Bus Error —
Execution Unit
at Instruction
Boundary
[Next instruction]SP
SP
INTERNAL INFORMATION
STATUS REGISTER
15 0
PROGRAM COUNTER
VECTOR OFFSET1010
+$06
+$02
INTERNAL REGISTER+$08 SPECIAL STATUS REGISTER+$0A INSTRUCTION PIPE STAGE C+$0C INSTRUCTION PIPE STAGE B+$0E
+$10
+$12 INTERNAL REGISTER
+$14 INTERNAL REGISTER
+$16
+$18
+$1A INTERNAL REGISTER+$1C
DATA CYCLE FAULT ADDRESS
DATA OUTPUT BUFFER
INTERNAL REGISTER+$1E
STATUS REGISTER
15 0
PROGRAM COUNTER
VECTOR OFFSET1011
+$06
+$02
INTERNAL REGISTER+$08 SPECIAL STATUS REGISTER+$0A INSTRUCTION PIPE STAGE C+$0C INSTRUCTION PIPE STAGE B+$0E
+$10
+$12 INTERNAL REGISTER
+$14 INTERNAL REGISTER
+$16
+$18
INTERNAL REGISTER,
4 WORDS
DATA CYCLE FAULT ADDRESS
DATA OUTPUT BUFFER
+$22
+$2A
+$24
INTERNAL REGISTERS,
2 WORDS
+$2C
+$30 INTERNAL REGISTERS,
3 WORDS
+$36
+$38
INTERNAL REGISTERS,
18 WORDS
+$5A
+$28
+$1A
+$1C
VERSION #
DATA INPUT BUFFER
STAGE B ADDRESS
SHORT BUS FAULT STACK FRAME
(16 WORDS) — FORMAT $A
Address Error or
Bus Error —
Instruction
Execution in
Progress
[Address of instruction in
execution when fault occurred —
may not be the instruction that
generated the faulted bus cycle]
LONG BUS FAULT STACK FRAME
(46 WORDS) — FORMAT $B
MOTOROLA M68020 USER’S MANUAL 7-1
SECTION 7
COPROCESSOR INTERFACE DESCRIPTION
The M68000 family of general-purpose microprocessors provides a level of performance
that satisfies a wide range of computer applications. Special-purpose hardware, however,
can often provide a higher level of performance for a specific application. The coprocessor
concept allows the capabilities and performance of a general-purpose processor to be
enhanced for a particular application without encumbering the main processor
architecture. A coprocessor can efficiently meet specific capability requirements that must
typically be implemented in software by a general-purpose processor. With a general-
purpose main processor and the appropriate coprocessor(s), the processing capabilities of
a system can be tailored to a specific application.
The MC68020/EC020 supports the M68000 coprocessor interface described in this
section. This section is intended for designers who are implementing coprocessors to
interface with the MC68020/EC020.
The designer of a system that uses one or more Motorola coprocessors (the MC68881 or
MC68882 floating-point coprocessor, for example) does not require a detailed knowledge
of the M68000 coprocessor interface. Motorola coprocessors conform to the interface
described in this section. Typically, they implement a subset of the interface, and that
subset is described in the coprocessor user's manual. These coprocessors execute
Motorola-defined instructions that are described in the user's manual for each
coprocessor.
7.1 INTRODUCTION
The distinction between standard peripheral hardware and an M68000 coprocessor is
important from a programming model perspective. The programming model of the main
processor consists of the instruction set, register set, and memory map. An M68000
coprocessor is a device or set of devices that communicates with the main processor
through the protocol defined as the M68000 coprocessor interface. The programming
model for a coprocessor is different than that for a peripheral device. A coprocessor adds
additional instructions and generally additional registers and data types to the
programming model that are not directly supported by the main processor architecture.
The additional instructions are dedicated coprocessor instructions that utilize the
coprocessor capabilities. The necessary interactions between the main processor and the
coprocessor that provide a given service are transparent to the programmer. That is, the
programmer does not need to know the specific communication protocol between the
main processor and the coprocessor because this protocol is implemented in hardware.
Thus, the coprocessor can provide capabilities to the user without appearing separate
from the main processor.
7-2 M68020 USER’S MANUAL MOTOROLA
In contrast, standard peripheral hardware is generally accessed through interface
registers mapped into the memory space of the main processor. To use the services
provided by the peripheral, the programmer accesses the peripheral registers with
standard processor instructions. While a peripheral could conceivably provide capabilities
equivalent to a coprocessor for many applications, the programmer must implement the
communication protocol between the main processor and the peripheral necessary to use
the peripheral hardware.
The communication protocol defined for the M68000 coprocessor interface is described in
7.2 Coprocessor Instruction Types. The algorithms that implement the M68000
coprocessor interface are provided in the microcode of the MC68020/EC020 and are
completely transparent to the MC68020/EC020 programming model. For example,
floating-point operations are not implemented in the MC68020/EC020 hardware. In a
system utilizing both the MC68020/EC020 and the MC68881 or MC68882 floating-point
coprocessor, a programmer can use any of the instructions defined for the coprocessor
without knowing that the actual computation is performed by the MC68881 or MC68882
hardware.
7.1.1 Interface Features
The M68000 coprocessor interface design incorporates a number of flexible capabilities.
The physical coprocessor interface uses the main processor external bus, which simplifies
the interface since no special-purpose signals are involved. With the MC68020/EC020, a
coprocessor uses the asynchronous bus transfer protocol. Since standard bus cycles
transfer information between the main processor and the coprocessor, the coprocessor
can be implemented in whatever technology is available to the coprocessor designer. A
coprocessor can be implemented as a VLSI device, as a separate system board, or even
as a separate computer system.
Since the main processor and a M68000 coprocessor can communicate using the
asynchronous bus, they can operate at different clock frequencies. The system designer
can choose the speeds of a main processor and coprocessor that provide the optimum
performance for a given system. Both the MC68881 and MC68882 floating-point
coprocessors use the asynchronous bus handshake protocol.
The M68000 coprocessor interface also facilitates the design of coprocessors. The
coprocessor designer must only conform to the coprocessor interface and does not need
an extensive knowledge of the architecture of the main processor. Also, the main
processor can operate with a coprocessor without having explicit provisions made in the
main processor for the capabilities of that coprocessor. This type of interface provides a
great deal of freedom in the implementation of a given coprocessor.
7.1.2 Concurrent Operation Support
The programming model for the M68000 family of microprocessors is based on
sequential, nonconcurrent instruction execution, which implies that the instructions in a
given sequence must appear to be executed in the order in which they occur. To maintain
a uniform programming model, any coprocessor extensions should also maintain the
MOTOROLA M68020 USER’S MANUAL 7-3
model of sequential, nonconcurrent instruction execution at the user level. Consequently,
the programmer can assume that the images of registers and memory affected by a given
instruction have been updated when the next instruction in the sequence accessing these
registers or memory locations is executed.
The M68000 coprocessor interface provides full support of all operations necessary for
nonconcurrent operation of the main processor and its associated coprocessors. Although
the M68000 coprocessor interface allows concurrency in coprocessor execution, the
coprocessor designer is responsible for implementing this concurrency while maintaining a
programming model based on sequential nonconcurrent instruction execution.
For example, if the coprocessor determines that instruction B does not use or alter
resources to be altered or used by instruction A, instruction B can be executed
concurrently (if the execution hardware is also available). Thus, the required instruction
interdependencies and sequences of the program are always respected. The MC68882
coprocessor offers concurrent instruction execution; whereas, the MC68881 coprocessor
does not. However, the MC68020/EC020 can execute instructions concurrently with
coprocessor instruction execution in the MC68881.
7.1.3 Coprocessor Instruction Format
The instruction set for a given coprocessor is defined by the design of that coprocessor.
When a coprocessor instruction is encountered in the main processor instruction stream,
the MC68020/EC020 hardware initiates communication with the coprocessor and
coordinates any interaction necessary to execute the instruction with the coprocessor. A
programmer needs to know only the instruction set and register set defined by the
coprocessor to use the functions provided by the coprocessor hardware.
The instruction set of an M68000 coprocessor uses a subset of the F-line operation words
in the M68000 instruction set. The operation word is the first word of any M68000 family
instruction. The F-line operation word contains ones in bits 15–12 (refer to Figure 7-1); the
remaining bits are coprocessor and instruction dependent. The F-line operation word may
be followed by as many extension words as are required to provide additional information
necessary for the execution of the coprocessor instruction.
15
0
1
1
1
14
13
12
1
11
CpID
9
8
TYPE
6
5
T
YPE DEPENDEN
T
Figure 7-1. F-Line Coprocessor Instruction Operation Word
As shown in Figure 7-1, bits 11–9 of the F-line operation word encode the coprocessor
identification (CpID) field. The MC68020/EC020 uses the CpID field to indicate the
coprocessor to which the instruction applies. F-line operation words, in which the CpID is
zero, are not coprocessor instructions for the MC68020/EC020. Instructions with a CpID of
zero and a nonzero type field are unimplemented instructions that cause the
7-4 M68020 USER’S MANUAL MOTOROLA
MC68020/EC020 to begin exception processing. The MC68020/EC020 never generates
coprocessor interface bus cycles with the CpID equal to zero (except via the MOVES
instruction).
CpID codes of 000–101 are reserved for current and future Motorola coprocessors, and
CpID codes of 110–111 are reserved for user-defined coprocessors. The Motorola CpID
code of 001 designates the MC68881 or MC68882 floating-point coprocessor. By default,
Motorola assemblers will use a CpID code of 001 when generating the instruction
operation codes for the MC68881 or MC68882.
The encoding of bits 8–0 of the coprocessor instruction operation word is dependent on
the particular instruction being implemented (refer to 7.2 Coprocessor Instruction
Types).
7.1.4 Coprocessor System Interface
The communication protocol between the main processor and coprocessor necessary to
execute a coprocessor instruction uses a group of interface registers, CIRs, resident
within the coprocessor. By accessing one of the CIRs, the MC68020/EC020 hardware
initiates coprocessor instructions. The coprocessor uses a set of response primitive codes
and format codes defined for the M68000 coprocessor interface to communicate status
and service requests to the main processor through these registers. The CIRs are also
used to pass operands between the main processor and the coprocessor. The CIR set,
response primitives, and format codes are discussed in 7.3 Coprocessor Interface
Register Set and 7.4 Coprocessor Response Primitives.
7.1.4.1 COPROCESSOR CLASSIFICATION. M68000 coprocessors can be classified into
two categories depending on their bus interface capabilities. The first category, non-DMA
coprocessors, consists of coprocessors that always operate as bus slaves. The second
category, DMA coprocessors, consists of coprocessors that operate as bus slaves while
communicating with the main processor across the coprocessor interface. These
coprocessors also have the ability to operate as bus masters, directly controlling the
system bus.
If the operation of a coprocessor does not require a large portion of the available bus
bandwidth or has special requirements not directly satisfied by the main processor, that
coprocessor can be efficiently implemented as a non-DMA coprocessor. Since non-DMA
coprocessors always operate as bus slaves, all external bus-related functions that the
coprocessor requires are performed by the main processor. The main processor transfers
operands from the coprocessor by reading the operand from the appropriate CIR and then
writing the operand to a specified effective address with the appropriate address space
specified on the FC2–FC0. Likewise, the main processor transfers operands to the
coprocessor by reading the operand from a specified effective address (and address
space) and then writing that operand to the appropriate CIR using the coprocessor
interface. The bus interface circuitry of a coprocessor operating as a bus slave is not as
complex as that of a device operating as a bus master.
MOTOROLA M68020 USER’S MANUAL 7-5
To improve the efficiency of operand transfers between memory and the coprocessor, a
coprocessor that requires a relatively high amount of bus bandwidth or has special bus
requirements can be implemented as a DMA coprocessor. The DMA coprocessor
provides all control, address, and data signals necessary to request and obtain the bus
and then performs DMA transfers using the bus. DMA coprocessors, however, must still
act as bus slaves when they require information or services of the main processor using
the M68000 coprocessor interface protocol.
7.1.4.2 PROCESSOR-COPROCESSOR INTERFACE. Figure 7-2 is a block diagram of
the signals involved in an asynchronous non-DMA M68000 coprocessor interface. Since
the CpID on signals A15–A13 of the address bus is used with other address signals to
select the coprocessor, the system designer can use several coprocessors of the same
type and assign a unique CpID to each one.
FC2–FC0
A19–A13
COPROCESSOR
DECODE
LOGIC
CS
COPROCESSOR
ASYNCHRONOUS
BUS
INTERFACE
LOGIC
AS
DS
R/W
A4–A1
D31–D0
DSACK1 / DSACK0
MAIN PROCESSOR
MC68020/EC020
FC2–FC0 = 111
A
19–A16 = 0010
A
15–A13 = xxx
A
4–A1 = rrrr
*Chip select logic may be integrated into the coprocessor.
Address lines not specified above are "0" during coprocessor access.
➧
➧
➧
➧
*
CPU SPACE CYCLE
C
OPROCESSOR ACCESS IN CPU SPACE
C
OPROCESSOR IDENTIFICATION
C
OPROCESSOR INFERFACE REGISTER SELECTOR
Figure 7-2. Asynchronous Non-DMA M68000
Coprocessor Interface Signal Usage
The MC68020/EC020 accesses the registers in the CIR set using standard asynchronous
bus cycles. Thus, the bus interface implemented by a coprocessor for its interface register
set must satisfy the MC68020/EC020 address, data, and control signal timing. The
MC68020/EC020 bus operation is described in detail in Section 5 Bus Operation.
7-6 M68020 USER’S MANUAL MOTOROLA
During coprocessor instruction execution, the MC68020/EC020 executes CPU space bus
cycles to access the CIR set. The MC68020/EC020 asserts FC2–FC0, identifying a CPU
space bus cycle. The CIR set is mapped into CPU space in the same manner that a
peripheral interface register set is generally mapped into data space. The information
encoded on FC2–FC0 an d t h e a d d r es s b u s o f the MC68020/EC020 during a coprocessor
access is used to generate the chip select signal for the coprocessor being accessed.
Other address lines select a register within the interface set. The information encoded on
the function code and address lines of the MC68020/EC020 during a coprocessor access
is illustrated in Figure 7-3.
0
C
I
R
CpID
0
1
0
0
0
0
0
0
0
0
0
0
0
0
1
0
0
0
0
0
0
0
4
5
12
13
15
19
20
31
0
2
0
0
0
16
1
1
F
UNCTIO
N
CODE ADDRESS
BUS
C
PU SPAC
E
T
YPE FIEL
D
Figure 7-3. MC68020/EC020 CPU Space Address Encodings
Signals A19–A16 of the MC68020/EC020 address bus specify the CPU space cycle type
for a CPU space bus cycle. The types of CPU space cycles currently defined for the
MC68020/EC020 are interrupt acknowledge, breakpoint acknowledge, module support
operations, and coprocessor access cycles. CPU space type $2 (A19–A16 = 0010)
specifies a coprocessor access cycle.
A15–A13 specify the CpID code for the coprocessor being accessed. This code is
transferred from bits 11–9 of the coprocessor instruction operation word (refer to Figure
7-1) to the address bus during each coprocessor access. Thus, decoding the
MC68020/EC020 FC2–FC0 and A19–A13 signals provides a unique chip select signal for
a given coprocessor. The FC2–FC0 and A19–A16 signals indicate a coprocessor access;
A15–A13 indicate which of the possible eight coprocessors (000–111) is being accessed.
Bits A31–A20 and A12–A5 of the MC68020 address bus and bits A23–A20 and A12–A5
of the MC68EC020 address bus are always zero during a coprocessor access.
7.1.4.3 COPROCESSOR INTERFACE REGISTER SELECTION . Figure 7-4 shows that
the value on the MC68020/EC020 address bus during a coprocessor access addresses a
unique region of the main processor's CPU address space. Signals A4–A0 of the
MC68020/EC020 address bus select the CIR being accessed. The register map for the
M68000 coprocessor interface is shown in Figure 7-5. The individual registers are
described in detail in 7.3 Coprocessor Interface Register Set.
MOTOROLA M68020 USER’S MANUAL 7-7
INTERFACE REGISTER SET
RESERVED
INTERFACE REGISTER SET
RESERVED
INTERFACE REGISTER SET
RESERVED
CPU SPACE ADDRESS
$20000
$2001F
$22000
$2201F
$24000
$2E000
$2E01F
ADDRESS SPACE FOR
C
OPROCESSOR WITH
C
pID = 0
ADDRESS SPACE FOR
C
OPROCESSOR WITH
C
pID = 1
ADDRESS SPACE FOR
C
OPROCESSOR WITH
C
pID = 7
Figure 7-4. Coprocessor Address Map in MC68020/EC020 CPU Space
31
15
0
$
0
0
$
0
4
$
0
8
$0
C
$
1
0
$
1
4
$
1
8
$1
C
O
PERAN
D
(RESERVED)
INSTRUCTION ADDRESS
OPERAND ADDRESS
REGISTER SELECT
C
ONDITIO
N
(RESERVED)
C
OMMAN
D
OPERATION WORD
R
ESTOR
E
S
AV
E
C
ONTRO
L
R
ESPONS
E
16
Figure 7-5. Coprocessor Interface Register Set Map
7.2 COPROCESSOR INSTRUCTION TYPES
The M68000 coprocessor interface supports four categories of coprocessor instructions:
general, conditional, context save, and context restore. The category name indicates the
type of operations provided by the coprocessor instructions in the category. The
instruction category also determines the CIR accessed by the MC68020/EC020 to initiate
instruction and communication protocols between the main processor and the
coprocessor necessary for instruction execution.
During the execution of instructions in the general or conditional categories, the
coprocessor uses the set of coprocessor response primitive codes defined for the M68000
coprocessor interface to request services from and indicate status to the main processor.
During the execution of the instructions in the context save and context
7-8 M68020 USER’S MANUAL MOTOROLA
restore categories, the coprocessor uses the set of coprocessor format codes defined for
the M68000 coprocessor interface to indicate its status to the main processor.
7.2.1 Coprocessor General Instructions
The coprocessor general instruction category contains data processing instructions and
other general-purpose instructions for a given coprocessor.
7.2.1.1 FORMAT. Figure 7-6 shows the format of a coprocessor general instruction.
C
OPROCESSOR COMMAN
D
1
15
1
14
1
13
1
12
11
CpID
9
0
8
0
7
0
6
5
E
FFECTIVE ADDRES
S
0
OPTIONAL EFFECTIVE ADDRESS OR COPROCESSOR-DEFINED EXTENSION WORDS
Figure 7-6. Coprocessor General Instruction Format (cpGEN)
The mnemonic cpGEN is a generic mnemonic used in this discussion for all general
instructions. The mnemonic of a specific general instruction usually suggests the type of
operation it performs and the coprocessor to which it applies. The actual mnemonic and
syntax used to represent a coprocessor instruction is determined by the syntax of the
assembler or compiler that generates the object code.
A coprocessor general instruction consists of at least two words. The first word of the
instruction is an F-line operation code (bits 15–12 = 1111). The CpID field of the F-line
operation code is used during the coprocessor access to indicate which coprocessor in
the system executes the instruction. During accesses to the CIRs (refer to 7.1.4.2
Processor-Coprocessor Interface), the processor places the CpID on address lines
A15–A13.
Bits 8–6 = 000 of the first word of an instruction indicate that the instruction is in the
general instruction category. Bits 5–0 of the F-line operation code sometimes encode a
standard M68000 effective address specifier (refer to M68000PM/AD,
M68000 Family
Programmer’s Reference Manual
). During the execution of a cpGEN instruction, the
coprocessor can use a coprocessor response primitive to request that the
MC68020/EC020 perform an effective address calculation necessary for that instruction.
Using the effective address specifier field of the F-line operation code, the processor then
determines the effective addressing mode. If a coprocessor never requests effective
address calculation, bits 5–0 can have any value (don't cares).
The second word of the general type instruction is the coprocessor command word. The
main processor writes this command word to the command CIR to initiate execution of the
instruction by the coprocessor.
An instruction in the coprocessor general instruction category optionally includes a
number of extension words following the coprocessor command word. These words can
provide additional information required for the coprocessor instruction. For example, if
MOTOROLA M68020 USER’S MANUAL 7-9
the coprocessor requests that the MC68020/EC020 calculate an effective address during
coprocessor instruction execution, information required for the calculation must be
included in the instruction format as effective address extension words.
7.2.1.2 PROTOCOL. The execution of a cpGEN instruction follows the protocol shown in
Figure 7-7. The main processor initiates communication with the coprocessor by writing
the instruction command word to the command CIR. The coprocessor decodes the
command word to begin processing the cpGEN instruction. Coprocessor design
determines the interpretation of the coprocessor command word; the MC68020/EC020
does not attempt to decode it.
While the coprocessor is executing an instruction, it requests any required services from
and communicates status to the main processor by placing coprocessor response
primitive codes in the response CIR. After writing to the command CIR, the main
processor reads the response CIR and responds appropriately. When the coprocessor
has completed the execution of an instruction or no longer needs the services of the main
processor to execute the instruction, it provides a response to release the main processor.
The main processor can then execute the next instruction in the instruction stream.
However, if a trace exception is pending, the MC68020/EC020 does not terminate
communication with the coprocessor until the coprocessor indicates that it has completed
all processing associated with the cpGEN instruction (refer to 7.5.2.5 Trace Exceptions).
The coprocessor interface protocol shown in Figure 7-7 allows the coprocessor to define
the operation of each coprocessor general type instruction. That is, the main processor
initiates the instruction execution by writing the instruction command word to the
command CIR and by reading the response CIR to determine its next action. The
execution of the coprocessor instruction is then defined by the internal operation of the
coprocessor and by its use of response primitives to request services from the main
processor. This instruction protocol allows a wide range of operations to be implemented
in the general instruction category.
7-10 M68020 USER’S MANUAL MOTOROLA
M
1 RECOGNIZE COPROCESSOR INSTRUCTION F-LIN
E
O
PERATION WORD
M
2 WRITE COPROCESSOR COMMAND WORD TO
C
OMMAND CIR
M
3 READ COPROCESSOR RESPONSE PRIMITIVE CO
DE
F
ROM RESPONSE CIR
1
) PERFORM SERVICE REQUESTED BY RESPONSE
P
RIMITIVE
2
) IF (COPROCESSOR RESPONSE PRIMITIVE
I
NDICATES "COME AGAIN") GO TO M3
(
SEE NOTE 1)
M
4 PROCEED WITH EXECUTION OF NEXT INSTRUCT
ION
(
SEE NOTE 2)
C
1 DECODE COMMAND WORD AND INITIATE
C
OMMAND EXECUTION
C
2 WHILE (MAIN PROCESSOR SERVICE IS REQUIRED
)
D
O STEPS 1) AND 2) BELOW
1
) REQUEST SERVICE BY PLACING APPROPRIATE
RESPONSE PRIMITIVE CODE IN RESPONSE CIR
2
) RECEIVE SERVICE FROM MAIN PROCESSOR
C
3 REFLECT "NO COME AGAIN" IN RESPONSE CIR
C
4 COMPLETE COMMAND EXECUTION
C
5 REFLECT "PROCESSING FINISHED" STATUS IN
R
ESPONSE CIR
MAIN PROCESSOR
C
OPROCESSO
R
NOTES: 1. "Come Again" indicates that further service of the main processor is being requested by the coprocessor.
2. The next instruction should be the operation word pointed to by the ScanPC at this point. The operation of
the MC68020/EC020 ScanPC is discussed in 7.4.1 ScanPC.
Figure 7-7. Coprocessor Interface Protocol
for General Category Instructions
7.2.2 Coprocessor Conditional Instructions
The conditional instruction category provides program control based on the operations of
the coprocessor. The coprocessor evaluates a condition and returns a true/false indicator
to the main processor. The main processor completes the execution of the instruction
based on this true/false condition indicator.
The implementation of instructions in the conditional category promotes efficient use of
both the main processor and the coprocessor hardware. The condition specified for the
instruction is related to the coprocessor operation and is therefore evaluated by the
coprocessor. However, the instruction completion following the condition evaluation is
directly related to the operation of the main processor. The main processor performs the
change of flow, the setting of a byte, or the TRAP operation, since its architecture explicitly
implements these operations for its instruction set.
Figure 7-8 shows the protocol for a conditional category coprocessor instruction. The main
processor initiates execution of an instruction in this category by writing a condition
selector to the condition CIR. The coprocessor decodes the condition selector to
determine the condition to evaluate. The coprocessor can use response primitives to
request that the main processor provide services required for the condition evaluation.
MOTOROLA M68020 USER’S MANUAL 7-11
After evaluating the condition, the coprocessor returns a true/false indicator to the main
processor by placing a null primitive (refer to 7.4.4 Null Primitive) in the response CIR.
The main processor completes the coprocessor instruction execution when it receives the
condition indicator from the coprocessor.
M
1 RECOGNIZE COPROCESSOR INSTRUCTION F-LIN
E
O
PERATION WORD
M
2 WRITE COPROCESSOR CONDITION SELECTOR TO
C
O NDITION CIR
M
3 READ COPROCESSOR RESPONSE PRIMITIVE CO
DE
F
ROM RESPONSE CIR
1
) PERFORM SERVICE REQUESTED BY RESPONSE
P
RIMITIVE
2
) IF (COPROCESSOR RESPONSE PRIMITIVE
I
NDICATES "COME AGAIN") GO TO M3
(
SEE NOTE)
M
4 COMPLETE EXECUTION OF INSTRUCTION BASED
O
N THE TRUE/FALSE CONDITION INDICATOR
R
ETURNED IN THE RESPONSE CIR
C
1 DECODE COMMAND WORD AND INITIATE
C
OMMAND EXECUTION
C
2 WHILE (MAIN PROCESSOR SERVICE IS REQUIRED)
D
O STEPS 1) AND 2) BELOW
1
) REQUEST SERVICE BY PLACING APPROPRIATE
RESPONSE PRIMITIVE CODE IN RESPONSE CIR
2
) RECEIVE SERVICE FROM MAIN PROCESSOR
C
3 COMPLETE CONDITION EVALUATION
C
4 REFLECT "NO COME AGAIN" STATUS WITH TRUE/FA
LSE
C
ONDITION INDICATOR IN RESPONSE CIR
MAIN PROCESSOR
C
OPROCESSO
R
NOTE: All coprocessor response primitives, except the Null primitive, that allow the "Come Again" primitive attribute
must indicate "Come Again" when used during the execution of a conditional category instruction. If a "Come
Again" attribute is not indicated in one of these primitives, the main processor will initiate protocol violation
exception processing (see 7.5.2.1 Protocol Violations).
Figure 7-8. Coprocessor Interface Protocol
for Conditional Category Instructions
7-12 M68020 USER’S MANUAL MOTOROLA
7.2.2.1 BRANCH ON COPROCESSOR CONDITION INSTRUCTION. The conditional
instruction category includes two formats of the M68000 family branch instruction. These
instructions branch on conditions related to the coprocessor operation. They execute
similarly to the conditional branch instructions provided in the M68000 family instruction
set.
7.2.2.1.1 Format. Figure 7-9 shows the format of the branch on coprocessor condition
instruction that provides a word-length displacement. Figure 7-10 shows the format of this
instruction that includes a long-word displacement.
D
ISPLACEMEN
T
1
15
1
14
1
13
1
12
11
CpID
9
0
8
1
7
0
6
5
C
ONDITION SELECTOR
0
O
PTIONAL COPROCESSOR-DEFINED EXTENSION WORD
S
Figure 7-9. Branch on Coprocessor Condition
Instruction Format (cpBcc.W)
DISPLACEMENT — HIGH
1
15
1
14
1
13
1
12
11
CpID
9
0
8
1
7
1
6
5
C
ONDITION SELECTOR
0
O
PTIONAL COPROCESSOR-DEFINED EXTENSION WORD
S
DISPLACEMENT — LOW
Figure 7-10. Branch on Coprocessor Condition
Instruction Format (cpBcc.L)
The first word of the branch on coprocessor condition instruction is the F-line operation
word. Bits 15–12 = 1111 and bits 11–9 contain the CpID code of the coprocessor that is to
evaluate the condition. The value in bits 8–6 identifies either the word or the long-word
displacement format of the branch instruction, which is specified by the cpBcc.W or
cpBcc.L mnemonic, respectively. Bits 5–0 of the F-line operation word contain the
coprocessor condition selector field. The MC68020/EC020 writes the entire operation
word to the condition CIR to initiate execution of the branch instruction by the
coprocessor. The coprocessor uses bits 5–0 to determine which condition to evaluate.
If the coprocessor requires additional information to evaluate the condition, the branch
instruction format can include this information in extension words. Following the F-line
operation word, the number of extension words is determined by the coprocessor design.
The final word(s) of the cpBcc instruction format contains the displacement used by the
main processor to calculate the destination address when the branch is taken.
7.2.2.1.2 Protocol. Figure 7-8 shows the protocol for the cpBcc.L and cpBcc.W
instructions. The main processor initiates the instruction by writing the F-line operation
word to the condition CIR to transfer the condition selector to the coprocessor. The main
MOTOROLA M68020 USER’S MANUAL 7-13
processor then reads the response CIR to determine its next action. The coprocessor can
7-14 M68020 USER’S MANUAL MOTOROLA
return a response primitive to request services necessary to evaluate the condition. If the
coprocessor returns the false condition indicator, the main processor executes the next
instruction in the instruction stream. If the coprocessor returns the true condition indicator,
the main processor adds the displacement to the MC68020/EC020 scanPC (refer to 7.4.1
ScanPC) to determine the address of the next instruction for the main processor to
execute. The scanPC must be pointing to the location of the first word of the displacement
in the instruction stream when the address is calculated. The displacement is a twos-
complement integer that can be either a 16-bit word or a 32-bit long word. The main
processor sign-extends the 16-bit displacement to a long-word value for the destination
address calculation.
7.2.2.2 SET ON COPROCESSOR CONDITION INSTRUCTION. The set on coprocessor
condition instruction sets or resets a flag (a data alterable byte) according to a condition
evaluated by the coprocessor. The operation of this instruction type is similar to the
operation of the Scc instruction in the M68000 family instruction set. Although the Scc
instruction and the cpScc instruction do not explicitly cause a change of program flow,
they are often used to set flags that control program flow.
7.2.2.2.1 Format . Figure 7-11 shows the format of the set on coprocessor condition
instruction, denoted by the cpScc mnemonic.
1
15
1
14
1
13
1
12
11
CpID
9
0
8
0
7
1
6
5
E
FFECTIVE ADDRES
S
0
O
PTIONAL COPROCESSOR-DEFINED EXTENSION WORD
S
O
PTIONAL EFFECTIVE ADDRESS EXTENSION WORDS (0–5 WORDS
)
C
ONDITION SELECTOR
R
ESERVE
D
Figure 7-11. Set on Coprocessor Condition Instruction Format (cpScc)
The first word of the cpScc instruction, the F-line operation word, contains the CpID field in
bits 11–9 and 001 in bits 8–6 to identify the cpScc instruction. Bits 5–0 of the F-line
operation word are used to encode an M68000 family effective addressing mode (refer to
M68000PM/AD,
M68000 Family Programmer’s Reference Manual
).
The second word of the cpScc instruction format contains the coprocessor condition
selector field in bits 5–0. Bits 15–6 of this word are reserved by Motorola and should be
zero to ensure compatibility with future M68000 products. This word is written to the
condition CIR to initiate execution of the cpScc instruction.
If the coprocessor requires additional information to evaluate the condition, the instruction
can include extension words to provide this information. The number of these extension
words, which follow the word containing the coprocessor condition selector field, is
determined by the coprocessor design.
MOTOROLA M68020 USER’S MANUAL 7-15
The final portion of the cpScc instruction format contains zero to five effective address
extension words. These words contain any additional information required to calculate the
effective address specified by bits 5–0 of the F-line operation word.
7.2.2.2.2 Protocol. Figure 7-8 shows the protocol for the cpScc instruction. The
MC68020/EC020 transfers the condition selector to the coprocessor by writing the word
following the F-line operation word to the condition CIR. The main processor then reads
the response CIR to determine its next action. The coprocessor can return a response
primitive to request services necessary to evaluate the condition. The operation of the
cpScc instruction depends on the condition evaluation indicator returned to the main
processor by the coprocessor. When the coprocessor returns the false condition indicator,
the main processor evaluates the effective address specified by bits 5–0 of the F-line
operation word and sets the byte at that effective address to FALSE (all bits cleared).
When the coprocessor returns the true condition indicator, the main processor sets the
byte at the effective address to TRUE (all bits set to one).
7.2.2.3 TEST COPROCESSOR CONDITION, DECREMENT, AND BRANCH
INSTRUCTION. The operation of the test coprocessor condition, decrement, and branch
instruction is similar to that of the DBcc instruction provided in the M68000 family
instruction set. This operation uses a coprocessor-evaluated condition and a loop counter
in the main processor. It is useful for implementing DO UNTIL constructs used in many
high-level languages.
7.2.2.3.1 Format. Figure 7-12 shows the format of the test coprocessor condition,
decrement, and branch instruction, denoted by the cpDBcc mnemonic.
1
15
1
14
1
13
1
12
11
CpID
9
0
8
0
7
1
6
5
R
EGISTE
R
0
O
PTIONAL COPROCESSOR-DEFINED EXTENSION WORD
S
D
ISPLACEMEN
T
C
ONDITION SELECTOR
0
5
0
4
1
3
2
(
RESERVED
)
Figure 7-12. Test Coprocessor Condition, Decrement, and Branch
Instruction Format (cpDBcc)
The first word of the cpDBcc instruction, F-line operation word, contains the CpID field in
bits 11–9 and 001001 in bits 8–3 to identify the cpDBcc instruction. Bits 2–0 of this
operation word specify the main processor data register used as the loop counter during
the execution of the instruction.
The second word of the cpDBcc instruction format contains the coprocessor condition
selector field in bits 5–0 and should contain zeros in bits 15–6 (reserved by Motorola) to
maintain compatibility with future M68000 products. This word is written to the condition
CIR to initiate execution of the cpDBcc instruction.
7-16 M68020 USER’S MANUAL MOTOROLA
If the coprocessor requires additional information to evaluate the condition, the cpDBcc
instruction can include this information in extension words. These extension words follow
the word containing the coprocessor condition selector field in the cpDBcc instruction
format.
The last word of the instruction contains the displacement for the cpDBcc instruction. This
displacement is a twos-complement 16-bit value that is sign-extended to long-word size
when it is used in a destination address calculation.
7.2.2.3.2 Protocol. Figure 7-8 shows the protocol for the cpDBcc instructions. The
MC68020/EC020 transfers the condition selector to the coprocessor by writing the word
following the operation word to the condition CIR. The main processor then reads the
response CIR to determine its next action. The coprocessor can use a response primitive
to request any services necessary to evaluate the condition. If the coprocessor returns the
true condition indicator, the main processor executes the next instruction in the instruction
stream. If the coprocessor returns the false condition indicator, the main processor
decrements the low-order word of the register specified by bits 2–0 of the F-line operation
word. If this register contains minus one (–1) after being decremented, the main processor
executes the next instruction in the instruction stream. If the register does not contain
minus one (–1) after being decremented, the main processor branches to the destination
address to continue instruction execution.
The MC68020/EC020 adds the displacement to the scanPC (refer to 7.4.1 ScanPC) to
determine the address of the next instruction. The scanPC must point to the 16-bit
displacement in the instruction stream when the destination address is calculated.
7.2.2.4 TRAP ON COPROCESSOR CONDITION INSTRUCTION. The trap on
coprocessor condition instruction allows the programmer to initiate exception processing
based on conditions related to the coprocessor operation.
7.2.2.4.1 Format. Figure 7-13 shows the format of the trap on coprocessor condition
instruction, denoted by the cpTRAPcc mnemonic.
1
15
1
14
1
13
1
12
11
CpID
9
0
8
0
7
1
6
5
O
PMOD
E
0
O
PTIONAL COPROCESSOR-DEFINED EXTENSION WORD
S
O
PTIONAL WOR
D
C
ONDITION SELECTOR
1
5
1
4
1
3
2
(
RESERVED
)
O
R LONG-WORD OPERAN
D
Figure 7-13. Trap on Coprocessor Condition
Instruction Format (cpTRAPcc)
The first word of the cpTRAPcc instruction, the F-line operation word contains the CpID
field in bits 11–9 and 001111 in bits 8–3 to identify the cpTRAPcc instruction. Bits 2–0 of
the cpTRAPcc F-line operation word specify the opmode, which selects the instruction
format. The instruction format can include zero, one, or two operand words.
MOTOROLA M68020 USER’S MANUAL 7-17
The second word of the cpTRAPcc instruction format contains the coprocessor condition
selector in bits 5–0 and should contain zeros in bits 15–6 (these bits are reserved by
Motorola) to maintain compatibility with future M68000 products. This word is written to the
condition CIR to initiate execution of the cpTRAPcc instruction.
If the coprocessor requires additional information to evaluate a condition, the instruction
can include this information in extension words. These extension words follow the word
containing the coprocessor condition selector field in the cpTRAPcc instruction format.
The operand words of the cpTRAPcc F-line operation word follow the coprocessor-defined
extension words. These operand words are not explicitly used by the MC68020/EC020,
but can be used to contain information referenced by the cpTRAPcc exception handling
routines. The valid encodings for bits 2–0 of the F-line operation word and the
corresponding numbers of operand words are listed in Table 7-1. Other encodings of
these bits are invalid for the cpTRAPcc instruction.
Table 7-1. cpTRAPcc Opmode Encodings
Opmode Operand Words in Instruction Format
010 One
011 Two
100 Zero
7.2.2.4.2 Protocol. Figure 7-8 shows the protocol for the cpTRAPcc instructions. The
MC68020/EC020 transfers the condition selector to the coprocessor by writing the word
following the operation word to the condition CIR. The main processor then reads the
response CIR to determine its next action. The coprocessor can return a response
primitive to request any services necessary to evaluate the condition. If the coprocessor
returns the true condition indicator, the main processor initiates exception processing for
the cpTRAPcc exception (refer to 7.5.2.4 cpTRAPcc Instruction Traps). If the
coprocessor returns the false condition indicator, the main processor executes the next
instruction in the instruction stream.
7.2.3 Coprocessor Context Save and Restore Instructions
The coprocessor context save and context restore instruction categories in the M68000
coprocessor interface support multitasking programming environments. In a multitasking
environment, the context of a coprocessor may need to be changed asynchronously with
respect to the operation of that coprocessor. That is, the coprocessor may be interrupted
at any point in the execution of an instruction in the general or conditional category to
begin context change operations.
In contrast to the general and conditional instruction categories, the context save and
context restore instruction categories do not use the coprocessor response primitives. A
set of format codes defined by the M68000 coprocessor interface communicates status
7-18 M68020 USER’S MANUAL MOTOROLA
information to the main processor during the execution of these instructions. These
coprocessor format codes are discussed in detail in 7.2.3.2 Coprocessor Format Words.
7.2.3.1 COPROCESSOR INTERNAL STATE FRAMES. The context save (cpSAVE) and
context restore (cpRESTORE) instructions transfer an internal coprocessor state frame
between memory and a coprocessor. This internal coprocessor state frame represents the
state of coprocessor operations. Using the cpSAVE and cpRESTORE instructions, it is
possible to interrupt coprocessor operation, save the context associated with the current
operation, and initiate coprocessor operations with a new context.
A cpSAVE instruction stores a coprocessor internal state frame as a sequence of long -
word entries in memory. Figure 7-14 shows the format of a coprocessor state frame. The
format and length fields of the coprocessor state frame format comprise the format word.
During execution of the cpSAVE instruction, the MC68020/EC020 calculates the state
frame effective address from information in the operation word of the instruction and
stores a format word at this effective address. The processor writes the long words that
form the coprocessor state frame to descending memory addresses, beginning with the
address specified by the sum of the effective address and the length field multiplied by
four. During execution of the cpRESTORE instruction, the MC68020/EC020 reads the
state frame from ascending addresses beginning with the effective address specified in
the instruction operation word.
31
FORMAT
24
23
LENGTH
16
15
(UNUSED, RESERVED)
0
COPROCESSOR-DEPENDENT INFORMATION
0
SAVE
O
RDE
R
0
R
ESTOR
E
ORDER
n
1
n
–
1
2
n
–
2
3
1
n
Figure 7-14. Coprocessor State Frame Format in Memory
The processor stores the coprocessor format word at the lowest address of the state
frame in memory, and this word is the first word transferred for both the cpSAVE and
cpRESTORE instructions. The word following the format word does not contain
information relevant to the coprocessor state frame, but serves to keep the information in
the state frame a multiple of four bytes in size. The number of entries following the format
word (at higher addresses) is determined by the format word length for a given
coprocessor state.
MOTOROLA M68020 USER’S MANUAL 7-19
The information in a coprocessor state frame describes a context of operation for that
coprocessor. This description of a coprocessor context includes the program invisible
state information and, optionally, the program visible state information. The program
invisible state information consists of any internal registers or status information that
cannot be accessed by the program but is necessary for the coprocessor to continue its
operation at the point of suspension. Program visible state information includes the
contents of all registers that appear in the coprocessor programming model and that can
be directly accessed using the coprocessor instruction set. The information saved by the
cpSAVE instruction must include the program invisible state information. If cpGEN
instructions are provided to save the program visible state of the coprocessor, the
cpSAVE and cpRESTORE instructions should only transfer the program invisible state
information to minimize interrupt latency during a save or restore operation.
7.2.3.2 COPROCESSOR FORMAT WORDS. The coprocessor communicates status
information to the main processor during the execution of cpSAVE and cpRESTORE
instructions using coprocessor format words. The format words defined for the M68000
coprocessor interface are listed in Table 7-2.
Table 7-2. Coprocessor Format Word Encodings
Format Code Length Meaning
$00 $xx Empty/Reset
$01 $xx Not Ready, Come Again
$02 $xx Invalid Format
$03–$OF $xx Undefined, Reserved
$10–$FF Length Valid Format, Coprocessor Defined
xx—Don’t care
The upper byte of the coprocessor format word contains the code used to communicate
coprocessor status information to the main processor. The MC68020/EC020 recognizes
four types of format words: empty/reset, not ready, invalid format, and valid format. The
MC68020/EC020 interprets the reserved format codes ($03–$0F) as invalid format words.
The lower byte of the coprocessor format word specifies the size in bytes (which must be
a multiple of four) of the coprocessor state frame. This value is only relevant when the
code byte contains the valid format code (refer to 7.2.3.2.4 Valid Format Word).
7.2.3.2.1 Empty/Reset Format Word . The coprocessor returns the empty/reset format
code during a cpSAVE instruction to indicate that the coprocessor contains no user-
specific information. That is, no coprocessor instructions have been executed since either
a previous cpRESTORE of an empty/reset format code or the previous hardware reset. If
the main processor reads the empty/reset format word from the save CIR during the
initiation of a cpSAVE instruction, it stores the format word at the effective address
specified in the cpSAVE instruction and executes the next instruction.
7-20 M68020 USER’S MANUAL MOTOROLA
When the main processor reads the empty/reset format word from memory during the
execution of the cpRESTORE instruction, it writes the format word to the restore CIR. The
main processor then reads the restore CIR and, if the coprocessor returns the empty/reset
format word, executes the next instruction. The main processor can then initialize the
coprocessor by writing the empty/reset format code to restore the CIR. When the
coprocessor receives the empty/reset format code, it terminates any current operations
and waits for the main processor to initiate the next coprocessor instruction. In particular,
after the cpRESTORE of the empty/reset format word, the execution of a cpSAVE should
cause the empty/reset format word to be returned when a cpSAVE instruction is executed
before any other coprocessor instructions. Thus, an empty/reset state frame consists only
of the format word and the following reserved word in memory (refer to Figure 7-14).
7.2.3.2.2 Not-Ready Format Word. When the main processor initiates a cpSAVE
instruction by reading the save CIR, the coprocessor can delay the save operation by
returning a not-ready format word. The main processor then services any pending
interrupts and reads the save CIR again. The not-ready format word delays the save
operation until the coprocessor is ready to save its internal state. The cpSAVE instruction
can suspend execution of a general or conditional coprocessor instruction; the
coprocessor can resume execution of the suspended instruction when the appropriate
state is restored with a cpRESTORE. If no further main processor services are required to
complete coprocessor instruction execution, it may be more efficient to complete the
instruction and thus reduce the size of the saved state. The coprocessor designer should
consider the efficiency of completing the instruction or of suspending and later resuming
the instruction when the main processor executes a cpSAVE instruction.
When the main processor initiates a cpRESTORE instruction by writing a format word to
the restore CIR, the coprocessor should usually terminate any current operations and
restore the state frame supplied by the main processor. Thus, the not-ready format word
should usually not be returned by the coprocessor during the execution of a cpRESTORE
instruction. If the coprocessor must delay the cpRESTORE operation for any reason, it
can return the not-ready format word when the main processor reads the restore CIR. If
the main processor reads the not-ready format word from the restore CIR during the
cpRESTORE instruction, it reads the restore CIR again without servicing any pending
interrupts.
7.2.3.2.3 Invalid Format Word. When the format word placed in the restore CIR to initiate
a cpRESTORE instruction does not describe a valid coprocessor state frame, the
coprocessor returns the invalid format word in the restore CIR. When the main processor
reads this format word during the cpRESTORE instruction, it sets the abort bit in the
control CIR and initiates format error exception processing.
A coprocessor usually should not place an invalid format word in the save CIR when the
main processor initiates a cpSAVE instruction. A coprocessor, however, may not be able
to support the initiation of a cpSAVE instruction while it is executing a previously initiated
cpSAVE or cpRESTORE instruction. In this situation, the coprocessor can return the
invalid format word when the main processor reads the save CIR to initiate the cpSAVE
instruction while either another cpSAVE or cpRESTORE instruction is executing. If the
MOTOROLA M68020 USER’S MANUAL 7-21
main processor reads an invalid format word from the save CIR, it writes the abort mask to
the control CIR and initiates format error exception processing (refer to 7.5.1.5 Format
Errors).
7.2.3.2.4 Valid Format Word. When the main processor reads a valid format word from
the save CIR during the cpSAVE instruction, it uses the length field to determine the size
of the coprocessor state frame to save. The length field in the lower eight bits of a format
word is relevant only in a valid format word. During the cpRESTORE instruction, the main
processor uses the length field in the format word read from the effective address in the
instruction to determine the size of the coprocessor state frame to restore.
The length field of a valid format word, representing the size of the coprocessor state
frame, must contain a multiple of four. If the main processor detects a value that is not a
multiple of four in a length field during the execution of a cpSAVE or cpRESTORE
instruction, the main processor writes the abort mask (refer to 7.2.3.2.3 Invalid Format
Word) to the control CIR and initiates format error exception processing.
7.2.3.3 COPROCESSOR CONTEXT SAVE INSTRUCTION. The M68000 coprocessor
context save instruction category consists of one instruction. The coprocessor context
save instruction, denoted by the cpSAVE mnemonic, saves the context of a coprocessor
dynamically without relation to the execution of coprocessor instructions in the general or
conditional instruction categories. During the execution of a cpSAVE instruction, the
coprocessor communicates status information to the main processor by using the
coprocessor format codes.
7.2.3.3.1 Format. Figure 7-15 shows the format of the cpSAVE instruction. The first word
of the instruction, the F-line operation word, contains the CpID code in bits 11–9 and an
M68000 effective address code in bits 5–0. The effective address encoded in the cpSAVE
instruction is the address at which the state frame associated with the current context of
the coprocessor is saved in memory.
1
15
1
14
1
13
1
12
11
CpID
9
1
8
0
7
0
6
5
E
FFECTIVE ADDRES
S
0
E
FFECTIVE ADDRESS EXTENSION WORDS (0–5 WORDS
)
Figure 7-15. Coprocessor Context Save Instruction Format (cpSAVE)
The control alterable and predecrement addressing modes are valid for the cpSAVE
instruction. Other addressing modes cause the MC68020/EC020 to initiate F-line emulator
exception processing as described in 7.5.2.2 F-Line Emulator Exceptions.
The instruction can include as many as five effective address extension words following
the F-line operation word. These words contain any additional information required to
calculate the effective address specified by bits 5–0 of the F-line operation word.
7-22 M68020 USER’S MANUAL MOTOROLA
7.2.3.3.2 Protocol . Figure 7-16 shows the protocol for the coprocessor context save
instruction. The main processor initiates execution of the cpSAVE instruction by reading
the save CIR. Thus, the cpSAVE instruction is the only coprocessor instruction that begins
by reading from a CIR. All other coprocessor instructions write to a CIR to initiate
execution of the instruction by the coprocessor. The coprocessor communicates status
information associated with the context save operation to the main processor by placing
coprocessor format codes in the save CIR.
M
1 RECOGNIZE COPROCESSOR INSTRUCTION F-LINE
O
PERATION WORD
M
2 READ SAVE CIR TO INITIATE THE cpSAVE INSTRUCTI
ON
M
3 IF (FORMAT = NOT READY) DO STEPS 1) AND 2) BELO
W
1
) SERVICE PENDING INTERRUPTS
2
) GO TO M2
M
4 EVALUATE EFFECTIVE ADDRESS SPECIFIED IN
F
-LINE OPWORD AND STORE FORMAT WORD AT
E
FFECTIVE ADDRESS
M
5 IF (FORMAT = EMPTY) GO TO M6 ELSE, TRANSFER
N
UMBER OF BYTES INDICATED IN FORMAT WORD
F
ROM OPERAND CIR TO EFFECTIVE ADDRESS
M
6 PROCEED WITH EXECUTION OF NEXT INSTRUCTION
MAIN PROCESSOR
C
OPROCESSO
R
Figure 7-16. Coprocessor Context Save Instruction Protocol
If the coprocessor is not ready to suspend its current operation when the main processor
reads the save CIR, it returns a not-ready format code. The main processor services any
pending interrupts and then reads the save CIR again. After placing the not-ready format
code in the save CIR, the coprocessor should either suspend or complete the instruction it
is currently executing.
Once the coprocessor has suspended or completed the instruction it is executing, it places
a format code representing the internal coprocessor state in the save CIR. When the main
processor reads the save CIR, it transfers the format word to the effective address
specified in the cpSAVE instruction. The lower byte of the coprocessor format word
specifies the number of bytes of state information, not including the format word and
associated null word, to be transferred from the coprocessor to the effective address
specified. If the state information is not a multiple of four bytes in size, the
MC68020/EC020 initiates format error exception processing (refer to 7.5.1.5 Format
Errors). The coprocessor and main processor coordinate the transfer of the internal state
of the coprocessor using the operand CIR. The MC68020/EC020 completes the
coprocessor context save by repeatedly reading the operand CIR and writing the
MOTOROLA M68020 USER’S MANUAL 7-23
information obtained into memory until all the bytes specified in the coprocessor format
word have been transferred. Following a cpSAVE instruction, the coprocessor should be
in an idle state—that is, not executing any coprocessor instructions.
The cpSAVE instruction is a privileged instruction. When the MC68020/EC020 identifies a
cpSAVE instruction, it checks the S-bit in the SR to determine whether it is operating at
the supervisor privilege level. If the MC68020/EC020 attempts to execute a cpSAVE
instruction while at the user privilege level (S-bit in the SR is clear), it initiates privilege
violation exception processing without accessing any of the CIRs (refer to 7.5.2.3
Privilege Violations).
The MC68020/EC020 initiates format error exception processing if it reads an invalid
format word (or a valid format word whose length field is not a multiple of four bytes) from
the save CIR during the execution of a cpSAVE instruction (refer to 7.2.3.2.3 Invalid
Format Word). The MC68020/EC020 writes an abort mask (refer to 7.2.3.2.3 Invalid
Format Word) to the control CIR to abort the coprocessor instruction prior to beginning
exception processing. Figure 7-16 does not include this case since a coprocessor usually
returns either a not-ready or a valid format code in the context of the cpSAVE instruction.
The coprocessor can return the invalid format word, however, if a cpSAVE is initiated
while the coprocessor is executing a cpSAVE or cpRESTORE instruction and the
coprocessor is unable to support the suspension of these two instructions.
7.2.3.4 COPROCESSOR CONTEXT RESTORE INSTRUCTION. The M68000
coprocessor context restore instruction category includes one instruction. The
coprocessor context restore instruction, denoted by the cpRESTORE mnemonic, forces a
coprocessor to terminate any current operations and to restore a former state. During
execution of a cpRESTORE instruction, the coprocessor can communicate status
information to the main processor by placing format codes in the restore CIR.
7.2.3.4.1 Format. Figure 7-17 shows the format of the cpRESTORE instruction.
1
15
1
14
1
13
1
12
11
CpID
9
1
8
0
7
1
6
5
E
FFECTIVE ADDRES
S
0
E
FFECTIVE ADDRESS EXTENSION WORDS (0–5 WORDS
)
Figure 7-17. Coprocessor Context Restore
Instruction Format (cpRESTORE)
The first word of the instruction, the F-line operation word, contains the CpID code in bits
11–9 and an M68000 effective addressing code in bits 5–0. The effective address
encoded in the cpRESTORE instruction is the starting address in memory where the
coprocessor context is stored. The effective address is that of the coprocessor format
word that applies to the context to be restored to the coprocessor.
7-24 M68020 USER’S MANUAL MOTOROLA
The instruction can include as many as five effective address extension words following
the F-line operation word in the cpRESTORE instruction format. These words contain any
additional information required to calculate the effective address specified by bits 5–0 of
the F-line operation word.
All memory addressing modes except the predecrement addressing mode are valid.
Invalid effective address encodings cause the MC68020/EC020 to initiate F-line emulator
exception processing (refer to 7.5.2.2 F-Line Emulator Exceptions).
7.2.3.4.2 Protocol. Figure 7-18 shows the protocol for the coprocessor context restore
instruction. When the main processor executes a cpRESTORE instruction, it first reads
the coprocessor format word from the effective address in the instruction. This format
word contains a format code and a length field. During cpRESTORE operation, the main
processor retains a copy of the length field to determine the number of bytes to be
transferred to the coprocessor during the cpRESTORE operation and writes the format
word to the restore CIR to initiate the coprocessor context restore.
M
1 RECOGNIZE COPROCESSOR INSTRUCTION F-LINE
O
PERATION WORD
M
2 READ COPROCESSOR FORMAT CODE FROM
E
FFECTIVE ADDRESS SPECIFIED IN OPERATION WORD
M
3 WRITE COPROCESSOR FORMAT WORD TO
R
ESTORE CIR
M
4 READ RESTORE CIR
M
5 IF (FORMAT = INVALID FORMAT) WRITE $0001 ABORT
C
ODE TO CONTROL CIR AND INITIATE FORMAT ERROR
E
XCEPTION PROCESSING (SEE NOTE 1)
M
6 IF (FORMAT = EMPTY/RESET) GO TO M7; ELSE, TRANSFER
N
UMBER OF BYTES SPECIFIED BY FORMAT WORD TO
O
PERAND CIR (SEE NOTE 2)
M
7 PROCEED WITH EXECUTION OF NEXT INSTRUCTION
C
1 TERMINATE CURRENT OPERATIONS AND EVALUATE
F
ORMAT WORD
C
2 IF (INVALID FORMAT) PLACE INVALID FORMAT CODE
I
N THE RESTORE CIR
C
3 IF (VALID FORMAT) RECEIVE NUMBER OF BYTES
I
NDICATED IN FORMAT WORD THROUGH OPERAND CIR
MAIN PROCESSOR
C
OPROCESSO
R
NOTES: 1. See 7.6.1.5 Format Error.
2. The MC68020/EC020 uses the length field in the format word read during M2 to determine the number of
bytes to read from memory and write to the operand CIR.
Figure 7-18. Coprocessor Context Restore Instruction Protocol
When the coprocessor receives the format word in the restore CIR, it must terminate any
current operations and evaluate the format word. If the format word represents a valid
coprocessor context as determined by the coprocessor design, the coprocessor returns
the format word to the main processor through the restore CIR and prepares to receive
the number of bytes specified in the format word through its operand CIR.
MOTOROLA M68020 USER’S MANUAL 7-25
After writing the format word to the restore CIR, the main processor continues
cpRESTORE dialog by reading that same register. If the coprocessor returns a valid
format word, the main processor transfers the number of bytes specified by the format
word at the effective address to the operand CIR.
If the format word written to the restore CIR does not represent a valid coprocessor state
frame, the coprocessor places an invalid format word in the restore CIR and terminates
any current operations. The main processor receives the invalid format code, writes an
abort mask (refer to 7.2.3.2.3 Invalid Format Word) to the control CIR, and initiates
format error exception processing (refer to 7.5.1.5 Format Errors).
The cpRESTORE instruction is a privileged instruction. When the MC68020/EC020
accesses a cpRESTORE instruction, it checks the S-bit in the SR. If the MC68020/EC020
attempts to execute a cpRESTORE instruction while at the user privilege level (S-bit in the
SR is clear), it initiates privilege violation exception processing without accessing any of
the CIRs (refer to 7.5.2.3 Privilege Violations).
7.3 COPROCESSOR INTERFACE REGISTER SET
The instructions of the M68000 coprocessor interface use registers of the CIR set to
communicate with the coprocessor. These CIRs are not directly related to the coprocessor
programming model.
Figure 7-4 is a memory map of the CIR set. The response, control, save, restore,
command, condition, and operand registers must be included in a coprocessor interface
that implements all four coprocessor instruction categories. The complete register model
must be implemented if the system uses all coprocessor response primitives defined for
the M68000 coprocessor interface.
The following paragraphs contain detailed descriptions of the registers.
7.3.1 Response CIR
The coprocessor uses the 16-bit response CIR to communicate all service requests
(coprocessor response primitives) to the main processor. The main processor reads the
response CIR to receive the coprocessor response primitives during the execution of
instructions in the general and conditional instruction categories. The offset from the base
address of the CIR set for the response CIR is $00. Refer to 7.4 Coprocessor Response
Primitives for additional information.
7.3.2 Control CIR
The main processor writes to the 2-bit control CIR to acknowledge coprocessor-requested
exception processing or to abort the execution of a coprocessor instruction. The offset
from the base address of the CIR set for the control CIR is $02. The control CIR occupies
the two least significant bits of the word at that offset. The 14 most significant bits of the
word are undefined and reserved by Motorola. Figure 7-19 shows the format of this
register.
7-26 M68020 USER’S MANUAL MOTOROLA
15
(UNDEFINED, RESERVED)
2
XA
1
AB
0
Figure 7-19. Control CIR Format
When the MC68020/EC020 receives one of the three take exception coprocessor
response primitives, it acknowledges the primitive by setting the exception acknowledge
bit (XA) in the control CIR. The MC68020/EC020 sets the abort bit (AB) in the control CIR
to abort any coprocessor instruction in progress. (The 14 most significant bits of both
masks are undefined.) The MC68020/EC020 aborts a coprocessor instruction when it
detects one of the following exception conditions:
• An F-line emulator exception condition after reading a response primitive
• A privilege violation exception as it performs a supervisor check in response to a
supervisor check primitive
• A format error exception when it receives an invalid format word or a valid format
word that contains an invalid length
7.3.3 Save CIR
The coprocessor uses the 16-bit save CIR to communicate status and state frame format
information to the main processor while executing a cpSAVE instruction. The main
processor reads the save CIR to initiate execution of the cpSAVE instruction by the
coprocessor. The offset from the base address of the CIR set for the save CIR is $04.
Refer to 7.2.3.2 Coprocessor Format Words for more information on the save CIR.
7.3.4 Restore CIR
The main processor initiates the cpRESTORE instruction by writing a coprocessor format
word to the 16-bit restore register. During the execution of the cpRESTORE instruction,
the coprocessor communicates status and state frame format information to the main
processor through the restore CIR. The offset from the base address of the CIR set for the
restore CIR is $06. Refer to 7.2.3.2 Coprocessor Format Words for more information on
the restore CIR.
7.3.5 Operation Word CIR
The main processor writes the F-line operation word of the instruction in progress to the
16-bit operation word CIR in response to a transfer operation word coprocessor response
primitive (refer to 7.4.6 Transfer Operation Word Primitive). The offset from the base
address of the CIR set for the operation word CIR is $08.
7.3.6 Command CIR
The main processor initiates a coprocessor general category instruction by writing the
instruction command word, which follows the instruction F-line operation word in the
instruction stream, to the 16-bit command CIR . The offset from the base address of the
CIR set for the command CIR is $0A.
MOTOROLA M68020 USER’S MANUAL 7-27
7.3.7 Condition CIR
The main processor initiates a conditional category instruction by writing the condition
selector to bits 5–0 of the 16-bit condition CIR. Bits 15–6 are undefined and reserved by
Motorola. The offset from the base address of the CIR set for the condition CIR is $0E.
Figure 7-20 shows the format of the condition CIR.
15
(UNDEFINED, RESERVED)
0
CONDITION SELECTOR
5
6
Figure 7-20. Condition CIR Format
7.3.8 Operand CIR
When the coprocessor requests the transfer of an operand, the main processor performs
the transfer by reading from or writing to the 32-bit operand CIR. The offset from the base
address of the CIR set for the operand CIR is $10.
The MC68020/EC020 aligns all operands transferred to and from the operand CIR to the
most significant byte of this CIR. The processor performs a sequence of long-word
transfers to read or write any operand larger than four bytes. If the operand size is not a
multiple of four bytes, the portion remaining after the initial long-word transfer is aligned to
the most significant byte of the operand CIR. Figure 7-21 shows the operand alignment
used by the MC68020/EC020 when accessing the operand CIR.
0
31
7
N
O TRANSFE
R
W
ORD OPERAN
D
T
HREE-BYTE OPERAN
D
L
ONG-WORD OPERAN
D
23
15
NO TRANSFER
NO TRANSFER
NO TRANSFER
O
PERAN
D
B
YTE
-
T
EN
-
B
YTE OPERAN
D
24
16
8
Figure 7-21. Operand Alignment for Operand CIR Accesses
7-28 M68020 USER’S MANUAL MOTOROLA
7.3.9 Register Select CIR
When the coprocessor requests the transfer of one or more main processor registers or a
group of coprocessor registers, the main processor reads the 16-bit register select CIR to
identify the number or type of registers to be transferred. The offset from the base address
of the CIR set for the register select CIR is $14. The format of this register depends on the
primitive that is currently using it (refer to 7.4 Coprocessor Response Primitives).
7.3.10 Instruction Address CIR
When the coprocessor requests the address of the instruction it is currently executing, the
main processor transfers this address to the 32-bit instruction address CIR. Any transfer of
the scanPC is also performed through the instruction address CIR (refer to 7.4.17
Transfer Status Register and ScanPC Primitive). The offset from the base address of
the CIR set for the instruction address CIR is $18.
7.3.11 Operand Address CIR
When a coprocessor requests an operand address transfer between the main processor
and the coprocessor, the address is transferred through the 32-bit operand address CIR.
The offset from the base address of the CIR set for the operand address CIR is $1C.
7.4 COPROCESSOR RESPONSE PRIMITIVES
The response primitives are primitive instructions that the coprocessor issues to the main
processor during the execution of a coprocessor instruction. The coprocessor uses
response primitives to communicate status information and service requests to the main
processor. In response to an instruction command word written to the command CIR or a
condition selector in the condition CIR, the coprocessor returns a response primitive in the
response CIR. Within the general and conditional instruction categories, individual
instructions are distinguished by the operation of the coprocessor hardware and by
services specified by coprocessor response primitives and provided by the main
processor.
Subsequent paragraphs, beginning with 7.4.2 Coprocessor Response Primitive
General Format, consist of detailed descriptions of the M68000 coprocessor response
primitives supported by the MC68020/EC020. Any response primitive that the
MC68020/EC020 does not recognize causes it to initiate protocol violation exception
processing (refer to 7.5.2.1 Protocol Violations). This processing of undefined primitives
supports emulation of extensions to the M68000 coprocessor response primitive set by
the protocol violation exception handler. Exception processing related to the coprocessor
interface is discussed in 7.5 Exceptions.
MOTOROLA M68020 USER’S MANUAL 7-29
7.4.1 ScanPC
Several of the response primitives involve the scanPC, and many of them require the main
processor to use it while performing services requested. These paragraphs describe the
scanPC and its operation.
During the execution of a coprocessor instruction, the PC in the MC68020/EC020 contains
the address of the F-line operation word of that instruction. A second register, called the
scanPC, sequentially addresses the remaining words of the instruction.
If the main processor requires extension words to calculate an effective address or
destination address of a branch operation, it uses the scanPC to address these extension
words in the instruction stream. Also, if a coprocessor requests the transfer of extension
words, the scanPC addresses the extension words during the transfer. As the processor
references each word, it increments the scanPC to point to the next word in the instruction
stream. When an instruction has completed, the processor transfers the value in the
scanPC to the PC to address the operation word of the next instruction.
The value in the scanPC when the main processor reads the first response primitive after
beginning to execute an instruction depends on the instruction being executed. For a
cpGEN instruction, the scanPC points to the word following the coprocessor command
word. For the cpBcc instructions, the scanPC points to the word following the instruction
F-line operation word. For the cpScc, cpTRAPcc, and cpDBcc instructions, the scanPC
points to the word following the coprocessor condition specifier word.
If a coprocessor implementation uses optional instruction extension words with a general
or conditional instruction, the coprocessor must use these words consistently so that the
scanPC is updated accordingly during the instruction execution. Specifically, during the
execution of general category instructions, when the coprocessor terminates the
instruction protocol, the MC68020/EC020 assumes that the scanPC is pointing to the
operation word of the next instruction to be executed. During the execution of conditional
category instructions, when the coprocessor terminates the instruction protocol, the
MC68020/EC020 assumes that the scanPC is pointing to the word following the last of
any coprocessor-defined extension words in the instruction format.
7.4.2 Coprocessor Response Primitive General Format
The M68000 coprocessor response primitives are encoded in a 16-bit word that is
transferred to the main processor through the response CIR. Figure 7-22 shows the
format of the coprocessor response primitives.
15
FUNCTION
0
PARAMETERCA
PC
DR
14
13
12
8
7
Figure 7-22. Coprocessor Response Primitive Format
7-30 M68020 USER’S MANUAL MOTOROLA
The encoding of bits 12–0 of a coprocessor response primitive depends on the individual
primitive. Bits 15–13, however, specify optional additional operations that apply to most of
the primitives defined for the M68000 coprocessor interface.
The CA bit specifies the come-again operation of the main processor. When the main
processor reads a response primitive from the response CIR with the CA bit set, it
performs the service indicated by the primitive and then reads the response CIR again.
Using the CA bit, a coprocessor can transfer several response primitives to the main
processor during the execution of a single coprocessor instruction.
The PC bit specifies the pass program counter operation. When the main processor reads
a primitive with the PC bit set from the response CIR, the main processor immediately
passes the current value in its program counter to the instruction address CIR as the first
operation in servicing the primitive request. The value in the program counter is the
address of the F-line operation word of the coprocessor instruction currently executing.
The PC bit is implemented in all coprocessor response primitives currently defined for the
M68000 coprocessor interface.
When an undefined primitive or a primitive that requests an illegal operation is passed to
the main processor, the main processor initiates exception processing for either an F-line
emulator or a protocol violation exception (refer to 7.5.2 Main-Processor-Detected
Exceptions). If the PC bit is set in one of these response primitives, however, the main
processor passes the program counter to the instruction address CIR before it initiates
exception processing.
When the main processor initiates a cpGEN instruction that can be executed concurrently
with main processor instructions, the PC bit is usually set in the first primitive returned by
the coprocessor. Since the main processor proceeds with instruction stream execution
once the coprocessor releases it, the coprocessor must record the instruction address to
support any possible exception processing related to the instruction. Exception processing
related to concurrent coprocessor instruction execution is discussed in 7.5.1
Coprocessor-Detected Exceptions.
The DR bit is the direction bit. It applies to operand transfers between the main processor
and the coprocessor. If the DR bit is clear, the direction of transfer is from the main
processor to the coprocessor (main processor write). If the DR bit is set, the direction of
transfer is from the coprocessor to the main processor (main processor read). If the
operation indicated by a given response primitive does not involve an explicit operand
transfer, the value of this bit depends on the particular primitive encoding.
MOTOROLA M68020 USER’S MANUAL 7-31
7.4.3 Busy Primitive
The busy response primitive causes the main processor to reinitiate a coprocessor
instruction. This primitive applies to instructions in the general and conditional categories.
Figure 7-23 shows the format of the busy primitive.
15
0
1
PC
1
14
13
12
0
0
11
1
10
0
9
0
8
0
7
0
6
0
5
0
4
0
3
0
2
0
1
0
Figure 7-23. Busy Primitive Format
The busy primitive uses the PC bit as described in 7.4.2 Coprocessor Response
Primitive General Format.
Coprocessors that can operate concurrently with the main processor but cannot buffer
write operations to their command or condition CIR use the busy primitive. A coprocessor
may execute a cpGEN instruction concurrently with an instruction in the main processor. If
the main processor attempts to initiate an instruction in the general or conditional
instruction category while the coprocessor is executing a cpGEN instruction, the
coprocessor can place the busy primitive in the response CIR. When the main processor
reads this primitive, it services pending interrupts using a preinstruction exception stack
frame (refer to Figure 7-41). The processor then restarts the general or conditional
coprocessor instruction that it had attempted to initiate earlier.
The busy primitive should only be used in response to a write to the command or condition
CIR. It should be the first primitive returned after the main processor attempts to initiate a
general or conditional category instruction. In particular, the busy primitive should not be
issued after program-visible resources have been altered by the instruction. (Program-
visible resources include coprocessor and main processor program-visible registers and
operands in memory, but not the scanPC.) The restart of an instruction after it has altered
program-visible resources causes those resources to have inconsistent values when the
processor reinitiates the instruction.
The MC68020/EC020 responds to the busy primitive differently in a special case that can
occur during a breakpoint operation (refer to Section 6 Exception Processing). This
special case occurs when a breakpoint acknowledge cycle initiates a coprocessor F-line
instruction, the coprocessor returns the busy primitive in response to the instruction
initiation, and an interrupt is pending. When these three conditions are met, the processor
reexecutes the breakpoint acknowledge cycle after completion of interrupt exception
processing. A design that uses a breakpoint to monitor the number of passes through a
loop by incrementing or decrementing a counter may not work correctly under these
conditions. This special case may cause several breakpoint acknowledge cycles to be
executed during a single pass through a loop.
7-32 M68020 USER’S MANUAL MOTOROLA
7.4.4 Null Primitive
The null coprocessor response primitive communicates coprocessor status information to
the main processor. This primitive applies to instructions in the general and conditional
categories. Figure 7-24 shows the format of the null primitive.
15
0
CA
PC
0
14
13
12
0
1
11
0
10
0
9
IA
8
0
7
0
6
0
5
0
4
0
3
0
2
PF
1
TF
Figure 7-24. Null Primitive Format
The null primitive uses the CA and PC bits as described in 7.4.2 Coprocessor Response
Primitive General Format.
The IA bit specifies the interrupts allowed optional operation. This bit determines whether
the MC68020/EC020 services pending interrupts prior to rereading the response CIR after
receiving a null primitive. Interrupts are allowed when the IA bit is set.
The PF bit shows the processing-finished status of the coprocessor. That is, PF = 1
indicates that the coprocessor has completed all processing associated with an
instruction.
The TF bit indicates the true/false condition during execution of a conditional category
instruction. TF = 1 is the true condition specifier; TF = 0 is the false condition specifier.
The TF bit is only relevant for null primitives with CA = 0 that are used by the coprocessor
during the execution of a conditional instruction.
The MC68020/EC020 processes a null primitive with CA = 1 in the same manner whether
executing a general or conditional category coprocessor instruction. If the coprocessor
sets CA and IA in the null primitive, the main processor services pending interrupts using
a midinstruction stack frame (refer to Figure 7-43) and reads the response CIR again. If
the coprocessor sets CA and clears IA in the null primitive, the main processor reads the
response CIR again without servicing any pending interrupts.
A null primitive with CA = 0 provides a condition evaluation indicator to the main processor
during the execution of a conditional instruction and ends the dialogue between the main
processor and coprocessor for that instruction. The main processor completes the
execution of a conditional category coprocessor instruction when it receives the primitive.
The PF bit is not relevant during conditional instruction execution since the primitive itself
implies completion of processing.
Usually, when the main processor reads any primitive that does not have CA = 1 while
executing a general category instruction, it terminates the dialogue between the main
processor and coprocessor. If a trace exception is pending, however, the main processor
does not terminate the instruction dialogue until it reads a null primitive with CA = 0 and
PF = 1 from the response CIR (refer to 7.5.2.5 Trace Exceptions). Thus, the main
processor continues to read the response CIR until it receives a null primitive with CA = 0
MOTOROLA M68020 USER’S MANUAL 7-33
and PF = 1, and then performs trace exception processing. When IA = 1, the main
processor services pending interrupts before reading the response CIR again.
A coprocessor can be designed to execute a cpGEN instruction concurrently with the
execution of main processor instructions and, also, buffer one write operation to either its
command or condition CIR. This type of coprocessor issues a null primitive with CA = 1
when it is concurrently executing a cpGEN instruction, and the main processor initiates
another general or conditional coprocessor instruction. This primitive indicates that the
coprocessor is busy and the main processor should read the response CIR again without
reinitiating the instruction. The IA bit of this null primitive usually should be set to minimize
interrupt latency while the main processor is waiting for the coprocessor to complete the
general category instruction.
Table 7-3 summarizes the encodings of the null primitive.
Table 7-3. Null Coprocessor Response Primitive Encodings
CA PC IA P F TF General Instructions Conditional Instructions
x1xxxPass Program Counter to Instruction
Address CIR, Clear PC Bit, and Proceed
with Operation Specified by CA, IA, PF,
and TF Bits
Same as General Category
1 0 0 x x Reread Response CIR, Do Not Service
Pending Interrupts Same as General Category
101xxService Pending Interrupts and Reread the
Response CIR Same as General Category
0000cIf (Trace Pending) Reread Response CIR;
Else, Execute Next Instruction Main Processor Completes Instruction
Execution Based on TF = c
0010cIf (Trace Pending) Service Pending
Interrupts and Reread Response CIR;
Else, Execute Next Instruction
Main Processor Completes Instruction
Execution Based on TF = c
00x1cCoprocessor Instruction Completed;
Service Pending Exceptions or Execute
Next Instruction
Main Processor Completes Instruction
Execution Based on TF = c.
x = Don't Care
c = 1 or 0 Depending on Coprocessor Condition Evaluation
7-34 M68020 USER’S MANUAL MOTOROLA
7.4.5 Supervisor Check Primitive
The supervisor check primitive verifies that the main processor is operating in the
supervisor privilege level while executing a coprocessor instruction. This primitive applies
to instructions in the general and conditional coprocessor instruction categories. Figure
7-25 shows the format of the supervisor check primitive.
15
0
1
PC
0
14
13
12
0
0
11
1
10
0
9
0
8
0
7
0
6
0
5
0
4
0
3
0
2
0
1
0
Figure 7-25. Supervisor Check Primitive Format
The supervisor check primitive uses the PC bit as described in 7.4.2 Coprocessor
Response Primitive General Format. Bit 15 is shown as one, but during execution of a
general category instruction, this primitive performs the same operations, regardless of the
value of bit 15. However, if this primitive is issued with bit 15 = 0 during a conditional
category instruction, the main processor initiates protocol violation exception processing.
When the MC68020/EC020 reads the supervisor check primitive from the response CIR, it
checks the value of the S-bit in the SR. If S = 0 (main processor operating at user privilege
level), the main processor aborts the coprocessor instruction by writing an abort mask to
the control CIR (refer to 7.3.2 Control CIR). The main processor then initiates privilege
violation exception processing (refer to 7.5.2.3 Privilege Violations). If the main
processor is at the supervisor privilege level when it receives this primitive, it reads the
response CIR again.
The supervisor check primitive allows privileged instructions to be defined in the
coprocessor general and conditional instruction categories. This primitive should be the
first one issued by the coprocessor during the dialog for an instruction that is implemented
as privileged.
7.4.6 Transfer Operation Word Primitive
The transfer operation word primitive requests a copy of the coprocessor instruction
operation word for the coprocessor. This primitive applies to general and conditional
category instructions. Figure 7-26 shows the format of the transfer operation word
primitive.
15
0
CA
PC
0
14
13
12
0
0
11
1
10
1
9
1
8
0
7
0
6
0
5
0
4
0
3
0
2
0
1
0
Figure 7-26. Transfer Operation Word Primitive Format
MOTOROLA M68020 USER’S MANUAL 7-35
The transfer operation word primitive uses the CA and PC bits as described in 7.4.2
Coprocessor Response Primitive General Format. If this primitive is issued with CA = 0
during a conditional category instruction, the main processor initiates protocol violation
exception processing.
When the main processor reads this primitive from the response CIR, it transfers the
F-line operation word of the currently executing coprocessor instruction to the operation
word CIR. The value of the scanPC is not affected by this primitive.
7.4.7 Transfer from Instruction Stream Primitive
The transfer from instruction stream primitive initiates transfers of operands from the
instruction stream to the coprocessor. This primitive applies to general and conditional
category instructions. Figure 7-27 shows the format of the transfer from instruction stream
primitive.
15
0
CA
PC
0
14
13
12
0
1
11
1
10
1
9
1
8
7
LENGTH
Figure 7-27. Transfer from Instruction Stream Primitive Format
The transfer from instruction stream primitive uses the CA and PC bits as described in
7.4.2 Coprocessor Response Primitive General Format. If this primitive is issued with
CA = 0 during a conditional category instruction, the main processor initiates protocol
violation exception processing.
The length field of this primitive specifies the length, in bytes, of the operand to be
transferred from the instruction stream to the coprocessor. The length must be an even
number of bytes. If an odd length is specified, the main processor initiates protocol
violation exception processing (refer to 7.5.2.1 Protocol Violations).
This primitive transfers coprocessor-defined extension words to the coprocessor. When
the main processor reads this primitive from the response CIR, it copies the number of
bytes indicated by the length field from the instruction stream to the operand CIR. The first
word or long word transferred is at the location pointed to by the scanPC when the
primitive is read by the main processor. The scanPC is incremented after each word or
long word is transferred. When execution of the primitive has completed, the scanPC has
been incremented by the total number of bytes transferred and points to the word
following the last word transferred. The main processor transfers the operands from the
instruction stream, using a sequence of long-word writes, to the operand CIR. If the length
field is not an even multiple of four bytes, the last two bytes from the instruction stream are
transferred using a word write to the operand CIR.
7-36 M68020 USER’S MANUAL MOTOROLA
7.4.8 Evaluate and Transfer Effective Address Primitive
The evaluate and transfer effective address primitive evaluates the effective address
specified in the coprocessor instruction operation word and transfers the result to the
coprocessor. This primitive applies to general category instructions. If this primitive is
issued by the coprocessor during the execution of a conditional category instruction, the
main processor initiates protocol violation exception processing. Figure 7-28 shows the
format of the evaluate and transfer effective address primitive.
15
0
CA
PC
0
14
13
12
0
1
11
0
10
1
9
0
8
0
7
0
6
0
5
0
4
0
3
0
2
0
1
0
Figure 7-28. Evaluate and Transfer Effective Address Primitive Format
The evaluate and transfer effective address primitive uses the CA and PC bits as
described in 7.4.2 Coprocessor Response Primitive General Format.
When the main processor reads this primitive while executing a general category
instruction, it evaluates the effective address specified in the instruction. At this point, the
scanPC contains the address of the first of any required effective address extension
words. The main processor increments the scanPC by two after it references each of
these extension words. After the effective address is calculated, the resulting 32-bit value
is written to the operand address CIR.
The MC68020/EC020 only calculates effective addresses for control alterable addressing
modes in response to this primitive. If the addressing mode in the operation word is not a
control alterable mode, the main processor aborts the instruction by writing a $0001 to the
control CIR and initiates F-line emulation exception processing (refer to 7.5.2.2 F-Line
Emulator Exceptions).
7.4.9 Evaluate Effective Address and Transfer Data Primitive
The evaluate effective address and transfer data primitive transfers an operand between
the coprocessor and the effective address specified in the coprocessor instruction
operation word. This primitive applies to general category instructions. If the coprocessor
issues this primitive during the execution of a conditional category instruction, the main
processor initiates protocol violation exception processing. Figure 7-29 shows the format
of the evaluate effective address and transfer data primitive.
15
0
CA
PC
DR
14
13
12
1
0
11
10
9
VALID EA
8
7
LENGTH
Figure 7-29. Evaluate Effective Address and
Transfer Data Primitive Format
This primitive uses the CA, PC, and DR bits as described in 7.4.2 Coprocessor
Response Primitive General Format.
MOTOROLA M68020 USER’S MANUAL 7-37
The valid EA field of the primitive format specifies the valid effective address categories
for this primitive. If the effective address specified in the instruction operation word is not a
member of the class specified by the valid EA field, the main processor aborts the
coprocessor instruction by writing an abort mask to the control CIR (refer to 7.3.2 Control
CIR) and by initiating F-line emulation exception processing. Table 7-4 lists the valid
effective address field encodings.
Table 7-4. Valid Effective Address Field Codes
Field Category
000 Control Alterable
001 Data Alterable
010 Memory Alterable
011 Alterable
100 Control
101 Data
110 Memory
111 Any Effective Address (No Restriction)
Even when the valid EA fields specified in the primitive and in the instruction operation
word match, the MC68020/EC020 initiates protocol violation exception processing if the
primitive requests a write to an unalterable effective address.
The length in bytes of the operand to be transferred is specified by the length field of the
primitive format. Several restrictions apply to operand lengths for certain effective
addressing modes. If the effective address is a main processor register (register direct
mode), only operand lengths of one, two, or four bytes are valid; all other lengths cause
the main processor to initiate protocol violation exception processing. Operand lengths of
0–255 bytes are valid for the memory addressing modes.
The length of 0–255 bytes does not apply to an immediate operand. The length of an
immediate operand must be one byte or an even number of bytes (less than 256), and the
direction of transfer must be to the coprocessor; otherwise, the main processor initiates
protocol violation exception processing.
When the main processor receives the evaluate effective address and transfer data
primitive during the execution of a general category instruction, it verifies that the effective
address encoded in the instruction operation word is in the category specified by the
primitive. If so, the processor calculates the effective address using the appropriate
effective address extension words at the current scanPC address and increments the
scanPC by two for each word referenced. Using long-word transfers whenever possible,
the main processor then transfers the number of bytes specified in the primitive between
the operand CIR and the effective address. Refer to 7.3.8 Operand CIR for information
concerning operand alignment for transfers involving the operand CIR.
7-38 M68020 USER’S MANUAL MOTOROLA
The DR bit specifies the direction of the operand transfer. DR = 0 requests a transfer from
the main processor to the coprocessor, and DR = 1 specifies a transfer from the
coprocessor to the main processor.
If the effective addressing mode specifies the predecrement mode, the address register
used is decremented by the size of the operand before the transfer. The bytes within the
operand are then transferred to or from ascending addresses beginning with the location
specified by the decremented address register. In this mode, if A7 is used as the address
register and the operand length is one byte, A7 is decremented by two to maintain a word-
aligned stack.
For the postincrement effective addressing mode, the address register used is
incremented by the size of the operand after the transfer. The bytes within the operand
are transferred to or from ascending addresses beginning with the location specified by
the address register. In this mode, if A7 is used as the address register and the operand
length is one byte, A7 is incremented by two after the transfer to maintain a word-aligned
stack. Transferring odd length operands longer than one byte using the –(A7) or (A7)+
addressing modes can result in a stack pointer that is not word aligned.
The processor repeats the effective address calculation each time this primitive is issued
during the execution of a given instruction. The calculation uses the current contents of
any required address and data registers. The instruction must include a set of effective
address extension words for each repetition of a calculation that requires them. The
processor locates these words at the current scanPC location and increments the scanPC
by two for each word referenced in the instruction stream.
The MC68020/EC020 sign-extends a byte or word-sized operand to a long-word value
when it is transferred to an address register (A7–A0) using this primitive with the register
direct effective addressing mode. A byte or word-sized operand transferred to a data
register (D7–D0) only overwrites the lower byte or word of the data register.
7.4.10 Write to Previously Evaluated Effective Address Primitive
The write to previously evaluated effective address primitive transfers an operand from the
coprocessor to a previously evaluated effective address. This primitive applies to general
category instructions. If the coprocessor uses this primitive during the execution of a
conditional category instruction, the main processor initiates protocol violation exception
processing. Figure 7-30 shows the format of the write to previously evaluated effective
address primitive.
15
0
CA
PC
1
14
13
12
0
0
11
10
9
8
7
LENGTH
0
0
0
Figure 7-30. Write to Previously Evaluated Effective Address Primitive Format
MOTOROLA M68020 USER’S MANUAL 7-39
The write to previously evaluated effective address primitive uses the CA and PC bits as
described in 7.4.2 Coprocessor Response Primitive General Format.
The length field of the primitive format specifies the length of the operand in bytes. The
MC68020/EC020 transfers operands of 0–255 bytes in length.
When the main processor receives this primitive during the execution of a general
category instruction, it transfers an operand from the operand CIR to an effective address
specified by a temporary register within the MC68020/EC020. When a previous primitive
for the current instruction has evaluated the effective address, this temporary register
contains the evaluated effective address. Primitives that store an evaluated effective
address in a temporary register of the main processor are the evaluate and transfer
effective address, evaluate effective address and transfer data, and transfer multiple
coprocessor registers primitive. If this primitive is used during an instruction in which the
effective address specified in the instruction operation word has not been calculated, the
effective address used for the write is undefined. Also, if the previously evaluated effective
address was register direct, the address written to in response to this primitive is
undefined.
The function code value during the write operation indicates either supervisor or user data
space, depending on the value of the S-bit in the MC68020/EC020 SR when the
processor reads this primitive. While a coprocessor should request writes to only alterable
effective addressing modes, the MC68020/EC020 does not check the type of effective
address used with this primitive. For example, if the previously evaluated effective address
was PC relative and the MC68020/EC020 is at the user privilege level (S = 0 in SR), the
MC68020/EC020 writes to user data space at the previously calculated program relative
address (the 32-bit value in the temporary internal register of the processor).
Operands longer than four bytes are transferred in increments of four bytes (operand
parts) when possible. The main processor reads a long-word operand part from the
operand CIR and transfers this part to the current effective address. The transfers
continue in this manner using ascending memory locations until all of the long-word
operand parts are transferred, and any remaining operand part is then transferred using a
one-, two-, or three-byte transfer as required. The operand parts are stored in memory
using ascending addresses beginning with the address in the MC68020/EC020 temporary
register, which is internal to the processor and not for user use.
The execution of this primitive does not modify any of the registers in the
MC68020/EC020 programming model, even if the previously evaluated effective address
mode is the predecrement or postincrement mode. If the previously evaluated effective
addressing mode used any of the MC68020/EC020 internal address or data registers, the
effective address value used is the final value from the preceding primitive. That is, this
primitive uses the value from an evaluate and transfer effective address, evaluate effective
address and transfer data, or transfer multiple coprocessor registers primitive without
modification.
7-40 M68020 USER’S MANUAL MOTOROLA
The take address and transfer data primitive described in 7.4.11 Take Address and
Transfer Data Primitive does not replace the effective address value that has been
calculated by the MC68020/EC020. The address that the main processor obtains in
response to the take address and transfer data primitive is not available to the write to
previously evaluated effective address primitive.
A coprocessor can issue an evaluate effective address and transfer data primitive followed
by this primitive to perform a read-modify-write operation that is not indivisible. The bus
cycles for this operation are normal bus cycles that can be interrupted, and the bus can be
arbitrated between the cycles.
7.4.11 Take Address and Transfer Data Primitive
The take address and transfer data primitive transfers an operand between the
coprocessor and an address supplied by the coprocessor. This primitive applies to general
and conditional category instructions. Figure 7-31 shows the format of the take address
and transfer data primitive.
15
0
CA
PC
DR
14
13
12
0
0
11
10
9
8
7
LENGTH
1
0
1
Figure 7-31. Take Address and Transfer Data Primitive Format
The take address and transfer data primitive uses the CA, PC, and DR bits as described
in 7.4.2 Coprocessor Response Primitive General Format. If the coprocessor issues
this primitive with CA = 0 during a conditional category instruction, the main processor
initiates protocol violation exception processing.
The length field of the primitive format specifies the operand length, which can be from
0–255 bytes.
The main processor reads a 32-bit address from the operand address CIR. Using a series
of long-word transfers, the processor transfers the operand between this address and the
operand CIR. The DR bit determines the direction of the transfer. The processor reads or
writes the operand parts to ascending addresses, starting at the address from the operand
address CIR. If the operand length is not a multiple of four bytes, the final operand part is
transferred using a one-, two-, or three-byte transfer as required.
The function code used with the address read from the operand address CIR indicates
either supervisor or user data space according to the value of the S-bit in the
MC68020/EC020 SR.
MOTOROLA M68020 USER’S MANUAL 7-41
7.4.12 Transfer to/from Top of Stack Primitive
The transfer to/from top of stack primitive transfers an operand between the coprocessor
and the top of the active system stack of the main processor. This primitive applies to
general and conditional category instructions. Figure 7-32 shows the format of the transfer
to/from top of stack primitive.
15
0
CA
PC
DR
14
13
12
0
1
11
10
9
8
7
LENGTH
1
1
0
Figure 7-32. Transfer to/from Top of Stack Primitive Format
The transfer to/from top of stack primitive uses the CA, PC, and DR bits as described in
7.4.2 Coprocessor Response Primitive General Format. If the coprocessor issues this
primitive with CA = 0 during a conditional category instruction, the main processor initiates
protocol violation exception processing.
The length field of the primitive format specifies the length in bytes of the operand to be
transferred. The operand may be one, two, or four bytes in length; other length values
cause the main processor to initiate protocol violation exception processing.
If DR = 0, the main processor transfers the operand from the active system stack to the
operand CIR. The implied effective address mode used for the transfer is the (A7)+
addressing mode. A one-byte operand causes the stack pointer to be incremented by two
after the transfer to maintain word alignment of the stack.
If DR = 1, the main processor transfers the operand from the operand CIR to the active
system stack. The implied effective address mode used for the transfer is the –(A7)
addressing mode. A one-byte operand causes the stack pointer to be decremented by two
before the transfer to maintain word alignment of the stack.
7.4.13 Transfer Single Main Processor Register Primitive
The transfer single main processor register primitive transfers an operand between one of
the main processor's data or address registers and the coprocessor. This primitive applies
to general and conditional category instructions. Figure 7-33 shows the format of the
transfer single main processor register primitive.
15
CA
PC
DR
14
13
12
0
1
11
10
9
0
7
1
0
2
REGISTER
3
D
/
A
4
0
5
0
6
0
0
0
8
Figure 7-33. Transfer Single Main Processor Register Primitive Format
The transfer single main processor register primitive uses the CA, PC, and DR bits as
described in 7.4.2 Coprocessor Response Primitive General Format. If the
coprocessor issues this primitive with CA = 0 during a conditional category instruction, the
main processor initiates protocol violation exception processing.
7-42 M68020 USER’S MANUAL MOTOROLA
The D/A bit specifies whether the primitive transfers an address or data register. D/A = 0
indicates a data register, and D/A = 1 indicates an address register. The register field
contains the register number.
If DR = 0, the main processor writes the long-word operand in the specified register to the
operand CIR. If DR = 1, the main processor reads a long-word operand from the operand
CIR and transfers it to the specified data or address register.
7.4.14 Transfer Main Processor Control Register Primitive
The transfer main processor control register primitive transfers a long-word operand
between one of its control registers and the coprocessor. This primitive applies to general
and conditional category instructions. Figure 7-34 shows the format of the transfer main
processor control register primitive.
15
CA
PC
DR
14
13
12
0
1
11
10
9
0
7
1
0
2
0
3
0
4
0
5
0
6
0
0
1
8
0
0
1
Figure 7-34. Transfer Main Processor Control Register Primitive Format
The transfer main processor control register primitive uses the CA, PC, and DR bits as
described in 7.4.2 Coprocessor Response Primitive General Format. If the
coprocessor issues this primitive with CA = 0 during a conditional category instruction, the
main processor initiates protocol violation exception processing.
When the main processor receives this primitive, it reads a control register select code
from the register select CIR. This code determines which main processor control register
is transferred. Table 7-5 lists the valid control register select codes. If the control register
select code is not valid, the MC68020/EC020 initiates protocol violation exception
processing (refer to 7.5.2.1 Protocol Violations).
Table 7-5. Main Processor Control
Register Select Codes
Select Code Control Register
$x000 SFC
$x001 DFC
$x002 CACR
$x800 USP
$x801 VBR
$x802 CAAR
$x803 MSP
$x804 ISP
All other codes cause a protocol violation exception.
MOTOROLA M68020 USER’S MANUAL 7-43
After reading a valid code from the register select CIR, if DR = 0, the main processor
writes the long-word operand from the specified control register to the operand CIR. If
DR = 1, the main processor reads a long-word operand from the operand CIR and places
it in the specified control register.
7.4.15 Transfer Multiple Main Processor Registers Primitive
The transfer multiple main processor registers primitive transfers long-word operands
between one or more of its data or address registers and the coprocessor. This primitive
applies to general and conditional category instructions. Figure 7-35 shows the format of
the transfer multiple main processor registers primitive.
15
CA
PC
DR
14
13
12
0
0
11
10
9
0
7
1
1
2
0
3
0
4
0
5
0
6
0
0
0
8
0
0
1
Figure 7-35. Transfer Multiple Main Processor Registers Primitive Format
The transfer multiple main processor registers primitive uses the CA, PC, and DR bits as
described in 7.4.2 Coprocessor Response Primitive General Format. If the
coprocessor issues this primitive with CA = 0 during a conditional category instruction, the
main processor initiates protocol violation exception processing.
When the main processor receives this primitive, it reads a 16-bit register select mask
from the register select CIR. The format of the register select mask is shown in Figure
7 -36. A register is transferred if the bit corresponding to the register in the register select
mask is set. The selected registers are transferred in the order D7–D0 and then A7–A0.
15
A7
A6
A5
14
13
12
A4
A3
11
10
9
0
7
A2
A1
2
D1
3
D3
4
D4
5
D5
6
D6
D7
A0
8
D2
D0
1
Figure 7-36. Register Select Mask Format
If DR = 0, the main processor writes the contents of each register indicated in the register
select mask to the operand CIR using a sequence of long-word transfers. If DR = 1, the
main processor reads a long-word operand from the operand CIR into each register
indicated in the register select mask. The registers are transferred in the same order,
regardless of the direction of transfer indicated by the DR bit.
7.4.16 Transfer Multiple Coprocessor Registers Primitive
The transfer multiple coprocessor registers primitive transfers from 0–16 operands
between the effective address specified in the coprocessor instruction and the
coprocessor. This primitive applies to general category instructions. If the coprocessor
issues this primitive during the execution of a conditional category instruction, the main
processor initiates protocol violation exception processing. Figure 7-37 shows the format
of the transfer multiple coprocessor registers primitive.
7-44 M68020 USER’S MANUAL MOTOROLA
15
0
CA
PC
DR
14
13
12
0
0
11
10
9
8
7
LENGTH
0
0
1
Figure 7-37. Transfer Multiple Coprocessor Registers Primitive Format
The transfer multiple coprocessor registers primitive uses the CA, PC, and DR bits as
described in 7.4.2 Coprocessor Response Primitive General Format.
The length field of the primitive format indicates the length in bytes of each operand
transferred. The operand length must be an even number of bytes; odd length operands
cause the MC68020/EC020 to initiate protocol violation exception processing (refer to
7.5.2.1 Protocol Violations).
When the main processor reads this primitive, it calculates the effective address specified
in the coprocessor instruction. The scanPC should be pointing to the first of any necessary
effective address extension words when this primitive is read from the response CIR; the
scanPC is incremented by two for each extension word referenced during the effective
address calculation. For transfers from the effective address to the coprocessor (DR = 0),
the control addressing modes and the postincrement addressing mode are valid. For
transfers from the coprocessor to the effective address (DR = 1), the control alterable and
predecrement addressing modes are valid. Invalid addressing modes cause the
MC68020/EC020 to abort the instruction by writing an abort mask to the control CIR (refer
to 7.3.2 Control CIR) and to initiate F-line emulator exception processing (refer to 7.5.2.2
F-Line Emulator Exceptions).
After performing the effective address calculation, the MC68020/EC020 reads a 16-bit
register select mask from the register select CIR. The coprocessor uses the register select
mask to specify the number of operands to transfer; the MC68020/EC020 counts the
number of ones in the register select mask to determine the number of operands. The
order of the ones in the register select mask is not relevant to the operation of the main
processor. As many as 16 operands can be transferred by the main processor in response
to this primitive. The total number of bytes transferred is the product of the number of
operands transferred and the length of each operand specified in the length field of the
primitive format.
If DR = 1, the main processor reads the number of operands specified in the register
select mask from the operand CIR and writes these operands to the effective address
specified in the instruction using long-word transfers whenever possible. If DR = 0, the
main processor reads the number of operands specified in the register select mask from
the effective address and writes them to the operand CIR.
For the control addressing modes, the operands are transferred to or from memory using
ascending addresses. For the postincrement addressing mode, the operands are read
from memory with ascending addresses also, and the address register used is
incremented by the size of an operand after each operand is transferred. The address
register used with the (An)+ addressing mode is incremented by the total number of bytes
transferred during the primitive execution.
MOTOROLA M68020 USER’S MANUAL 7-45
For the predecrement addressing mode, the operands are written to memory with
descending addresses, but the bytes within each operand are written to memory with
ascending addresses. As an example, Figure 7-38 shows the format in long-word-
oriented memory for two 12-byte operands transferred from the coprocessor to the
effective address using the –(An) addressing mode. The processor decrements the
address register by the size of an operand before the operand is transferred. It writes the
bytes of the operand to ascending memory addresses. When the transfer is complete, the
address register has been decremented by the total number of bytes transferred. The
MC68020/EC020 transfers the data using long-word transfers whenever possible.
31
15
0
O
P1, BYTE (0
)
7
23
O
P0, BYTE (0
)
O
P1, BYTE (L – 1
)
O
P0, BYTE (L – 1
)
A
n – LENGT
H
INITIAL A
n
OP0, Byte (0) is the first byte written to memory
OP0, Byte (L–1) is the last byte of the first operand written to memory
OP1, Byte (0) is the first byte of the second operand written to memory
OP1, Byte (L–1) is the last byte written to memory
NOTE:
16
8
24
An – 2 LENGT
H
= FINAL A
n
*
Figure 7-38. Operand Format in Memory for Transfer to –(An)
7.4.17 Transfer Status Register and ScanPC Primitive
The transfer status register and the scanPC primitive transfers values between the
coprocessor and the MC68020/EC020 SR. On an optional basis, the scanPC also makes
transfers. This primitive applies to general category instructions. If the coprocessor issues
this primitive during the execution of a conditional category instruction, the main processor
initiates protocol violation exception processing. Figure 7-39 shows the format of the
transfer status register and scanPC primitive.
15
CA
PC
DR
14
13
12
0
0
11
10
9
0
7
0
1
2
0
3
0
4
0
5
0
6
0
0
SP
8
0
0
1
Figure 7-39. Transfer Status Register and ScanPC Primitive Format
The transfer status register and scanPC primitive uses the CA, PC, and DR bits as
described in 7.4.2 Coprocessor Response Primitive General Format.
The SP bit selects the scanPC option. If SP = 1, the primitive transfers both the scanPC
and SR. If SP = 0, only the SR is transferred.
7-46 M68020 USER’S MANUAL MOTOROLA
If SP = 0 and DR = 0, the main processor writes the 16-bit SR value to the operand CIR. If
SP = 0 and DR = 1, the main processor reads a 16-bit value from the operand CIR into the
main processor SR.
If SP = 1 and DR = 0, the main processor writes the long-word value in the scanPC to the
instruction address CIR and then writes the SR value to the operand CIR. If SP = 1 and
DR = 1, the main processor reads a 16-bit value from the operand CIR into the SR and
then reads a long-word value from the instruction address CIR into the scanPC.
With this primitive, a general category instruction can change the main processor program
flow by placing a new value in the SR, in the scanPC, or new values in both the SR and
the scanPC. By accessing the SR, the coprocessor can determine and manipulate the
main processor condition codes, supervisor status, trace modes, selection of the active
stack, and interrupt mask level.
The MC68020/EC020 discards any instruction words that have been prefetched beyond
the current scanPC location when this primitive is issued with DR = 1 (transfer to main
processor). The MC68020/EC020 then refills the instruction pipe from the scanPC
address in the address space indicated by the S-bit of the SR.
If the MC68020/EC020 is operating in the trace on change of flow mode (T1, T0 in the SR
= 01) when the coprocessor instruction begins to execute and if this primitive is issued
with DR = 1 (from coprocessor to main processor), the MC68020/EC020 prepares to take
a trace exception. The trace exception occurs when the coprocessor signals that it has
completed all processing associated with the instruction. Changes in the trace modes due
to the transfer of the SR to the main processor take effect on execution of the next
instruction.
7.4.18 Take Preinstruction Exception Primitive
The take preinstruction exception primitive initiates exception processing using a
coprocessor-supplied exception vector number and the preinstruction exception stack
frame format. This primitive applies to general and conditional category instructions.
Figure 7-40 shows the format of the take preinstruction exception primitive.
15
0
0
PC
0
14
13
12
1
1
11
10
9
8
7
V
ECTOR NUMBE
R
1
0
0
Figure 7-40. Take Preinstruction Exception Primitive Format
The take preinstruction exception primitive uses the PC bit as described in 7.4.2
Coprocessor Response Primitive General Format. The vector number field contains
the exception vector number used by the main processor to initiate exception processing.
When the main processor receives this primitive, it acknowledges the coprocessor
exception request by writing an exception acknowledge mask to the control CIR (refer to
7.3.2 Control CIR ). The MC68020/EC020 then proceeds with exception processing as
MOTOROLA M68020 USER’S MANUAL 7-47
described in Section 6 Exception Processing. The vector number for the exception is
taken from the vector number field of the primitive, and the MC68020/EC020 uses the
four-word stack frame format shown in Figure 7-41.
0
11
12
15
S
TATUS REGISTE
R
0
0
0
0
V
ECTOR NUMBE
R
PROGRAM COUNTER
+0
6
+0
2
SP
Figure 7-41. MC68020/EC020 Preinstruction Stack Frame
The value of the PC saved in this stack frame is the F-line operation word address of the
coprocessor instruction during which the primitive was received. Thus, if the exception
handler routine does not modify the stack frame, an RTE instruction causes the
MC68020/EC020 to return and reinitiate execution of the coprocessor instruction.
The take preinstruction exception primitive can be used when the coprocessor does not
recognize a value written to either its command CIR or condition CIR to initiate a
coprocessor instruction. This primitive can also be used if an exception occurs in the
coprocessor instruction before any program-visible resources are modified by the
instruction operation. This primitive should not be used during a coprocessor instruction if
program-visible resources have been modified by that instruction. Otherwise, since the
MC68020/EC020 reinitiates the instruction when it returns from exception processing, the
restarted instruction receives the previously modified resources in an inconsistent state.
One of the most important uses of the take preinstruction exception primitive is to signal
an exception condition in a cpGEN instruction that was executing concurrently with the
main processor's instruction execution. If the coprocessor no longer requires the services
of the main processor to complete a cpGEN instruction and if the concurrent instruction
completion is transparent to the programming model, the coprocessor can release the
main processor by issuing a primitive with CA = 0. The main processor usually executes
the next instruction in the instruction stream, and the coprocessor completes its operations
concurrently with the main processor operation. If an exception occurs while the
coprocessor is executing an instruction concurrently, the exception is not processed until
the main processor attempts to initiate the next general or conditional instruction. After the
main processor writes to the command or condition CIR to initiate a general or conditional
instruction, it then reads the response CIR. At this time, the coprocessor can return the
take preinstruction exception primitive. This protocol allows the main processor to proceed
with exception processing related to the previous concurrently executing coprocessor
instruction and then return and reinitiate the coprocessor instruction during which the
exception was signaled. The coprocessor should record the addresses of all general
category instructions that can be executed concurrently with the main processor and that
support exception recovery. Since the exception is not reported until the next coprocessor
instruction is initiated, the processor usually requires the instruction address to determine
7-48 M68020 USER’S MANUAL MOTOROLA
which instruction the coprocessor was executing when the exception occurred. A
coprocessor can record the instruction address by setting PC = 1 in one of the primitives it
uses before releasing the main processor.
7.4.19 Take Midinstruction Exception Primitive
The take midinstruction exception primitive initiates exception processing using a
coprocessor-supplied exception vector number and the midinstruction exception stack
frame format. This primitive applies to general and conditional category instructions.
Figure 7-42 shows the format of the take midinstruction exception primitive.
15
0
0
PC
0
14
13
12
1
1
11
10
9
8
7
V
ECTOR NUMBE
R
1
0
1
Figure 7-42. Take Midinstruction Exception Primitive Format
The take midinstruction exception primitive uses the PC bit as described in 7.4.2
Coprocessor Response Primitive General Format. The vector number field contains
the exception vector number used by the main processor to initiate exception processing.
When the main processor receives this primitive, it acknowledges the coprocessor
exception request by writing an exception acknowledge mask (refer to 7.3.2 Control CIR)
to the control CIR. The MC68020/EC020 then performs exception processing as
described in Section 6 Exception Processing. The vector number for the exception is
taken from the vector number field of the primitive, and the MC68020/EC020 uses the
10-word stack frame format shown in Figure 7-43.
0
11
12
15
S
TATUS REGISTE
R
1
0
0
1
V
ECTOR NUMBE
R
SCAN PC
+0
6
+0
2
SP
PROGRAM COUNTER
+0
C
O
PERATION WOR
D
EFFECTIVE ADDRESS
+1
0
I
NTERNAL REGISTE
R
+0
8
+
0
E
Figure 7-43. MC68020/EC020 Midinstruction Stack Frame
MOTOROLA M68020 USER’S MANUAL 7-49
The PC value saved in this stack frame is the operation word address of the coprocessor
instruction during which the primitive is received. The scanPC field contains the value of
the MC68020/EC020 scanPC when the primitive is received. If the current instruction does
not evaluate an effective address prior to the exception request primitive, the value of the
effective address field in the stack frame is undefined.
The coprocessor uses this primitive to request exception processing for an exception
during the instruction dialog with the main processor. If the exception handler does not
modify the stack frame, the MC68020/EC020 returns from the exception handler and
reads the response CIR. Thus, the main processor attempts to continue executing the
suspended instruction by reading the response CIR and processing the primitive it
receives.
7.4.20 Take Postinstruction Exception Primitive
The take postinstruction exception primitive initiates exception processing using a
coprocessor-supplied exception vector number and the postinstruction exception stack
frame format. This primitive applies to general and conditional category instructions.
Figure 7-44 shows the format of the take postinstruction exception primitive.
15
0
0
PC
0
14
13
12
1
1
11
10
9
8
7
V
ECTOR NUMBE
R
1
1
0
Figure 7-44. Take Postinstruction Exception Primitive Format
The take postinstruction exception primitive uses the PC bit as described in 7.4.2
Coprocessor Response Primitive General Format. The vector number field contains
the exception vector number used by the main processor to initiate exception processing.
When the main processor receives this primitive, it acknowledges the coprocessor
exception request by writing an exception acknowledge mask to the control CIR (refer to
7.3.2 Control CIR). The MC68020/EC020 then performs exception processing as
described in Section 6 Exception Processing. The vector number for the exception is
taken from the vector number field of the primitive, and the MC68020/EC020 uses the six-
word stack frame format shown in Figure 7-45.
0
11
12
15
S
TATUS REGISTE
R
0
0
1
0
V
ECTOR NUMBE
R
SCAN PC
+0
6
+0
2
SP
PROGRAM COUNTER
+0
8
Figure 7-45. MC68020/EC020 Postinstruction Stack Frame
7-50 M68020 USER’S MANUAL MOTOROLA
The value in the main processor scanPC at the time this primitive is received is saved in
the scanPC field of the postinstruction exception stack frame. The value of the PC saved
is the F-line operation word address of the coprocessor instruction during which the
primitive is received.
When the MC68020/EC020 receives the take postinstruction exception primitive, it
assumes that the coprocessor either completed or aborted the instruction with an
exception. If the exception handler does not modify the stack frame, the MC68020/EC020
returns from the exception handler to begin execution at the location specified by the
scanPC field of the stack frame. This location should be the address of the next instruction
to be executed.
The coprocessor uses this primitive to request exception processing when it completes or
aborts an instruction while the main processor is awaiting a normal response. For a
general category instruction, the response is a release; for a conditional category
instruction, it is an evaluated true/false condition indicator. Thus, the operation of the
MC68020/EC020 in response to this primitive is compatible with standard M68000 family
instruction related exception processing (for example, the divide-by-zero exception).
7.5 EXCEPTIONS
Various exception conditions related to the execution of coprocessor instructions may
occur. Whether an exception is detected by the main processor or by the coprocessor, the
main processor coordinates and performs exception processing. Servicing these
coprocessor-related exceptions is an extension of the protocol used to service standard
M68000 family exceptions. That is, when either the main processor detects an exception
or is signaled by the coprocessor that an exception condition has occurred, the main
processor proceeds with exception processing as described in Section 6 Exception
Processing.
7.5.1 Coprocessor-Detected Exceptions
Coprocessor interface exceptions that the coprocessor detects, as well as those that the
main processor detects, are usually classified as coprocessor-detected exceptions.
Coprocessor-detected exceptions can occur during M68000 coprocessor interface
operations, internal operations, or other system-related operations of the coprocessor.
Most coprocessor-detected exceptions are signaled to the main processor through the use
of one of the three take exception primitives defined for the M68000 coprocessor
interface. The main processor responds to these primitives as described in 7.4.18 Take
Preinstruction Exception Primitive, 7.4.19 Take Midinstruction Exception Primitive,
and 7.4.20 Take Postinstruction Exception Primitive. However, not all coprocessor -
detected exceptions are signaled by response primitives. Coprocessor-detected format
errors during the cpSAVE or cpRESTORE instruction are signaled to the main processor
using the invalid format word described in 7.2.3.2.3 Invalid Format Words.
MOTOROLA M68020 USER’S MANUAL 7-51
7.5.1.1 COPROCESSOR-DETECTED PROTOCOL VIOLATIONS. Protocol violation
exceptions are communication failures between the main processor and coprocessor
across the M68000 coprocessor interface. Coprocessor-detected protocol violations occur
when the main processor accesses entries in the CIR set in an unexpected sequence.
The sequence of operations that the main processor performs for a given coprocessor
instruction or coprocessor response primitive has been described previously in this
section.
A coprocessor can detect protocol violations in various ways. According to the M68000
coprocessor interface protocol, the main processor always accesses the operation word,
operand, register select, instruction address, or operand address CIRs synchronously with
respect to the operation of the coprocessor. That is, the main processor accesses these
five registers in a certain sequence, and the coprocessor expects them to be accessed in
that sequence. As a minimum, all M68000 coprocessors should detect a protocol violation
if the main processor accesses any of these five registers when the coprocessor is
expecting an access to either the command or condition CIR. Likewise, if the coprocessor
is expecting an access to the command or condition CIR and the main processor
accesses one of these five registers, the coprocessor should detect and signal a protocol
violation.
According to the M68000 coprocessor interface protocol, the main processor can perform
a read of either the save CIR or response CIR or a write of either the restore CIR or
control CIR asynchronously with respect to the operation of the coprocessor. That is, an
access to one of these registers without the coprocessor explicitly expecting that access
at that point can be a valid access. Although the coprocessor can anticipate certain
accesses to the restore, response, and control CIRs, these registers can be accessed at
other times also.
The coprocessor cannot signal a protocol violation to the main processor during execution
of a cpSAVE or cpRESTORE instruction. If a coprocessor detects a protocol violation
during execution of the cpSAVE or cpRESTORE instruction, it should signal the exception
to the main processor when the next coprocessor instruction is initiated.
The main philosophy of the coprocessor-detected protocol violation is that the
coprocessor should always acknowledge an access to one of its interface registers. If the
coprocessor determines that the access is not valid, it should assert DSACK1/DSACK0 to
the main processor and signal a protocol violation when the main processor next reads
the response CIR. If the coprocessor fails to assert DSACK1/DSACK0, the main
processor waits for the assertion of that signal (or some other bus termination signal)
indefinitely. The protocol previously described ensures that the coprocessor cannot halt
the main processor.
The coprocessor can signal a protocol violation to the main processor with the take
midinstruction exception primitive. To maintain consistency, the vector number should be
13, as it is for a protocol violation detected by the main processor. When the main
processor reads this primitive, it proceeds as described in 7.4.19 Take Midinstruction
Exception Primitive . If the exception handler does not modify the stack frame, the
MC68020/EC020 returns from the exception handler and reads the response CIR.
7-52 M68020 USER’S MANUAL MOTOROLA
7.5.1.2 COPROCESSOR-DETECTED ILLEGAL COMMAND OR CONDITION WORDS.
Illegal coprocessor command or condition words are values written to the command CIR
or condition CIR that the coprocessor does not recognize. If a value written to either of
these registers is not valid, the coprocessor should return the take preinstruction
exception primitive in the response CIR. When it receives this primitive, the main
processor takes a preinstruction exception as described in 7.4.18 Take Preinstruction
Exception Primitive. If the exception handler does not modify the main processor stack
frame, an RTE instruction causes the MC68020/EC020 to reinitiate the instruction that
took the exception. The coprocessor designer should ensure that the state of the
coprocessor is not irrecoverably altered by an illegal command or condition exception if
the system supports emulation of the unrecognized command or condition word.
All M68000 coprocessors signal illegal command and condition words by returning the
take preinstruction exception primitive with the F-line emulator exception vector number
11.
7.5.1.3 COPROCESSOR DATA-PROCESSING-RELATED EXCEPTIONS. Exceptions
related to the internal operation of a coprocessor are classified as data-processing-related
exceptions. These exceptions are analogous to the divide-by-zero exception defined by
M68000 microprocessors and should be signaled to the main processor using one of the
three take exception primitives containing an appropriate exception vector number. Which
of these three primitives is used to signal the exception is usually determined by the point
in the instruction operation where the main processor should continue the program flow
after exception processing. Refer to 7.4.18 Take Preinstruction Exception Primitives ,
7.4.19 Take Midinstruction Exception Primitive, and 7.4.20 Take Postinstruction
Exception Primitive.
7.5.1.4 COPROCESSOR SYSTEM-RELATED EXCEPTIONS. System-related exceptions
detected by a DMA coprocessor include those associated with bus activity and any other
exceptions (interrupts, for example) occurring external to the coprocessor. The actions
taken by the coprocessor and the main processor depend on the type of exception that
occurs.
When an address or bus error is detected by a DMA coprocessor, the coprocessor should
store any information necessary for the main processor exception handling routines in
system-accessible registers. The coprocessor should place one of the three take
exception primitives encoded with an appropriate exception vector number in the
response CIR. Which of the three primitives is used depends upon the point in the
coprocessor instruction at which the exception was detected and the point in the
instruction execution at which the main processor should continue after exception
processing. Refer to 7.4.18 Take Preinstruction Exception Primitives, 7.4.19 Take
Midinstruction Exception Primitive, and 7.4.20 Take Postinstruction Exception
Primitive.
MOTOROLA M68020 USER’S MANUAL 7-53
7.5.1.5 FORMAT ERRORS. Format errors are the only coprocessor-detected exceptions
that are not signaled to the main processor with a response primitive. When the main
processor writes a format word to the restore CIR during the execution of a cpRESTORE
instruction, the coprocessor decodes this word to determine if it is valid (refer to 7.2.3.3
Coprocessor Context Save Instruction). If the format word is not valid, the coprocessor
places the invalid format code in the restore CIR. When the main processor reads the
invalid format code, it aborts the coprocessor instruction by writing an abort mask to the
control CIR (refer to 7.3.2 Control CIR ). The main processor then performs exception
processing using a four-word preinstruction stack frame and the format error exception
vector number 14. Thus, if the exception handler does not modify the stack frame, the
MC68020/EC020 restarts the cpRESTORE instruction when the RTE instruction in the
handler is executed. If the coprocessor returns the invalid format code when the main
processor reads the save CIR to initiate a cpSAVE instruction, the main processor
performs format error exception processing as outlined for the cpRESTORE instruction.
7.5.2 Main-Processor-Detected Exceptions
A number of exceptions related to coprocessor instruction execution are detected and
serviced by the main processor instead of the coprocessor. These exceptions can be
related to the execution of coprocessor response primitives, communication across the
M68000 coprocessor interface, or completion of conditional coprocessor instructions by
the main processor.
7.5.2.1 PROTOCOL VIOLATIONS. The main processor detects a protocol violation when
it reads a primitive from the response CIR that is not a valid primitive. The protocol
violations that can occur in response to the primitives defined for the M68000 coprocessor
interface are summarized in Table 7-6.
7-54 M68020 USER’S MANUAL MOTOROLA
Table 7-6. Exceptions Related to Primitive Processing
Primitive Protocol F-Line Other
Busy
Null
Supervisory Check*
Other: Privilege Violation if S-Bit in the SR = 0 X
Transfer Operation Word*
Transfer from Instruction Stream*
Protocol: If Length Field Is Odd (Zero Length Legal) X
Evaluate and Transfer Effective Address
Protocol: If Used with Conditional Instruction
F-Line: If EA in Opword Is NOT Control Alterable XX
Evaluate Effective Address and Transfer Data
Protocol:
1. If Used with Conditional Instructions
2. Length Is Not 1, 2, or 4 and EA = Register Direct
3. If EA = Immediate and Length Odd and Greater Than 1
4. Attempt to Write to Unalterable Address
Even if Address Declared Legal in Primitive
F-Line: Valid EA Field Does Not Match EA in Opword
X
X
Write to Previously Evaluated Effective Address
Protocol: If Used with Conditional Instruction X
Take Address and Transfer Data*
Transfer to/from Top of Stack*
Protocol: Length Field Other Than 1, 2, or 4 X
Transfer Single Main Processor Register*
Transfer Main Processor Control Register
Protocol: Invalid Control Register Select Code X
Transfer Multiple Main Processor Registers*
Transfer Multiple Coprocessor Registers
Protocol:
1. If Used with Conditional Instructions
2. Odd Length Value
F-Line:
1. EA Not Control Alterable or (An)+ for CP to Memory Transfer
2. EA Not Control Alterable or –(An) for Memory to CP Transfer
X
X
Transfer Status and ScanPC
Protocol: If Used with Conditional Instruction
Other:
1. Trace—Trace Made Pending if MC68020/EC020 in “Trace on Change of
Flow” Mode and DR = 1
2. Address Error—If Odd Value Written to ScanPC
XX
Take Preinstruction, Midinstruction, or Postinstruction Exception
Exception Depends on Vector Supplies in Primitive X X X
*Use of this primitive with CA = 0 will cause protocol violation on conditional instructions.
Abbreviations:
EA—Effective Address
CP—Coprocessor
MOTOROLA M68020 USER’S MANUAL 7-55
When the MC68020/EC020 detects a protocol violation, it does not automatically notify the
coprocessor of the resulting exception by writing to the control CIR. However, the
exception handling routine may use the MOVES instruction to read the response CIR and
thus determine the primitive that caused the MC68020/EC020 to initiate protocol violation
exception processing. The main processor initiates exception processing using the
midinstruction stack frame (refer to Figure 7-43) and the coprocessor protocol violation
exception vector number 13. If the exception handler does not modify the stack frame, the
main processor reads the response CIR again following the execution of an RTE
instruction to return from the exception handler. This protocol allows extensions to the
M68000 coprocessor interface to be emulated in software by a main processor that does
not provide hardware support for these extensions. Thus, the protocol violation is
transparent to the coprocessor if the primitive execution can be emulated in software by
the main processor.
7.5.2.2 F-LINE EMULATOR EXCEPTIONS. The F-line emulator exceptions detected by
the MC68020/EC020 are either explicitly or implicitly related to the encodings of F-line
operation words in the instruction stream. If the main processor determines that an F-line
operation word is not valid, it initiates F-line emulator exception processing. Any F-line
operation word with bits 8–6 = 110 or 111 causes the MC68020/EC020 to initiate
exception processing without initiating any communication with the coprocessor for that
instruction. Also, an operation word with bits 8–6 = 000–101 that does not map to one of
the valid coprocessor instructions in the instruction set causes the MC68020/EC020 to
initiate F-line emulator exception processing. If the F-line emulator exception is either of
these two situations, the main processor does not write to the control CIR prior to initiating
exception processing.
F-line exceptions can also occur if the operations requested by a coprocessor response
primitive are not compatible with the effective address type in bits 5–0 of the coprocessor
instruction operation word. The F-line emulator exceptions that can result from the use of
the M68000 coprocessor response primitives are summarized in Table 7-6. If the
exception is caused by receiving an invalid primitive, the main processor aborts the
coprocessor instruction in progress by writing an abort mask (refer to 7.3.2 Control CIR )
to the control CIR prior to F-line emulator exception processing.
Another type of F-line emulator exception occurs when a bus error occurs during the CIR
access that initiates a coprocessor instruction. The main processor assumes that the
coprocessor is not present and takes the exception.
When the main processor initiates F-line emulator exception processing, it uses the four-
word preinstruction exception stack frame (refer to Figure 7-41) and the F-line emulator
exception vector number 11. Thus, if the exception handler does not modify the stack
frame, the main processor attempts to restart the instruction that caused the exception
after it executes an RTE instruction to return from the exception handler.
If the cause of the F-line exception can be emulated in software, the handler stores the
results of the emulation in the appropriate registers of the programming model and in the
status register field of the saved stack frame. The exception handler adjusts the program
7-56 M68020 USER’S MANUAL MOTOROLA
counter field of the saved stack frame to point to the next instruction operation word and
executes the RTE instruction. The MC68020/EC020 then executes the instruction
following the instruction that was emulated.
The exception handler should also check the copy of the SR on the stack to determine
whether tracing is enabled. If tracing is enabled, the trace exception processing should
also be emulated. Refer to Section 6 Exception Processing for additional information.
7.5.2.3 PRIVILEGE VIOLATIONS. Privilege violations can result from the cpSAVE and
cpRESTORE instructions and from the supervisor check coprocessor response primitive.
The MC68020/EC020 initiates privilege violation exception processing if it attempts to
execute either the cpSAVE or cpRESTORE instruction when it is in the user state (S = 0
in the SR). The main processor initiates this exception processing prior to any
communication with the coprocessor associated with the cpSAVE or cpRESTORE
instructions.
If the main processor is executing a coprocessor instruction in the user state when it reads
the supervisor check primitive, it aborts the coprocessor instruction in progress by writing
an abort mask to the control CIR (refer to 7.3.2 Control CIR). The main processor then
performs privilege violation exception processing.
If a privilege violation occurs, the main processor initiates exception processing using the
four-word preinstruction stack frame (refer to Figure 7-41) and the privilege violation
exception vector number 8. Thus, if the exception handler does not modify the stack
frame, the main processor attempts to restart the instruction during which the exception
occurred after it executes an RTE to return from the handler.
7.5.2.4 cpTRAPcc INSTRUCTION TRAPS. If, during the execution of a cpTRAPcc
instruction, the coprocessor returns the TRUE condition indicator to the main processor
with a null primitive, the main processor initiates trap exception processing. The main
processor uses the six-word postinstruction exception stack frame (refer to Figure 7-45)
and the trap exception vector number 7. The scanPC field of this stack frame contains the
address of the instruction following the cpTRAPcc instruction. The processing associated
with the cpTRAPcc instruction can then proceed, and the exception handler can locate
any immediate operand words encoded in the cpTRAPcc instruction using the information
contained in the six-word stack frame. If the exception handler does not modify the stack
frame, the main processor executes the instruction following the cpTRAPcc instruction
after it executes an RTE instruction to exit from the handler.
7.5.2.5 TRACE EXCEPTIONS. The MC68020/EC020 supports two modes of instruction
tracing, as discussed in Section 6 Exception Processing. In the trace on instruction
execution mode, the MC68020/EC020 takes a trace exception after completing each
instruction. In the trace on change of flow mode, the MC68020/EC020 takes a trace
exception after each instruction that alters the SR or places an address other than the
address of the next instruction in the PC.
MOTOROLA M68020 USER’S MANUAL 7-57
The protocol used to execute coprocessor cpSAVE, cpRESTORE, or conditional category
instructions does not change when a trace exception is pending in the main processor.
The main processor performs a pending trace on instruction execution exception after
completing the execution of that instruction. If the main processor is in the trace on
change of flow mode and an instruction places an address other than that of the next
instruction in the PC, the processor takes a trace exception after it executes the
instruction.
If a trace exception is not pending during a general category instruction, the main
processor terminates communication with the coprocessor after reading any primitive with
CA = 0. Thus, the coprocessor can complete a cpGEN instruction concurrently with the
execution of instructions by the main processor. When a trace exception is pending,
however, the main processor must ensure that all processing associated with a cpGEN
instruction has been completed before it takes the trace exception. In this case, the main
processor continues to read the response CIR and to service the primitives until it receives
either a null primitive with CA = 0 and PF = 1 or until exception processing caused by a
take postinstruction exception primitive has completed. The coprocessor should return the
null primitive with CA = 0 and PF = 0 while it is completing the execution of the cpGEN
instruction. The main processor may service pending interrupts between reads of the
response CIR if IA = 1 in these primitives (refer to Table 7-3). This protocol ensures that a
trace exception is not taken until all processing associated with a cpGEN instruction has
completed.
If T1, T0 = 01 in the MC68020/EC020 SR (trace on change of flow mode) when a general
category instruction is initiated, a trace exception is taken for the instruction only when the
coprocessor issues a transfer status register and scanPC primitive with DR = 1 during the
execution of that instruction. In this case, it is possible that the coprocessor is still
executing the cpGEN instruction concurrently when the main processor begins execution
of the trace exception handler. A cpSAVE instruction executed during the trace on change
of flow exception handler could thus suspend the execution of a concurrently operating
cpGEN instruction.
7.5.2.6 INTERRUPTS. Interrupt processing, discussed in Section 6 Exception
Processing, can occur at any instruction boundary. Interrupts are also serviced during the
execution of a general or conditional category instruction under either of two conditions. If
the main processor reads a null primitive with CA = 1 and IA = 1, it services any pending
interrupts prior to reading the response CIR. Similarly, if a trace exception is pending
during cpGEN instruction execution and the main processor reads a null primitive with CA
= 0, IA = 1, and PF = 0 (refer to 7.5.2.5 Trace Exceptions), the main processor services
pending interrupts prior to reading the response CIR again.
The MC68020/EC020 uses the 10-word midinstruction stack frame (see Figure 7-43)
when it services interrupts during the execution of a general or conditional category
coprocessor instruction. Since it uses this stack frame, the main processor can perform all
necessary processing and then return to read the response CIR. Thus, it can continue
execution of the coprocessor instruction during which the interrupt exception occurred.
7-58 M68020 USER’S MANUAL MOTOROLA
The MC68020/EC020 also services interrupts if it reads the not-ready format word from
the save CIR during a cpSAVE instruction. The MC68020/EC020 uses the normal four-
word preinstruction stack frame (see Figure 7-41) when it services interrupts after reading
the not-ready format word. Thus, the processor can service any pending interrupts and
execute an RTE to return and reinitiate the cpSAVE instruction by reading the save CIR.
7.5.2.7 FORMAT ERRORS. The MC68020/EC020 can detect a format error while
executing a cpSAVE or cpRESTORE instruction if the length field of a valid format word is
not a multiple of four bytes. If the MC68020/EC020 reads a format word with an invalid
length field from the save CIR during the cpSAVE instruction, it aborts the coprocessor
instruction by writing an abort mask to the control CIR (refer to 7.3.2 Control CIR) and
initiates format error exception processing. If the MC68020/EC020 reads a format word
with an invalid length field from the effective address specified in the cpRESTORE
instruction, the MC68020/EC020 writes that format word to the restore CIR and then reads
the coprocessor response from the restore CIR. The MC68020/EC020 then aborts the
cpRESTORE instruction by writing an abort mask to the control CIR (refer to 7.3.2
Control CIR) and initiates format error exception processing.
The MC68020/EC020 uses the four-word preinstruction stack frame (see Figure 7-41) and
the format error vector number 14 when it initiates format error exception processing.
Thus, if the exception handler does not modify the stack frame, the main processor, after it
executes an RTE to return from the handler, attempts to restart the instruction during
which the exception occurred.
7.5.2.8 ADDRESS AND BUS ERRORS. Coprocessor-instruction-related bus faults can
occur during main processor bus cycles to CPU space to communicate with a coprocessor
or during memory cycles run as part of the coprocessor instruction execution. If a bus
error occurs during the CIR access that is used to initiate a coprocessor instruction, the
main processor assumes that the coprocessor is not present and takes an F-line emulator
exception as described in 7.5.2.2 F-Line Emulator Exceptions . That is, the processor
takes an F-line emulator exception when a bus error occurs during the initial access to a
CIR by a coprocessor instruction. If a bus error occurs on any other coprocessor access
or on a memory access made during the execution of a coprocessor instruction, the main
processor performs bus error exception processing as described in Section 6 Exception
Processing. After the exception handler has corrected the cause of the bus error, the
main processor can return to the point in the coprocessor instruction at which the fault
occurred.
An address error occurs if the MC68020/EC020 attempts to prefetch an instruction from
an odd address. This can occur if the calculated destination address of a cpBcc or cpDBcc
instruction is odd or if an odd value is transferred to the scanPC with the transfer status
register and the scanPC response primitive. If an address error occurs, the
MC68020/EC020 performs exception processing for a bus fault as described in Section 6
Exception Processing.
MOTOROLA M68020 USER’S MANUAL 7-59
7.5.3 Coprocessor Reset
Either an external reset signal or a RESET instruction can reset the external devices of a
system. The system designer can design a coprocessor to be reset and initialized by both
reset types or by external reset signals only. To be consistent with the MC68020/EC020
design, the coprocessor should be affected by external reset signals only and not by
RESET instructions, because the coprocessor is an extension to the main processor
programming model and to the internal state of the MC68020/EC020.
7.6 COPROCESSOR SUMMARY
Coprocessor instruction formats are included with the instruction formats in the
M68000PM/AD,
M68000 Family Programmer's Reference Manual
.
The M68000 coprocessor response primitive formats are shown in this section. Any
response primitive with bits 13–8 = $00 or $3F causes a protocol violation exception.
Response primitives with bits 13–8 = $0B, $18–$1B, $1F, $28–$2B, and $38–3B currently
cause protocol violation exceptions; they are undefined and reserved for future use by
Motorola.
7-60 M68020 USER’S MANUAL MOTOROLA
15
1
PC
1
14
13
12
0
0
11
10
9
0
7
1
0
2
0
3
0
4
0
5
0
6
0
0
0
8
0
0
1
15
CA
PC
DR
14
13
12
0
0
11
10
9
0
7
0
0
LENGTH
1
8
15
CA
PC
DR
14
13
12
0
0
11
10
9
0
7
0
1
2
0
3
0
4
0
5
0
6
0
0
SP
8
0
0
1
15
1
PC
0
14
13
12
0
0
11
10
9
0
7
1
0
2
0
3
0
4
0
5
0
6
0
0
0
8
0
0
1
15
CA
PC
DR
14
13
12
0
0
11
10
9
0
7
1
1
2
0
3
0
4
0
5
0
6
0
0
0
8
0
0
1
15
CA
PC
0
14
13
12
0
0
11
10
9
0
7
1
1
2
0
3
0
4
0
5
0
6
0
0
1
8
0
0
1
15
CA
PC
0
14
13
12
0
1
11
10
9
0
7
0
0
2
PF
3
0
4
0
5
0
6
0
0
IA
8
0
TF
1
15
CA
PC
0
14
13
12
0
1
11
10
9
0
7
0
1
2
0
3
0
4
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5
0
6
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0
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8
0
0
1
15
CA
PC
DR
14
13
12
0
0
11
10
9
0
7
1
0
LENGTH
1
8
Busy
Transfer Multiple Coprocessor Registers
Transfer Status Register and ScanPC
Supervisor Check
Take Address and Transfer Data
Transfer Multiple Main Processor Registers
Transfer Operation Word
Null
Evaluate and Transfer Effective Address
MOTOROLA M68020 USER’S MANUAL 7-61
15
CA
PC
DR
14
13
12
0
1
11
10
9
0
7
1
1
LENGTH
0
8
15
CA
PC
DR
14
13
12
0
1
11
10
9
0
7
1
0
2
0
3
0
4
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5
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6
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1
8
0
0
1
15
CA
PC
DR
14
13
12
0
1
11
10
9
0
7
1
0
2
REGISTER
3
D
/
A
4
0
5
0
6
0
0
0
8
15
CA
PC
0
14
13
12
0
1
11
10
9
0
7
1
1
LENGTH
1
8
15
CA
PC
DR
14
13
12
1
0
11
10
9
0
7
VALID EA LENGTH
8
15
0
PC
0
14
13
12
1
1
11
10
9
0
7
1
0
V
ECTOR NUMBE
R
0
8
15
0
PC
0
14
13
12
1
1
11
10
9
0
7
1
0
V
ECTOR NUMBE
R
1
8
15
0
PC
0
14
13
12
1
1
11
10
9
0
7
1
1
V
ECTOR NUMBE
R
0
8
15
CA
PC
1
14
13
12
0
0
11
10
9
0
7
0
0
LENGTH
0
8
Transfer Single Main Processor Register
Transfer Main Processor Control Register
Transfer to/from Top of Stack
Transfer from Instruction Stream
Evaluate Effective Address and Transfer Data
Take Preinstruction Exception
Take Midinstruction Exception
Take Postinstruction Exception
Write to Previously Evaluated Effective Address
MOTOROLA M68020 USER’S MANUAL 8-1
SECTION 8
INSTRUCTION EXECUTION TIMING
This section describes the instruction execution and operations (table searches, etc.) of
the MC68020/EC020 in terms of external clock cycles. It provides accurate execution and
operation timing guidelines but not exact timings for every possible circumstance. This
approach is used since exact execution time for an instruction or operation is highly
dependent on memory speeds and other variables. The timing numbers presented in this
section allow the assembly language programmer or compiler writer to predict timings
needed to evaluate the performance of the MC68020/EC020.
In this section, instruction and operation times are shown in clock cycles, which eliminates
clock frequency dependencies.
8.1 TIMING ESTIMATION FACTORS
The advanced architecture of the MC68020/EC020 makes exact instruction timing
calculations difficult due to the effects of:
1. An On-Chip Instruction Cache and Instruction Prefetch
2. Operand Misalignment
3. Bus Controller/Sequence Concurrency
4. Instruction Execution Overlap
These factors make MC68020/EC020 instruction set timing difficult to calculate on a single
instruction basis since instructions vary in execution time from one context to another. A
detailed explanation of each of these factors follows.
8.1.1 Instruction Cache and Prefetch
The on-chip cache of the MC68020/EC020 is an instruction-only cache. Its purpose is to
increase execution efficiency by providing a quick store for instructions.
Instruction prefetches that hit in the cache will occur with no delay in instruction execution.
Instruction prefetches that miss in the cache will cause an external memory cycle to be
performed, which may overlap with internal instruction execution. Thus, while the
execution unit of the microprocessor is busy, the bus controller prefetches the next
instruction from external memory. Both cases are illustrated in later examples.
8-2 M68020 USER’S MANUAL MOTOROLA
When prefetching instructions from external memory, the microprocessor will utilize long-
word read cycles. When the read is aligned on a long-word address boundary, the
processor reads two words, which may load two instructions at once or two words of a
multiword instruction. The subsequent instruction prefetch will find the second word is
already available, and there is no need to run an external bus cycle (read).
The MC68020/EC020 always prefetches long words. When an instruction prefetch falls on
an odd-word boundary (e.g., due to a branch to an odd-word location), the
MC68020/EC020 will read the even word associated with the long-word base address at
the same time as (32-bit memory) or before (8- or 16-bit memory) the odd word is read.
When an instruction prefetch falls on an even-word boundary (as would be the normal
case), the MC68020/EC020 reads both words at the long-word address, thus effectively
prefetching the next two words.
8.1.2 Operand Misalignment
Another significant factor affecting instruction timing is operand misalignment. Operand
misalignment has impact on performance when the microprocessor is reading or writing
external memory. In this case, the address of a word operand falls across a long-word
boundary, or a long-word operand falls on a byte or word address that is not a long-word
boundary. Although the MC68020/EC020 will automatically handle all occurrences of
operand misalignment, it must use multiple bus cycles to complete such transfers.
8.1.3 Bus/Sequencer Concurrency
The bus controller is responsible for all bus activity. The sequencer controls the bus
controller, instruction execution, and internal processor operation, such as calculation of
effective addresses and setting of condition codes.
The bus controller and sequencer can operate on an instruction concurrently. The bus
controller can perform a read or write while the sequencer controls an effective address
calculation or sets the condition codes. The sequencer may also request a bus cycle that
the bus controller cannot immediately perform. In this case, the bus cycle is queued and
the bus controller runs the cycle when the current cycle is complete.
MOTOROLA M68020 USER’S MANUAL 8-3
8.1.4 Instruction Execution Overlap
Overlap is the time, measured in clocks, when two instructions execute concurrently. In
Figure 8-1, instructions A and B execute concurrently, and the overlapped portion of
instruction B is absorbed in the instruction execution time of A (the previous instruction).
The overlap time is deducted from the execution time of instruction B. Similarly, there is an
overlap period between instruction B and instruction C, which reduces the attributed
execution time for C.
INSTRUCTION A
INSTRUCTION B
INSTRUCTION C
OVERLAP
OVERLAP
Figure 8-1. Concurrent Instruction Execution
The execution time attributed to instructions A, B, and C (after considering the overlap) is
depicted in Figure 8-2.
INSTRUCTION A
INSTRUCTION B
INSTRUCTION C
OVERLAP
PERIOD
(ABSORBED BY
I
NSTRUCTION B
)
OVERLAP
PERIOD
(ABSORBED BY
I
NSTRUCTION A
)
Figure 8-2. Instruction Execution for Instruction Timing Purposes
It is possible that the execution time of an instruction will be absorbed by the overlap with
a previous instruction for a net execution time of zero clocks.
Because of this overlap, a NOP is required between a write to a peripheral to clear an
interrupt request and a subsequent MOVE to SR instruction to lower the interrupt mask
level. Otherwise, the MOVE to SR instruction may complete before the write is
accomplished, and a new interrupt exception will be generated for an old interrupt request.
8-4 M68020 USER’S MANUAL MOTOROLA
8.1.5 Instruction Stream Timing Examples
A programming example allows a more detailed examination of these effects. The effect of
instruction execution overlap on instruction timing is illustrated by the following example
instruction stream.
Instruction
#1) MOVE.L D4,(A1)+
#2) ADD.L D4,D5
#3) MOVE.L (A1), –(A2)
#4) ADD.L D5,D6
Example 1
For the first example, the assumptions are:
1. The data bus is 32 bits,
2. The first instruction is prefetched from an odd-word address,
3. Memory access occurs with no wait states, and
4. The instruction cache is disabled.
For example 1, the instruction stream is positioned in 32-bit memory as follows:
Address n ••• MOVE #1
n + 4 ADD #2 MOVE #3
n + 8 ADD #4 •• •
Figure 8-3 shows processor activity on the first example instruction stream. It shows the
activity of the external bus, the bus controller, the sequencer, and the attributed instruction
execution time.
MOTOROLA M68020 USER’S MANUAL 8-5
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
CLOCK
BUS
ACTIVIT
Y
PREFETCH
WRITE
READ
PREFETCH
WRITE
PREFETCH
B
YTES n +
8
BUS
C
ONTROLLE
R
WRITE TO (A1)+
PREFETCH
BYTES n + 12
WRITE TO –(A2)
READ FROM (A1)
IDLE
PERFORM
MOVE #1
IDLE
PERFORM
ADD #2
CALCULATE
S
OURCE E
A
MOVE #3
CALCULATE
D
ESTINATIO
N
EA
MOVE #3
IDLE
PERFORM
MOVE #3
PERFORM
ADD #4
MOVE.L D4,(A1)+
MOVE.L (A1),–(A2)
ADD.L
D5,D
6
SEQUENCER
INSTRUCTION
E
XECUTION TIM
E
(6)
(9)
(1)
CLOCK
C
OUN
T
LEGEND:
1) MOVE.L D4,(A1)+
2) ADD.L D4,D5
3) MOVE.L (A1),–(A2)
4) ADD.L D5,D6
Figure 8-3. Processor Activity for Example 1
For the first three clocks of this example, the bus controller and sequencer are both
performing tasks associated with the MOVE #1 instruction. The next three clocks (clocks
4, 5, and 6) demonstrate instruction overlap. The bus controller is performing a write to
memory as part of the MOVE #1 instruction. The sequencer, on the other hand, is
performing the ADD #2 instruction for two clocks (clocks 4 and 5) and beginning source
effective address (EA) calculations for the MOVE #3 instruction. The bus controller activity
completely overlaps the execution of the ADD #2 instruction, causing the ADD #2
attributed execution time to be zero clocks. The overlap also shortens the effective
execution time of the MOVE #3 instruction by one clock because the bus controller
completes the MOVE #1 write operation while the sequencer begins the MOVE #3 EA
calculation.
The sequencer continues the source EA calculation for one more clock period (clock 7)
while the bus controller begins a read for MOVE #3. When counting instruction execution
time in bus clocks, the MOVE #1 completes at the end of clock 6, and the execution of
MOVE #3 begins on clock 7.
Both the sequencer and bus controller continue with MOVE #3 until the end of clock 14,
when the sequencer begins to perform ADD #4. Timing for MOVE #3 continues because
the bus controller is still performing the write to the destination of MOVE #3. The bus
activity for MOVE #3 completes at the end of clock 15. The effective execution time for
MOVE #3 is nine clocks.
The one clock cycle (clock 15) when the sequencer is performing ADD #4 and the bus
controller is writing to the destination of MOVE #3 is absorbed by the execution time of
MOVE #3. This overlap shortens the effective execution time of ADD #4 by one clock,
giving it an attributed execution time of one clock.
8-6 M68020 USER’S MANUAL MOTOROLA
Example 2
Using the same instruction stream, the second example demonstrates the different effects
of instruction execution overlap on instruction timing when the same instructions are
positioned slightly differently in 32-bit memory:
Address n MOVE #1 ADD #2
n + 4 MOVE #3 ADD #4
n + 8 ••• •••
The assumptions for example 2 (see Figure 8-4) are:
1. The data bus is 32 bits,
2. The first instruction is prefetched from an even-word address,
3. Memory access occurs with no wait states, and
4. The cache is disabled.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
CLOCK
BUS
ACTIVIT
Y
PREFETCH
WRITE
READ
PREFETCH
WRITE
PREFETCH
BYTES n + 1
2
BUS
C
ONTROLLE
R
WRITE TO (A1)+
PREFETCH
BYTES n + 8
WRITE TO –(A2)
READ FROM (A1)
IDLE
PERFORM
MOVE #1
PERFORM
ADD #2
CALCULATE
S
OURCE E
A
MOVE #3
CALCULATE
D
ESTINATIO
N
EA
MOVE #3
PERFORM
MOVE #3
MOVE.L D4,(A1)+
MOVE.L (A1),–(A2)
ADD.L D5,D6
SEQUENCER
INSTRUCTION
E
XECUTION TIM
E
(4)
(3)
(3)
CLOCK
COUNTE
R
LEGEND:
1) MOVE.L D4,(A1)+
2) ADD.L D4,D5
3) MOVE.L (A1),–(A2)
4) ADD.L D5,D6
IDLE
NEXT
I
NSTRUCTIO
N
ADD.L D4,D5
(6)
PERFORM
ADD #4
IDLE
Figure 8-4. Processor Activity for Example 2
Although the total execution time of the instruction segment does not change in this
example, the individual instruction times are significantly different. This example
demonstrates that the effects of overlap are not only instruction-sequence dependent but
are also dependent upon the alignment of the instruction stream in memory.
MOTOROLA M68020 USER’S MANUAL 8-7
Example 3
Both Figures 8-3 and 8-4 show instruction execution without benefit of the
MC68020/EC020 instruction cache. Figure 8-5 shows a third example for the same
instruction stream executing in the cache. Note that once the instructions are in the cache,
the original location in external memory is no longer a factor in timing.
The assumptions for Example 3 are:
1. The data bus is 32 bits,
2. The cache is enabled and instructions are in the cache, and
3. Memory access occurs with no wait states.
1
2
3
4
5
6
7
8
9
10
11
12
13
CLOCK
BUS
ACTIVIT
Y
WRITE
READ
WRITE
BUS
C
ONTROLLE
R
WRITE TO (A1)+
WRITE TO –(A2)
READ FROM (A1)
IDLE
PERFORM
MOVE #1
PERFORM
ADD #2
CALCULATE
S
OURCE E
A
MOVE #3
CALCULATE
D
ESTINATIO
N
EA
MOVE #3
PERFORM
MOVE #3
MOVE.L D4,(A1)+
MOVE.L (A1),–(A2)
ADD.L
D5,D
6
SEQUENCER
INSTRUCTION
E
XECUTION TIM
E
(4)
(1)
CLOCK
COUNTE
R
LEGEND:
1) MOVE.L D4,(A1)+
2) ADD.L D4,D5
3) MOVE.L (A1),–(A2)
4) ADD.L D5,D6
(7)
PERFORM
ADD #4
IDLE
IDLE
Figure 8-5. Processor Activity for Example 3
Figure 8-5 illustrates the benefits of the instruction cache. The total number of clock cycles
is reduced from 16 to 12 clocks. Since the instructions are resident in the cache, the
instruction prefetch activity does not require the bus controller to perform external bus
cycles. Since prefetch occurs with no delay, the bus controller is idle more often.
Example 4
Idle clock cycles, such as those shown in example 3, are useful in MC68020/EC020
systems that require wait states when accessing external memory. This fact is illustrated
in example 4 (see Figure 8-6) with the following assumptions:
1. The data bus is 32 bits,
2. The cache is enabled and instructions are in the cache, and
3. Memory access occurs with one wait state.
8-8 M68020 USER’S MANUAL MOTOROLA
1
2
3
4
5
6
7
8
9
10
11
12
13
14
CLOCK
BUS
ACTIVIT
Y
WRITE
READ
WRITE
BUS
C
ONTROLLE
R
WRITE TO (A1)+
WRITE TO –(A2)
READ FROM (A1)
IDLE
PERFORM
MOVE #1
PERFORM
ADD #2
CALCULATE
S
OURCE E
A
MOVE #3
PERFORM
MOVE #3
MOVE.L D4,(A1)+
MOVE.L (A1),–(A2)
SEQUENCER
INSTRUCTION
E
XECUTION TIM
E
(5)
CLOCK
COUNTE
R
LEGEND:
1) MOVE.L D4,(A1)+
2) ADD.L D4,D5
3) MOVE.L (A1),–(A2)
4) ADD.L D5,D6
IDLE
PERFORM
ADD #4
(8)
CALCULATE
D
ESTINATIO
N
EA
MOVE #3
Figure 8-6. Processor Activity for Example 4
Figure 8-6 shows the same instruction stream executing with four clocks for every read
and write. The idle bus cycles coincide with the wait states of the memory access;
therefore, the total execution time is only 13 clocks.
Examples 1–4 demonstrate the complexity of instruction timing calculation for the
MC68020/EC020. It is impossible to anticipate individual instruction timing as an absolute
number of clock cycles due to the dependency of overlap on the instruction sequence and
alignment as well as the number of wait states in memory. This can be seen by comparing
individual and composite time for Figures 8-3 through 8-6. These instruction timings are
compared in Table 8-1, where timing varies for each instruction as the context varies.
Table 8-1. Examples 1–4 Instruction Stream Execution Comparison
Instruction Example 1
(Odd Alignment) Example 2
(Even Alignment) Example 3
(Cache)
Example 4
(Cache with
Wait States)
#1) MOVE.L
#2) ADD.L
#3) MOVE.L
#4) ADD.L
D4,(A1)+
D4,D5
(A1),–(A2)
D5,D6
6
0
9
1
4
3
6
3
4
0
7
1
5
0
8
0
Total Clock Cycles 16 16 12 13
MOTOROLA M68020 USER’S MANUAL 8-9
8.2 INSTRUCTION TIMING TABLES
The instruction times given in the following illustration include the following assumptions
about the MC68020/EC020 system:
1. All operands are long-word aligned as is the stack,
2. The data bus is 32 bits, and
3. Memory access occurs with no wait states (three-cycle read/write).
There are three values given for each instruction and addressing mode:
1. The best case (BC), which reflects the time (in clocks) when the instruction is in the
cache and benefits from maximum overlap due to other instructions,
2. The cache-only case (CC) when the instruction is in the cache but has no overlap,
and
3. The worst case (WC) when the instruction is not in the cache or the cache is
disabled and there is no instruction overlap.
The only instances for which the size of the operand has any effect are the instructions
with immediate operands. Unless specified otherwise, immediate byte and word operands
have identical execution times.
Within each set or column of instruction timings are four sets of numbers, three of which
are enclosed in parentheses. The bolded outer number is the total number of clocks for
the instruction. The first number inside the parentheses is the number of operand read
cycles performed by the instruction. The second value inside parentheses is the number
of instruction accesses performed by the instruction, including all prefetches to keep the
instruction pipe filled. The third value within parentheses is the number of write cycles
performed by the instruction. One example from the instruction timing table is:
TOTAL NUMBER OF CLOCKS
NUMBER OF READ CYCLE
S
NUMBER OF INSTRUCTION ACCESS CYCLE
S
NUMBER OF WRITE CYCLE
S
24
(2
3
0)
/
/
The total number of bus-activity clocks for the previous example is derived in the following
way:
(2 Reads * 3 Clocks/Read) + (3 Instruction Accesses * 3 Clocks/Access)
+ (0 Writes * 3 Clocks/Write) = 15 Clocks of Bus Activity
24 Total Clocks – 15 Clocks (Bus Activity) = 9 Internal Clocks
The example used here was taken from a worst-case fetch effective address time. The
addressing mode was ([d32,B],I,d32). The same addressing mode under the best-case
8-10 M68020 USER’S MANUAL MOTOROLA
entry is 17 (2/0/0). For the best case, there are no instruction accesses because the cache
is enabled and the sequencer does not have to go to external memory for the instruction
words.
The first tables deal exclusively with fetching and calculating effective addresses and
immediate operands. The tables are arranged in this manner because some instructions
do not require effective address calculation or fetching. For example, the instruction CLR
<ea> (found in the table under 8.2.11 Single Operand Instructions) only needs to have a
calculated effective address time added to its table entry because no fetch of an operand
is required. This instruction only writes to memory or a register. Some instructions use
specific addressing modes which exclude timing for calculation or fetching of an operand.
When these instances arise, they are footnoted to indicate which other tables are needed
in the timing calculation.
Many two-word instructions (e.g., MULU.L, DIV.L, BFSET, etc.) include the fetch
immediate effective address time or the calculate immediate effective address time in the
execution time calculation. The timing for immediate data of word length (# <data>.W) is
used for these calculations. If the instruction has a source and a destination, the source
effective address is used for the table lookup. If the instruction is single operand, the
effective address of that operand is used.
The following example includes multiword instructions that refer to the fetch immediate
effective address and calculate immediate effective address tables in 8.2 Instruction
Timing Tables.
Instruction
#1) MULU.L D7,D1:D2
#2) BFCLR $6000{0:8}
#3) DIVS.L #$10000,D3:D4
CC
1.MULU.L (D7),D1:D2
#<data>.W,Dn 2
MUL.L EA,Dn 43
2.BFCLR $6000{0:8}
#<data>.W.,$XXX.W 5
BFCLR Mem (<5 bytes) 16
3.DIVS.L #$10000,D3:D4
#<data>.W,#<data>.L 6
DIVS.L EA, Dn 90
Execution time = 2 + 43 + 5 + 16 + 6 + 90
= 102 clock periods
MOTOROLA M68020 USER’S MANUAL 8-11
NOTE
This CC time is a maximum since the times given for the
MULU.L and DIVS.L are maximums.
The MOVE instruction timing tables include all necessary timing for extension word fetch,
address calculation, and operand fetch.
The instruction timing tables are used to calculate a best-case and worst-case bounds for
some target instruction stream. Calculating exact timing from the timing tables is
impossible because the tables cannot anticipate how the combination of factors will
influence every particular sequence of instructions. This is illustrated by comparing the
observed instruction timing from the prior four examples with instruction timing derived
from the instruction timing tables.
Table 8-2 lists the original instruction stream and the corresponding clock timing from the
appropriate timing tables for the best case, cache-only case, and worst case.
Table 8-2. Instruction Timings from Timing Tables
Instruction Best Case Cache Case Worst Case
#1) MOVE.L
#2) ADD.L
#3) MOVE.L
#4) ADD.L
D4,(A1)+
D4,D5
(A1),–(A2)
D5,D6
4
0
6
0
4
2
7
2
6
3
9
3
Total 10 15 21
Table 8-3 summarizes the observed instruction timings for the same instruction stream as
executed according to the assumptions of the four examples. For each example, Table 8-
3 shows which entry (BC/CC/WC) from the timing tables corresponds to the observed
timing for each of the four instructions. Some of the observed instruction timings cannot be
found in the timing tables and appear in Table 8-3 within parentheses in the most
appropriate column. These timings occur when instruction execution overlap dynamically
alters what would otherwise be a BC, CC, or WC timing.
Table 8-3. Observed Instruction Timings
Example 1 Example 2 Example 3 Example 4
Instruction BC CC WC BC CC WC BC CC WC BC CC WC
#1) MOVE.L
#2) ADD.L
#3) MOVE.L
#4) ADD.L
D4,(A1)+
D4,D5
(A1),–(A2)
D5,D6
0
(1)
6
9
4
63
3
0
(1)
4
70
0
(5)
(8)
Total (16) (16) (12) (13)
8-12 M68020 USER’S MANUAL MOTOROLA
Comparing Tables 8-2 and 8-3 demonstrates that calculation of instruction timing cannot
be a simple lookup of only BC or only WC timings. Even when the assumptions are known
and fixed, as in the four examples summarized in Table 8-3, the microprocessor can
sometimes achieve best-case timings under worst-case assumptions.
Looking across the four examples in Table 8-3 for an individual instruction, it is difficult to
predict which timing table entry is used, since the influence of instruction overlap may or
may not improve the BC, WC, or CC timings. When looking at the observed instruction
timings for one example, it is also difficult to determine which combination of BC/CC/WC
timing is required. Just how the instruction stream will fit and run with the cache enabled,
how instructions are positioned in memory, and the degree of instruction overlap are
factors that are impossible to account for in all combinations of the timing tables.
Although the timing tables cannot accurately predict the instruction timing that would be
observed when executing an instruction stream on the MC68020/EC020, the tables can
be used to calculate best-case and worst-case bounds for instruction timing. Absolute
instruction timing must be measured by using the microprocessor itself to execute the
target instruction stream.
MOTOROLA M68020 USER’S MANUAL 8-13
8.2.1 Fetch Effective Address
The fetch effective address table indicates the number of clock periods needed for the
processor to calculate and fetch the specified effective address. The total number of clock
cycles is outside the parentheses; the number of read, prefetch, and write cycles is given
inside the parentheses as (r/p/w). These cycles are included in the total clock cycle
number.
Address Mode Best Case Cache Case Worst Case
Dn 0(0/0/0) 0(0/0/0) 0(0/0/0)
An 0(0/0/0) 0(0/0/0) 0(0/0/0)
(An) 3(1/0/0) 4(1/0/0) 4(1/0/0)
(An)+ 4(1/0/0) 4(1/0/0) 4(1/0/0)
–(An) 3(1/0/0) 5(1/0/0) 5(1/0/0)
(d 16 ,An) of (d16,PC) 3(1/0/0) 5(1/0/0) 6(1/1/0)
(xxx).W 3(1/0/0) 4(1/0/0) 6(1/1/0)
(xxx).L 3(1/0/0) 4(1/0/0) 7(1/1/0)
#<data>.B 0(0/0/0) 2(0/0/0) 3(0/1/0)
#<data>.W 0(0/0/0) 2(0/0/0) 3(0/1/0)
#<data>.L 0(0/0/0) 4(0/0/0) 5(0/1/0)
(d 8,An,Xn) or (d8,PC,Xn) 4(1/0/0) 7(1/0/0) 8(1/1/0)
(d 16 ,An,Xn) or (d16,PC,Xn) 4(1/0/0) 7(1/0/0) 9(1/1/0)
(B) 4(1/0/0) 7(1/0/0) 9(1/1/0)
(d 16 ,B) 6(1/0/0) 9(1/0/0) 12(1/1/0)
(d 32 ,B) 10(1/0/0) 13(1/0/0) 16(1/2/0)
([B],I) 9(2/0/0) 12(2/0/0) 13(2/1/0)
([B],I,d16 )11(2/0/0) 14(2/0/0) 16(2/1/0)
([B],I,d32 )11(2/0/0) 14(2/0/0) 17(2/2/0)
([d16,B],I) 11(2/0/0) 14(2/0/0) 16(2/1/0)
([d16,B],I,d16 )13(2/0/0) 16(2/0/0) 19(2/2/0)
([d16,B],I,d32 )13(2/0/0) 16(2/0/0) 20(2/2/0)
([d32,B],I) 15(2/0/0) 18(2/0/0) 20(2/2/0)
([d32,B],I,d16 )17(2/0/0) 20(2/0/0) 22(2/2/0)
([d32,B],I,d32 )17(2/0/0) 20(2/0/0) 24(2/3/0)
B = Base address; 0, An, PC, Xn, An + Xn. Form does not affect timing.
I = Index; 0, Xn
NOTE: Xn cannot be in B and I at the same time. Scaling and size of Xn do not affect timing.
8-14 M68020 USER’S MANUAL MOTOROLA
8.2.2 Fetch Immediate Effective Address
The fetch immediate effective address table indicates the number of clock periods needed
for the processor to fetch the immediate source operand and calculate and fetch the
specified destination operand. The total number of clock cycles is outside the
parentheses; the number of read, prefetch, and write cycles is given inside the
parentheses as (r/p/w). These cycles are included in the total clock cycle number.
Address Mode Best Case Cache Case Worst Case
#<data>.W,Dn 0(0/0/0) 2(0/0/0) 3(0/1/0)
#<data>.L,Dn 1(0/0/0) 4(0/0/0) 5(0/1/0)
#<data>.W,(An) 3(1/0/0) 4(1/0/0) 4(1/1/0)
#<data>.L,(An) 3(1/0/0) 4(1/0/0) 7(1/1/0)
#<data>.W,(An)+ 4(1/0/0) 6(1/0/0) 7(1/1/0)
#<data>.L,(An)+ 5(1/0/0) 8(1/0/0) 9(1/1/0)
#<data>.W,–(An) 3(1/0/0) 5(1/0/0) 6(1/1/0)
#<data>.L,–(An) 4(1/0/0) 7(1/0/0) 8(1/1/0)
#<data>.W,(bd,An) 3(1/0/0) 5(1/0/0) 7(1/1/0)
#<data>.L,(bd,An) 4(1/0/0) 7(1/0/0) 10(1/2/0)
#<data>.W,xxx.W 3(1/0/0) 5(1/0/0) 7(1/1/0)
#<data>.L,xxx.W 4(1/0/0) 7(1/0/0) 10(1/2/0)
#<data>.W,xxx.L 3(1/0/0) 6(1/0/0) 10(1/2/0)
#<data>.L,xxx.L 4(1/0/0) 8(1/0/0) 12(1/2/0)
#<data>.W,#<data>.B,W 0(0/0/0) 4(0/0/0) 6(0/2/0)
#<data>.L,#<data>.B,W 1(0/0/0) 6(0/0/0) 8(0/2/0)
#<data>.W,#<data>.L 0(0/0/0) 6(0/0/0) 8(0/2/0)
#<data>.L,#(data>.L 1(0/0/0) 8(0/0/0) 10(0/2/0)
#<data>.W,(d8,An,Xn) or (d8,PC,Xn) 4(1/0/0) 9(1/0/0) 11(1/2/0)
#<data>.L,(d8,An,Xn) or (d8,PC,Xn) 5(1/0/0) 11(1/0/0) 13(1/2/0)
#<data>.W,(d16,An,Xn) or (d16,PC,Xn) 4(1/0/0) 9(1/0/0) 12(1/2/0)
#<data>.L,(d16,An,Xn) or (d16,PC,Xn) 5(1/0/0) 11(1/0/0) 15(1/2/0)
#<data>.W,(B) 4(1/0/0) 9(1/0/0) 12(1/1/0)
#<data>.L,(B) 5(1/0/0) 11(1/0/0) 14(1/2/0)
#<data>.W,(bd,PC) 10(1/0/0) 15(1/0/0) 19(1/3/0)
#<data>.L,(bd,PC) 11(1/0/0) 17(1/0/0) 21(1/3/0)
#<data>.W,(d16,B) 6(1/0/0) 11(1/0/0) 15(1/2/0)
#<data>.L,(d16,B) 7(1/0/0) 13(1/0/0) 17(1/2/0)
#<data>.W,(d32,B) 10(1/0/0) 15(1/0/0) 19(1/3/0)
#<data>.L,(d32,B) 11(1/0/0) 17(1/0/0) 21(1/3/0)
#<data>.W,([B],I) 9(2/0/0) 14(2/0/0) 16(2/2/0)
#<data>.L,([B],I) 10(2/0/0) 16(2/0/0) 18(2/2/0)
#<data>.W,([B],I,d16)11(2/0/0) 16(2/0/0) 19(2/2/0)
#<data>.L,([B],I,d16)12(2/0/0) 18(2/0/0) 21(2/2/0)
MOTOROLA M68020 USER’S MANUAL 8-15
Address Mode Best Case Cache Case Worst Case
#<data>.W,([B],I,d32)11(2/0/0) 16(2/0/0) 20(2/2/0)
#<data>.L,([d16,B],I,d32)12(2/0/0) 18(2/0/0) 22(2/3/0)
#<data>.W,([d16,B],I) 11(2/0/0) 16(2/0/0) 19(2/2/0)
#<data>.L,([d16,B],I) 12(2/0/0) 18(2/0/0) 21(2/2/0)
#<data>.W,([d16,B],I,d16)13(2/0/0) 18(2/0/0) 22(2/2/0)
#<data>.L,([d16,B],I,d16)14(2/0/0) 20(2/0/0) 24(2/3/0)
#<data>.W,([d32,B],I) 15(2/0/0) 20(2/0/0) 23(2/3/0)
#<data>.L,([d32,B],I) 16(2/0/0) 22(2/0/0) 25(2/3/0)
#<data>.W,([d32,B],I,d16)17(2/0/0) 22(2/0/0) 25(2/3/0)
#<data>.L,([d32,B],I,d16)18(2/0/0) 24(2/0/0) 27(2/3/0)
#<data>.W,([d32,b],I,d32)17(2/0/0) 22(2/0/0) 27(2/3/0)
#<data>.L,([d32,b],I,d32)18(2/0/0) 24(2/0/0) 29(2/4/0)
B = Base address; 0, An, PC, Xn, An + Xn. Form does not affect timing.
I = Index; 0, Xn
NOTE: Xn cannot be in B and I at the same time. Scaling and size of Xn do not affect timing.
8-16 M68020 USER’S MANUAL MOTOROLA
8.2.3 Calculate Effective Address
The calculate immediate effective address table indicates the number of clock periods
needed for the processor to calculate the specified effective address. Fetch time is only
included for the first level of indirection on memory indirect addressing modes. The total
number of clock cycles is outside the parentheses; the number of read, prefetch, and write
cycles is given inside the parentheses as (r/p/w). These cycles are included in the total
clock cycle number.
Address Mode Best Case Cache Case Worst Case
Dn 0(0/0/0) 0(0/0/0) 0(0/0/0)
An 0(0/0/0) 0(0/0/0) 0(0/0/0)
(An) 2(0/0/0) 2(0/0/0) 2(0/0/0)
(An)+ 2(0/0/0) 2(0/0/0) 2(0/0/0)
–(An) 2(0/0/0) 2(0/0/0) 2(0/0/0)
(d 16,An) or (d16,PC) 2(0/0/0) 2(0/0/0) 3(0/1/0)
<data>.W 2(0/0/0) 2(0/0/0) 3(0/1/0)
<data>.L 1(0/0/0) 4(0/0/0) 5(0/1/0)
(d 8,An,Xn) or (d8,PC,Xn) 1(0/0/0) 4(0/0/0) 5(0/1/0)
(d 16 ,An,Xn) or (d16,PC,Xn) 3(0/0/0) 6(0/0/0) 7(0/1/0)
(B) 3(0/0/0) 6(0/0/0) 7(0/1/0)
(d 16 ,B) 5(0/0/0) 8(0/0/0) 10(0/1/0)
(d 32 ,B) 9(0/0/0) 12(0/0/0) 15(0/2/0)
([B],I) 8(1/0/0) 11(1/0/0) 12(1/1/0)
([B],I,d16 )10(1/0/0) 13(1/0/0) 15(1/1/0)
([B],I,d32 )10(1/0/0) 13(1/0/0) 16(1/2/0)
([d16,B],I) 10(1/0/0) 13(1/0/0) 15(1/1/0)
([d16,B],I,d16 )12(1/0/0) 15(1/0/0) 18(1/2/0)
([d16,B],I,d32 )12(1/0/0) 15(1/0/0) 19(1/2/0)
([d32,B],I) 14(1/0/0) 17(1/0/0) 19(1/2/0)
([d32,B],I,d16 )16(1/0/0) 19(1/0/0) 21(1/2/0)
([d32,B],I,d32 )16(1/0/0) 19(1/0/0) 24(1/3/0)
B = Base address; 0, An, PC, Xn, An + Xn. Form does not affect timing.
I = Index; 0, Xn
NOTE: Xn cannot be in B and I at the same time. Scaling and size of Xn do not affect timing.
MOTOROLA M68020 USER’S MANUAL 8-17
8.2.4 Calculate Immediate Effective Address
The calculate immediate effective address table indicates the number of clock periods
needed for the processor to fetch the immediate source operand and calculate the
specified destination effective address. Fetch time is only included for the first level of
indirection on memory indirect addressing modes. The total number of clock cycles is
outside the parentheses; the number of read, prefetch, and write cycles is given inside the
parentheses as (r/p/w). These cycles are included in the total clock cycle number.
Address Mode Best Case Cache Case Worst Case
#<data>.W,Dn 0(0/0/0) 2(0/0/0) 3(0/1/0)
#<data>.L,Dn 1(0/0/0) 4(0/0/0) 5(0/1/0)
#<data>.W,(An) 0(0/0/0) 2(0/0/0) 3(0/1/0)
#<data>.L,(An) 1(0/0/0) 4(0/0/0) 5(0/1/0)
#<data>.W,(An)+ 2(0/0/0) 4(0/0/0) 5(0/1/0)
#<data>.L,(An)+ 3(0/0/0) 6(0/0/0) 7(0/1/0)
#<data>.W,(bd,An) 1(0/0/0) 4(0/0/0) 5(0/1/0)
#<data>.L,(bd,An) 3(0/0/0) 6(0/0/0) 8(0/2/0)
#<data>.W,xxx.W 1(0/0/0) 4(0/0/0) 5(0/1/0)
#<data>.L,xxx.W 3(0/0/0) 6(0/0/0) 8(0/2/0)
#<data>.W,xxx.L 2(0/0/0) 4(0/0/0) 6(0/2/0)
#<data>.L,xxx.L 3(0/0/0) 8(0/0/0) 10(0/2/0)
#<data>.W,(d8,An,Xn) or (d8,PC,Xn) 0(0/0/0) 6(0/0/0) 8(0/2/0)
#<data>.L,(d8,An,Xn) or (d8,PC,Xn) 2(0/0/0) 8(0/0/0) 10(0/2/0)
#<data>.W,(d16,An,Xn) or (d16,PC,Xn) 3(0/0/0) 8(0/0/0) 10(0/2/0)
#<data>.L,(d16,An,Xn) or (d16,PC,Xn) 4(0/0/0) 10(0/0/0) 12(0/2/0)
#<data>.W,(B) 3(0/0/0) 8(0/0/0) 10(0/1/0)
#<data>.L,(B) 4(0/0/0) 10(0/0/0) 12(0/2/0)
#<data>.W,(bd,PC) 9(0/0/0) 14(0/0/0) 18(0/3/0)
#<data>.L,(bd,PC) 10(0/0/0) 16(0/0/0) 20(0/3/0)
#<data>.W,(d16,B) 5(0/0/0) 10(0/0/0) 13(0/2/0)
#<data>.L,(d16,B) 6(0/0/0) 12(0/0/0) 15(0/2/0)
#<data>.W,(d32,B) 9(0/0/0) 14(0/0/0) 18(0/2/0)
#<data>.L,(d32,B) 10(0/0/0) 16(0/0/0) 20(0/3/0)
#<data>.W,([B],I) 8(1/0/0) 13(1/0/0) 15(1/2/0)
#<data>.L,([B],I) 9(1/0/0) 15(1/0/0) 17(1/2/0)
#<data>.W,([B],I,d16 )10(1/0/0) 15(1/0/0) 18(1/2/0)
#<data>.L,([B],I,d16)11(1/0/0) 17(1/0/0) 20(1/2/0)
#<data>.W,([B],I,d32 )10(1/0/0) 15(1/0/0) 19(1/2/0)
#<data>.L,([d16,B],I,d32 )11(1/0/0) 17(1/0/0) 21(1/3/0)
#<data>.W,([d16 ,B],I) 10(1/0/0) 15(1/0/0) 18(1/2/0)
#<data>.L,([d16,B],I) 11(1/0/0) 17(1/0/0) 20(1/2/0)
#<data>.W,([d16 ,B],I,d16 )12(1/0/0) 17(1/0/0) 21(1/2/0)
8-18 M68020 USER’S MANUAL MOTOROLA
Address Mode Best Case Cache Case Worst Case
#<data>.L,([d16,B],I,d16 )13(1/0/0) 19(1/0/0) 23(1/3/0)
#<data>.([d16 ,B],I,d32 )12(1/0/0) 17(1/0/0) 22(1/3/0)
#<data>.([d16 ,B],I,d32 )13(1/0/0) 19(1/0/0) 24(1/3/0)
#<data>.W,([d32 ,B],I) 14(1/0/0) 19(1/0/0) 22(1/3/0)
#<data>.L,([d32,B],I) 15(1/0/0) 21(1/0/0) 24(1/3/0)
#<data>.W,([d32 ,B],I,d16 )16(1/0/0) 21(1/0/0) 24(1/3/0)
#<data>.L,([d32,B],I,d16 )17(1/0/0) 23(1/0/0) 26(1/3/0)
#<data>.W,([d32 ,B],I,d32 )16(1/0/0) 21(1/0/0) 24(1/3/0)
#<data>.L,([d32,B],I,d32 )17(1/0/0) 23(1/0/0) 29(1/4/0)
B = Base address; 0, An, PC, Xn, An + Xn. Form does not affect timing.
I = Index; 0, Xn
NOTE: Xn cannot be in B and I at the same time. Scaling and size of Xn do not affect timing.
MOTOROLA M68020 USER’S MANUAL 8-19
8.2.5 Jump Effective Address
The jump effective address table indicates the number of clock periods needed for the
processor to calculate the specified effective address. Fetch time is only included for the
first level of indirection on memory indirect addressing modes. The total number of clock
cycles is outside the parentheses; the number or read, prefetch, and write cycles is given
inside the parentheses as (r/p/w). These cycles are included in the total clock cycle
number.
Address Mode Best Case Cache Case Worst Case
(An) 0(0/0/0) 2(0/0/0) 2(0/0/0)
(d 16 ,An) 1(0/0/0) 4(0/0/0) 4(0/0/0)
(xxx).W 0(0/0/0) 2(0/0/0) 2(0/0/0)
(xxx).L 0(0/0/0) 2(0/0/0) 2(0/0/0)
(d 8,An,Xn) or (d8,PC,Xn) 3(0/0/0) 6(0/0/0) 6(0/0/0)
(d 16 ,An,Xn) or (d16,PC,Xn) 3(0/0/0) 6(0/0/0) 6(0/0/0)
(B) 3(0/0/0) 6(0/0/0) 6(0/0/0)
(B,d16 )5(0/0/0) 8(0/0/0) 8(0/1/0)
(B,d32 )9(0/0/0) 12(0/0/0) 12(0/1/0)
([B],I) 8(1/0/0) 11(1/0/0) 11(1/1/0)
([B],I,d16 )10(1/0/0) 13(1/0/0) 14(1/1/0)
([B],I,d32 )10(1/0/0) 13(1/0/0) 14(1/1/0)
([d16,B],I) 10(1/0/0) 13(1/0/0) 14(1/1/0)
([d16,B],I,d16 )12(1/0/0) 15(1/0/0) 17(1/1/0)
([d16,B],I,d32 )12(1/0/0) 15(1/0/0) 17(1/1/0)
([d32,B],I) 14(1/0/0) 17(1/0/0) 19(1/2/0)
([d32,B],I,d16 )16(1/0/0) 19(1/0/0) 21(1/2/0)
([d32,B],I,d32 )16(1/0/0) 19(1/0/0) 23(1/2/0)
B = Base address; 0, An, PC, Xn, An + Xn, PC + Xn. Form does not affect timing.
I = Index; 0, Xn
NOTE: Xn cannot be in B and I at the same time. Scaling and size of Xn do not affect timing.
8-20 M68020 USER’S MANUAL MOTOROLA
8.2.6 MOVE Instruction
The MOVE instruction table indicates the number of clock periods needed for the
processor to fetch, calculate, and perform the MOVE or MOVEA with the specified source
and destination effective addresses, including both levels of indirection on memory indirect
addressing modes. No additional tables are needed to calculate the total effective
execution time for the MOVE or MOVEA instruction. The total number of clock cycles is
outside the parentheses; the number of read, prefetch, and write cycles is given inside the
parentheses as (r/p/w). These cycles are included in the total clock cycle number.
BEST CASE
Destination
Source Address Mode An Dn (An) (An)+ –(An) (d16 ,An) (xxx).W (xxx).L
Rn 0(0/0/0) 0(0/0/0) 3(0/0/1) 4(0/0/1) 3(0/0/1) 3(0/0/1) 3(0/0/1) 5(0/0/1)
#<data>.B,W 0(0/0/0) 0(0/0/0) 3(0/0/1) 4(0/0/1) 3(0/0/1) 3(0/0/1) 3(0/0/1) 5(0/0/1)
#<data>.L 0(0/0/0) 0(0/0/0) 3(0/0/1) 4(0/0/1) 3(0/0/1) 3(0/0/1) 3(0/0/1) 5(0/0/1)
(An) 3(1/0/0) 3(1/0/0) 6(1/0/1) 6(1/0/1) 6(1/0/1) 6(1/0/1) 6(1/0/1) 8(1/0/1)
(An)+ 4(1/0/0) 4(1/0/0) 7(1/0/1) 7(1/0/1) 7(1/0/1) 7(1/0/1) 7(1/0/1) 9(1/0/1)
–(An) 3(1/0/0) 3(1/0/0) 6(1/0/1) 6(1/0/1) 6(1/0/1) 6(1/0/1) 6(1/0/1) 8(1/0/1)
(d 16 ,An) or (d16,PC) 3(1/0/0) 3(1/0/0) 6(1/0/1) 6(1/0/1) 6(1/0/1) 6(1/0/1) 6(1/0/1) 8(1/0/1)
(xxx).W 3(1/0/0) 3(1/0/0) 6(1/0/1) 6(1/0/1) 6(1/0/1) 6(1/0/1) 6(1/0/1) 8(1/0/1)
(xxx).L 3(1/0/0) 3(1/0/0) 6(1/0/1) 6(1/0/1) 6(1/0/1) 6(1/0/1) 6(1/0/1) 8(1/0/1)
(d 8,An,Xn) or (d8,PC,Xn) 4(1/0/0) 4(1/0/0) 7(1/0/1) 7(1/0/1) 7(1/0/1) 7(1/0/1) 7(1/0/1) 9(1/0/1)
(d 16 ,An,Xn) or (d16,PC,Xn) 4(1/0/0) 4(1/0/0) 7(1/0/1) 7(1/0/1) 7(1/0/1) 7(1/0/1) 7(1/0/1) 9(1/0/1)
(B) 4(1/0/0) 4(1/0/0) 7(1/0/1) 7(1/0/1) 7(1/0/1) 7(1/0/1) 7(1/0/1) 9(1/0/1)
(d 16 ,B) 6(1/0/0) 6(1/0/0) 9(1/0/1) 9(1/0/1) 9(1/0/1) 9(1/0/1) 9(1/0/1) 11(1/0/1)
(d 32 ,B) 10(1/0/0) 10(1/0/0) 13(1/0/1) 13(1/0/1) 13(1/0/1) 13(1/0/1) 13(1/0/1) 15(1/0/1)
([B],I) 9(2/0/0) 9(2/0/0) 12(2/0/1) 12(2/0/1) 12(2/0/1) 12(2/0/1) 12(2/0/1) 14(2/0/1)
([B],I,d16 )11(2/0/0) 11(2/0/0) 14(2/0/1) 14(2/0/1) 14(2/0/1) 14(2/0/1) 14(2/0/1) 16(2/0/1)
([B],I,d32 )11(2/0/0) 11(2/0/0) 14(2/0/1) 14(2/0/1) 14(2/0/1) 14(2/0/1) 14(2/0/1) 16(2/0/1)
([d16,B],I) 11(2/0/0) 11(2/0/0) 14(2/0/1) 14(2/0/1) 14(2/0/1) 14(2/0/1) 14(2/0/1) 16(2/0/1)
([d16,B],I,d16 )13(2/0/0) 13(2/0/0) 16(2/0/1) 16(2/0/1) 16(2/0/1) 16(2/0/1) 16(2/0/1) 18(2/0/1)
([d16,B],d32 )13(2/0/0) 13(2/0/0) 16(2/0/1) 16(2/0/1) 16(2/0/1) 16(2/0/1) 16(2/0/1) 18(2/0/1)
([d32,B],I) 15(2/0/0) 15(2/0/0) 18(2/0/1) 18(2/0/1) 18(2/0/1) 18(2/0/1) 18(2/0/1) 20(2/0/1)
([d32,B],I,d16 )17(2/0/0) 17(2/0/0) 20(2/0/1) 20(2/0/1) 20(2/0/1) 20(2/0/1) 20(2/0/1) 22(2/0/1)
([d32,B],I,d32 )17(2/0/0) 17(2/0/0) 20(2/0/1) 20(2/0/1) 20(2/0/1) 20(2/0/1) 20(2/0/1) 22(2/0/1)
MOTOROLA M68020 USER’S MANUAL 8-21
BEST CASE (Continued)
Source Destination
Address Mode (d8,An,Xn) (d16 ,An,Xn) (B) (d16 ,B) (d32 ,B) ([B],I) ([B],I,d16) ([B],I,d32)
Rn 4(0/0/1) 6(0/0/1) 5(0/0/1) 7(0/0/1) 11(0/0/1) 9(1/0/1) 11(1/0/1) 12(1/0/1)
#<data>.B,W 4(0/0/1) 6(0/0/1) 5(0/0/1) 7(0/0/1) 11(0/0/1) 9(1/0/1) 11(1/0/1) 12(1/0/1)
#<data>.L 4(0/0/1) 6(0/0/1) 5(0/0/1) 7(0/0/1) 11(0/0/1) 9(1/0/1) 11(1/0/1) 12(1/0/1)
(An) 8(1/0/1) 10(1/0/1) 9(1/0/1) 11(1/0/1) 15(1/0/1) 13(2/0/1) 15(2/0/1) 16(2/0/1)
(An)+ 9(1/0/1) 11(1/0/1) 10(1/0/1) 12(1/0/1) 16(1/0/1) 14(2/0/1) 16(2/0/1) 17(2/0/1)
–(An) 8(1/0/1) 10(1/0/1) 9(1/0/1) 11(1/0/1) 15(1/0/1) 13(2/0/1) 15(2/0/1) 16(2/0/1)
(d 16 ,An) or
(d 16 ,PC) 8(1/0/1) 10(1/0/1) 9(1/0/1) 11(1/0/1) 15(1/0/1) 13(2/0/1) 15(2/0/1) 16(2/0/1)
(xxx).W 8(1/0/1) 10(1/0/1) 9(1/0/1) 11(1/0/1) 15(1/0/1) 13(2/0/1) 15(2/0/1) 16(2/0/1)
(xxx).L 8(1/0/1) 10(1/0/1) 9(1/0/1) 11(1/0/1) 15(1/0/1) 13(2/0/1) 15(2/0/1) 16(2/0/1)
(d 8,An,Xn) or
(d 8,PC,Xn) 9(1/0/1) 10(1/0/1) 10(1/0/1) 12(1/0/1) 16(1/0/1) 14(2/0/1) 16(2/0/1) 17(2/0/1)
(d 16 ,An,Xn) or
(d 16 ,PC,Xn) 9(1/0/1) 11(1/0/1) 10(1/0/1) 12(1/0/1) 16(1/0/1) 14(2/0/1) 16(2/0/1) 17(2/0/1)
(B) 9(1/0/1) 11(1/0/1) 10(1/0/1) 12(1/0/1) 16(1/0/1) 14(2/0/1) 16(2/0/1) 17(2/0/1)
(d 16 ,B) 11(1/0/1) 13(1/0/1) 12(1/0/1) 14(1/0/1) 18(1/0/1) 16(2/0/1) 18(2/0/1) 19(2/0/1)
(d 32 ,B) 15(1/0/1) 17(1/0/1) 18(1/0/1) 18(1/0/1) 22(1/0/1) 20(2/0/1) 22(2/0/1) 23(2/0/1)
([B],I) 14(2/0/1) 16(2/0/1) 17(2/0/1) 17(2/0/1) 21(2/0/1) 19(3/0/1) 21(3/0/1) 22(3/0/1)
([B],I,d16 )16(2/0/1) 18(2/0/1) 19(2/0/1) 19(2/0/1) 23(2/0/1) 21(3/0/1) 23(3/0/1) 24(3/0/1)
([B],I,d32 )16(2/0/1) 18(2/0/1) 19(2/0/1) 19(2/0/1) 23(2/0/1) 21(3/0/1) 23(3/0/1) 24(3/0/1)
([d16,B],I) 16(2/0/1) 18(2/0/1) 19(2/0/1) 19(2/0/1) 23(2/0/1) 21(3/0/1) 23(3/0/1) 24(3/0/1)
([d16,B],I,d16 )18(2/0/1) 20(2/0/1) 21(2/0/1) 21(2/0/1) 25(2/0/1) 23(3/0/1) 25(3/0/1) 26(3/0/1)
([d16,B],I,d32 )18(2/0/1) 20(2/0/1) 21(2/0/1) 21(2/0/1) 25(2/0/1) 23(3/0/1) 25(3/0/1) 26(3/0/1)
([d32,B],I) 20(2/0/1) 22(2/0/1) 23(2/0/1) 23(2/0/1) 27(2/0/1) 25(3/0/1) 27(3/0/1) 28(3/0/1)
([d32,B],I,d16 )22(2/0/1) 24(2/0/1) 25(2/0/1) 25(2/0/1) 29(2/0/1) 27(3/0/1) 29(3/0/1) 30(3/0/1)
([d32,B],I,d32 )22(2/0/1) 24(2/0/1) 25(2/0/1) 25(2/0/1) 29(2/0/1) 27(3/0/1) 29(3/0/1) 30(3/0/1)
8-22 M68020 USER’S MANUAL MOTOROLA
BEST CASE (Concluded)
Source Destination
Address Mode ([ d16 ,B],I) ([d16 ,B],I,d16) ([d16 ,B],I,d32 ) ([d32 ,B],I) ([d32 ,B],I,d16 ) ([d32 ,B],I,d32 )
Rn 11(1/0/1) 13(1/0/1) 14(1/0/1) 15(1/0/1) 17(1/0/1) 18(1/0/1)
#<data>.B,W 11(1/0/1) 13(1/0/1) 14(1/0/1) 15(1/0/1) 17(1/0/1) 18(1/0/1)
#<data>.L 11(1/0/1) 13(1/0/1) 14(1/0/1) 15(1/0/1) 17(1/0/1) 18(1/0/1)
(An) 15(2/0/1) 17(2/0/1) 18(2/0/1) 19(2/0/1) 21(2/0/1) 22(2/0/1)
(An)+ 16(2/0/1) 18(2/0/1) 19(2/0/1) 20(2/0/1) 22(2/0/1) 23(2/0/1)
–(An) 15(2/0/1) 17(2/0/1) 18(2/0/1) 19(2/0/1) 21(2/0/1) 22(2/0/1)
(d 16 ,An) or (d16,PC) 15(2/0/1) 17(2/0/1) 18(2/0/1) 19(2/0/1) 21(2/0/1) 22(2/0/1)
(xxx).W 15(2/0/1) 17(2/0/1) 18(2/0/1) 19(2/0/1) 21(2/0/1) 22(2/0/1)
(xxx).L 15(2/0/1) 17(2/0/1) 18(2/0/1) 19(2/0/1) 21(2/0/1) 22(2/0/1)
(d 8,An,Xn) or
(d 8,PC,Xn) 16(2/0/1) 18(2/0/1) 19(2/0/1) 20(2/0/1) 22(2/0/1) 23(2/0/1)
(d 16 ,An,Xn) or
(d 16 ,PC,Xn) 16(2/0/1) 18(2/0/1) 19(2/0/1) 20(2/0/1) 22(2/0/1) 23(2/0/1)
(B) 16(2/0/1) 18(2/0/1) 19(2/0/1) 20(2/0/1) 22(2/0/1) 23(2/0/1)
(d 16 ,B) 18(2/0/1) 20(2/0/1) 21(2/0/1) 22(2/0/1) 24(2/0/1) 25(2/0/1)
(d 32 ,B) 22(2/0/1) 24(2/0/1) 25(2/0/1) 26(2/0/1) 28(2/0/1) 29(2/0/1)
([B],I) 21(3/0/1) 23(3/0/1) 24(3/0/1) 25(3/0/1) 27(3/0/1) 28(3/0/1)
([B],I,d16 )23(3/0/1) 25(3/0/1) 26(3/0/1) 27(3/0/1) 29(3/0/1) 30(3/0/1)
([B],I,d32 )23(3/0/1) 25(3/0/1) 26(3/0/1) 27(3/0/1) 29(3/0/1) 30(3/0/1)
([d16,B],I) 23(3/0/1) 25(3/0/1) 26(3/0/1) 27(3/0/1) 29(3/0/1) 30(3/0/1)
([d16,B],I,d16 )25(3/0/1) 27(3/0/1) 28(3/0/1) 29(3/0/1) 31(3/0/1) 32(3/0/1)
([d16,B],I,d32 )25(3/0/1) 27(3/0/1) 28(3/0/1) 29(3/0/1) 31(3/0/1) 32(0/0/1)
([d32,B],I) 27(3/0/1) 29(3/0/1) 30(3/0/1) 31(3/0/1) 33(3/0/1) 34(3/0/1)
([d32,B],I,d16 )29(3/0/1) 31(3/0/1) 32(3/0/1) 33(3/0/1) 35(3/0/1) 36(3/0/1)
([d32,B],I,d32 )29(3/0/1) 31(3/0/1) 32(3/0/1) 33(3/0/1) 35(3/0/1) 36(3/0/1)
MOTOROLA M68020 USER’S MANUAL 8-23
CACHE CASE
Source Destination
Address Mode An Dn (An) (An)+ –(An) (d16 ,An) (xxx).W (xxx).L
Rn 2(0/0/0) 2(0/0/0) 4(0/0/1) 4(0/0/1) 5(0/0/1) 5(0/0/1) 4(0/0/1) 6(0/0/1)
#<data>.B,W 4(0/0/0) 4(0/0/0) 6(0/0/1) 6(0/0/1) 7(0/0/1) 7(0/0/1) 6(0/0/1) 8(0/0/1)
#<data>.L 6(0/0/0) 6(0/0/0) 8(0/0/1) 8(0/0/1) 9(0/0/1) 9(0/0/1) 8(0/0/1) 10(0/0/1)
(An) 6(1/0/0) 6(1/0/0) 7(1/0/1) 7(1/0/1) 7(1/0/1) 7(1/0/1) 7(1/0/1) 9(1/0/1)
(An)+ 6(1/0/0) 6(1/0/0) 7(1/0/1) 7(1/0/1) 7(1/0/1) 7(1/0/1) 7(1/0/1) 9(1/0/1)
–(An) 7(1/0/0) 7(1/0/0) 8(1/0/1) 8(1/0/1) 8(1/0/1) 8(1/0/1) 8(1/0/1) 10(1/0/1)
(d 16 ,An) or
(d 16 ,PC) 7(1/0/0) 7(1/0/0) 8(1/0/1) 8(1/0/1) 8(1/0/1) 8(1/0/1) 8(1/0/1) 10(1/0/1)
(xxx).W 6(1/0/0) 6(1/0/0) 7(1/0/1) 7(1/0/1) 7(1/0/1) 7(1/0/1) 7(1/0/1) 9(1/0/1)
(xxx).L 6(1/0/0) 6(1/0/0) 7(1/0/1) 7(1/0/1) 7(1/0/1) 7(1/0/1) 7(1/0/1) 9(1/0/1)
(d 8,An,Xn) or
(d 8,PC,Xn) 9(1/0/0) 9(1/0/0) 10(1/0/1) 10(1/0/1) 10(1/0/1) 10(1/0/1) 10(1/0/1) 12(1/0/1)
(d 16 ,An,Xn) or
(d 16 ,PC,Xn) 9(1/0/0) 9(1/0/0) 10(1/0/1) 10(1/0/1) 10(1/0/1) 10(1/0/1) 10(1/0/1) 12(1/0/1)
(B) 9(1/0/0) 9(1/0/0) 10(1/0/1) 10(1/0/1) 10(1/0/1) 10(1/0/1) 10(1/0/1) 12(1/0/1)
(d 16 ,B) 11(1/0/0) 11(1/0/0) 12(1/0/1) 12(1/0/1) 12(1/0/1) 12(1/0/1) 12(1/0/1) 14(1/0/1)
(d 32 ,B) 15(1/0/0) 15(1/0/0) 16(1/0/1) 16(1/0/1) 16(1/0/1) 16(1/0/1) 16(1/0/1) 18(1/0/1)
([B],I) 14(2/0/0) 14(2/0/0) 15(2/0/1) 15(2/0/1) 15(2/0/1) 15(2/0/1) 15(2/0/1) 17(2/0/1)
([B],I,d16 )16(2/0/0) 16(2/0/0) 17(2/0/1) 17(2/0/1) 17(2/0/1) 17(2/0/1) 17(2/0/1) 19(2/0/1)
([B],I,d32 )16(2/0/0) 16(2/0/0) 17(2/0/1) 17(2/0/1) 17(2/0/1) 17(2/0/1) 17(2/0/1) 19(2/0/1)
([d16,B],I) 16(2/0/0) 16(2/0/0) 17(2/0/1) 17(2/0/1) 17(2/0/1) 17(2/0/1) 17(2/0/1) 19(2/0/1)
([d16,B],I,d16 )18(2/0/0) 18(2/0/0) 19(2/0/1) 19(2/0/1) 19(2/0/1) 19(2/0/1) 19(2/0/1) 21(2/0/1)
([d16,B],d32 )18(2/0/0) 18(2/0/0) 19(2/0/1) 19(2/0/1) 19(2/0/1) 19(2/0/1) 19(2/0/1) 21(2/0/1)
([d32,B],I) 20(2/0/0) 20(2/0/0) 21(2/0/1) 21(2/0/1) 21(2/0/1) 21(2/0/1) 21(2/0/1) 23(2/0/1)
([d32,B],I,d16 )22(2/0/0) 22(2/0/0) 23(2/0/1) 23(2/0/1) 23(2/0/1) 23(2/0/1) 23(2/0/1) 25(2/0/1)
([d32,B],I,d32 )22(2/0/0) 22(2/0/0) 23(2/0/1) 23(2/0/1) 23(2/0/1) 23(2/0/1) 23(2/0/1) 25(2/0/1)
8-24 M68020 USER’S MANUAL MOTOROLA
CACHE CASE (Continued)
Source Destination
Address Mode (d8,An,Xn) (d16 ,An,Xn) (B) (d16 ,B) (d32 ,B) ([B],I) ([B],I,d16) ([B],I,d32)
Rn 7(0/0/1) 9(0/0/1) 8(0/0/1) 10(0/0/1) 14(0/0/1) 12(1/0/1) 14(1/0/1) 15(1/0/1)
#<data>.B,W 7(0/0/1) 9(0/0/1) 8(0/0/1) 10(0/0/1) 14(0/0/1) 12(1/0/1) 14(1/0/1) 15(1/0/1)
#<data>.L 9(0/0/1) 11(0/0/1) 10(0/0/1) 12(0/0/1) 16(0/0/1) 14(1/0/1) 16(1/0/1) 17(1/0/1)
(An) 9(1/0/1) 11(1/0/1) 10(1/0/1) 12(1/0/1) 16(1/0/1) 14(2/0/1) 16(2/0/1) 17(2/0/1)
(An)+ 9(1/0/1) 11(1/0/1) 10(1/0/1) 12(1/0/1) 16(1/0/1) 14(2/0/1) 16(2/0/1) 17(2/0/1)
–(An) 10(1/0/1) 12(1/0/1) 11(1/0/1) 13(1/0/1) 17(1/0/1) 15(2/0/1) 17(2/0/1) 18(2/0/1)
(d 16 ,An) or
(d 16 ,PC) 10(1/0/1) 12(2/0/1) 11(1/0/1) 13(1/0/1) 17(1/0/1) 15(2/0/1) 17(2/0/1) 18(2/0/1)
(xxx).W 9(1/0/1) 11(1/0/1) 10(1/0/1) 12(1/0/1) 16(1/0/1) 14(2/0/1) 16(2/0/1) 17(2/0/1)
(xxx).L 9(1/0/1) 11(1/0/1) 10(1/0/1) 12(1/0/1) 16(1/0/1) 14(2/0/1) 16(2/0/1) 17(2/0/1)
(d 8,An,Xn) or
(d 8,PC,Xn) 12(1/0/1) 14(1/0/1) 13(1/0/1) 15(1/0/1) 19(1/0/1) 17(2/0/1) 19(2/0/1) 20(2/0/1)
(d 16 ,An,Xn) or
(d 16 ,PC,Xn) 12(1/0/1) 14(1/0/1) 13(1/0/1) 15(1/0/1) 19(1/0/1) 17(2/0/1) 19(2/0/1) 20(2/0/1)
(B) 12(1/0/1) 14(1/0/1) 13(1/0/1) 15(1/0/1) 19(1/0/1) 17(2/0/1) 19(2/0/1) 20(2/0/1)
(d 16 ,B) 14(1/0/1) 16(1/0/1) 15(1/0/1) 17(1/0/1) 21(1/0/1) 19(2/0/1) 21(2/0/1) 22(2/0/1)
(d 32 ,B) 18(1/0/1) 20(1/0/1) 19(1/0/1) 21(1/0/1) 25(1/0/1) 23(2/0/1) 25(2/0/1) 26(2/0/1)
([B],I) 17(2/0/1) 19(2/0/1) 18(2/0/1) 20(2/0/1) 24(2/0/1) 22(3/0/1) 24(3/0/1) 25(3/0/1)
([B],I,d16 )19(2/0/1) 21(2/0/1) 20(2/0/1) 22(2/0/1) 26(2/0/1) 24(3/0/1) 26(3/0/1) 27(3/0/1)
([B],I,d32 )19(2/0/1) 21(2/0/1) 20(2/0/1) 22(2/0/1) 26(2/0/1) 24(3/0/1) 26(3/0/1
)
27(3/0/1)
([d16,B],I) 19(2/0/1) 21(2/0/1) 20(2/0/1) 22(2/0/1) 26(2/0/1) 24(3/0/1) 26(3/0/1) 27(3/0/1)
([d16,B],I,d16 )21(2/0/1) 23(2/0/1) 22(2/0/1) 24(2/0/1) 28(2/0/1) 26(3/0/1) 28(3/0/1) 29(3/0/1)
([d16,B],I,d32 )21(2/0/1) 23(2/0/1) 22(2/0/1) 24(2/0/1) 28(2/0/1) 26(3/0/1) 28(3/0/1) 29(3/0/1)
([d32,B],I) 23(2/0/1) 25(2/0/1) 24(2/0/1) 26(2/0/1) 30(2/0/1) 28(3/0/1) 30(3/0/1) 31(3/0/1)
([d32,B],I,d16 )25(2/0/1) 27(2/0/1) 26(2/0/1) 28(2/0/1) 32(2/0/1) 30(3/0/1) 32(3/0/1) 33(3/0/1)
([d32,B],I,d32 )25(2/0/1) 27(2/0/1) 26(2/0/1) 28(2/0/1) 32(2/0/1) 30(3/0/1) 32(3/0/1) 33(3/0/1)
MOTOROLA M68020 USER’S MANUAL 8-25
CACHE CASE (Concluded)
Source Destination
Address Mode ([ d16 ,B],I) ([d16 ,B],I,d16 ) ([d16 ,B],I,d32 ) ([d32 ,B],I) ([d32 ,B],I,d16 ) ([d32 ,B],I,d32 )
Rn 14(1/0/1) 16(1/0/1) 17(1/0/1) 18(1/0/1) 20(1/0/1) 21(1/0/1)
#<data>.B,W 14(1/0/1) 16(1/0/1) 17(1/0/1) 18(1/0/1) 20(1/0/1) 21(1/0/1)
#<data>.L 16(1/0/1) 18(1/0/1) 19(1/0/1) 20(1/0/1) 22(1/0/1) 23(1/0/1)
(An) 16(2/0/1) 18(2/0/1) 19(2/0/1) 20(2/0/1) 22(2/0/1) 23(2/0/1)
(An)+ 16(2/0/1) 18(2/0/1) 19(2/0/1) 20(2/0/1) 22(2/0/1) 23(2/0/1)
–(An) 17(2/0/1) 19(2/0/1) 20(2/0/1) 21(2/0/1) 23(2/0/1) 24(2/0/1)
(d 16 ,An) or (d16,PC) 17(2/0/1) 19(2/0/1) 20(2/0/1) 21(2/0/1) 23(2/0/1) 24(2/0/1)
(xxx).W 16(2/0/1) 18(2/0/1) 19(2/0/1) 20(2/0/1) 22(2/0/1) 23(2/0/1)
(xxx).L 16(2/0/1) 18(2/0/1) 19(2/0/1) 20(2/0/1) 22(2/0/1) 23(2/0/1)
(d 8,An,Xn) or
(d 8,PC,Xn) 19(2/0/1) 21(2/0/1) 22(2/0/1) 23(2/0/1) 25(2/0/1) 26(2/0/1)
(d 16 ,An,Xn) or
(d 16 ,PC,Xn) 19(2/0/1) 21(2/0/1) 22(2/0/1) 23(2/0/1) 25(2/0/1) 26(2/0/1)
(B) 19(2/0/1) 21(2/0/1) 22(2/0/1) 23(2/0/1) 25(2/0/1) 26(2/0/1)
(d 16 ,B) 21(2/0/1) 23(2/0/1) 24(2/0/1) 25(2/0/1) 27(2/0/1) 28(2/0/1)
(d 32 ,B) 25(2/0/1) 27(2/0/1) 28(2/0/1) 29(2/0/1) 31(2/0/1) 32(2/0/1)
([B],I) 24(3/0/1) 26(3/0/1) 27(3/0/1) 28(3/0/1) 30(3/0/1) 31(3/0/1)
([B],I,d16 )26(3/0/1) 28(3/0/1) 29(3/0/1) 30(3/0/1) 32(3/0/1) 33(3/0/1)
([B],I,d32 )26(3/0/1) 28(3/0/1) 29(3/0/1) 30(3/0/1) 32(3/0/1) 33(3/0/1)
([d16,B],I) 26(3/0/1) 28(3/0/1) 29(3/0/1) 30(3/0/1) 32(3/0/1) 33(3/0/1)
([d16,B],I,d16 )28(3/0/1) 30(3/0/1) 31(3/0/1) 32(3/0/1) 34(3/0/1) 35(3/0/1)
([d16,B],I,d32 )28(3/0/1) 30(3/0/1) 31(3/0/1) 32(3/0/1) 34(3/0/1) 35(3/0/1)
([d32,B],I) 30(3/0/1) 32(3/0/1) 33(3/0/1) 34(3/0/1) 36(3/0/1) 37(3/0/1)
([d32,B],I,d16 )32(3/0/1) 34(3/0/1) 35(3/0/1) 36(3/0/1) 38(3/0/1) 39(3/0/1)
([d32,B],I,d32 )32(3/0/1) 34(3/0/1) 35(3/0/1) 36(3/0/1) 38(3/0/1) 39(3/0/1)
8-26 M68020 USER’S MANUAL MOTOROLA
WORST CASE
Destination
Source Address Mode An Dn (An) (An)+ –(An) (d16 ,An) (xxx).W (xxx).L
Rn 3(0/1/0) 3(0/1/0) 5(0/1/0) 5(0/1/1) 6(0/1/1) 7(0/1/1) 7(0/1/1) 9(0/2/1)
#<data>.B,W 3(0/1/0) 3(0/1/0) 5(0/1/0) 8(0/1/1) 6(0/1/1) 7(0/1/1) 7(0/1/1) 9(0/2/1)
#<data>.L 5(0/1/0) 5(0/1/0) 7(0/0/1) 7(0/1/1) 8(0/1/1) 9(0/1/1) 9(0/1/1) 11(0/2/1)
(An) 7(1/1/0) 7(1/1/0) 9(1/1/1) 9(1/1/1) 9(1/1/1) 11(1/1/1) 11(1/1/1) 13(1/2/1)
(An)+ 7(1/1/0) 7(1/1/0) 9(1/1/1) 9(1/1/1) 9(1/1/1) 11(1/1/1) 11(1/1/1) 13(1/2/1)
–(An) 8(1/1/0) 8(1/1/0) 10(1/1/1) 10(1/1/1) 10(1/1/1) 12(1/1/1) 12(1/1/1) 14(1/2/1)
(d 16 ,An) or (d16,PC) 9(1/2/0) 9(1/2/0) 11(1/2/1) 11(1/2/1) 11(1/2/1) 13(1/2/1) 13(1/2/1) 15(1/3/1)
(xxx).W 8(1/2/0) 8(1/2/0) 10(1/2/1) 10(1/2/1) 10(1/2/1) 12(1/2/1) 12(1/2/1) 14(1/3/1)
(xxx).L 10(1/2/0) 10(1/2/0) 12(1/2/1) 12(1/2/1) 12(1/2/1) 14(1/2/1) 14(1/2/1) 16(1/3/1)
(d 8,An,Xn) or (d8,PC,Xn) 11(1/2/0) 11(1/2/0) 13(1/2/1) 13(1/2/1) 13(1/2/1) 15(1/2/1) 15(1/2/1) 17(1/3/1)
(d 16 ,An,Xn) or (d16,PC,Xn) 12(1/2/0) 12(1/2/0) 14(1/2/1) 14(1/2/1) 14(1/2/1) 16(1/2/1) 16(1/2/1) 18(1/3/1)
(B) 12(1/2/0) 12(1/2/0) 14(1/2/1) 14(1/2/1) 14(1/2/1) 16(1/2/1) 16(1/2/1) 18(1/3/1)
(d 16 ,B) 15(1/2/0) 15(1/2/0) 17(1/2/1) 17(1/2/1) 17(1/3/1) 19(1/2/1) 19(1/2/1) 21(1/3/1)
(d 32 ,B) 19(1/3/0) 19(1/3/0) 21(1/3/1) 21(1/3/1) 21(1/3/1) 23(1/3/1) 23(1/3/1) 25(1/4/1)
([B],I) 16(2/2/0) 16(2/2/0) 18(2/2/1) 18(2/2/1) 18(2/2/1) 20(2/2/1) 20(2/2/1) 22(2/3/1)
([B],I,d16 )19(2/2/0) 19(2/2/0) 21(2/2/1) 21(2/2/1) 21(2/2/1) 23(2/2/1) 23(2/2/1) 25(2/3/1)
([B],I,d32 )20(2/3/0) 20(2/3/0) 22(2/3/1) 22(2/3/1) 22(2/3/1) 24(2/3/1) 24(2/3/1) 26(2/4/1)
([d16,B],I) 19(2/2/0) 19(2/2/0) 21(2/2/1) 21(2/2/1) 21(2/2/1) 23(2/2/1) 23(2/2/1) 25(2/3/1)
([d16,B],I,d16 )22(2/3/0) 22(2/3/0) 24(2/3/1) 24(2/3/1) 24(2/3/1) 26(2/3/1) 26(2/3/1) 28(2/4/1)
([d16,B],d32 )23(2/3/0) 23(2/3/0) 25(2/3/1) 25(2/3/1) 25(2/3/1) 27(2/3/1) 27(2/3/1) 29(2/4/1)
([d32,B],I) 23(2/3/0) 23(2/3/0) 25(2/3/1) 25(2/3/1) 25(2/3/1) 27(2/3/1) 27(2/3/1) 29(2/4/1)
([d32,B],I,d16 )25(2/3/0) 25(2/3/0) 27(2/3/1) 27(2/3/1) 27(2/3/1) 29(2/3/1) 29(2/3/1) 31(2/4/1)
([d32,B],I,d32 )27(2/4/0) 27(2/4/0) 29(2/4/1) 29(2/4/1) 29(2/4/1) 31(2/4/1) 31(2/4/1) 33(2/5/1)
MOTOROLA M68020 USER’S MANUAL 8-27
WORST CASE (Continued)
Source Destination
Address Mode (d8,An,Xn) (d16,An,Xn) (B) (d16,B) (d32,B) ([B],I) ([B],I,d16) ([B],I,D32)
Rn 9(0/1/1) 12(0/2/1) 10(0/1/1) 14(0/2/1) 19(0/2/1) 14(1/1/1) 17(1/2/1) 20(1/2/1)
#<data>.B,W 9(0/1/1) 12(0/2/1) 10(0/1/1) 14(0/2/1) 19(0/2/1) 14(1/1/1) 17(1/2/1) 20(1/2/1)
#<data>.L 11(0/1/1) 14(0/2/1) 12(0/1/1) 16(0/2/1) 21(0/2/1) 16(1/1/1) 19(1/2/1) 22(1/2/1)
(An) 11(1/1/1) 14(1/2/1) 12(1/1/1) 16(1/2/1) 21(1/2/1) 12(2/1/1) 19(2/2/1) 22(2/2/1)
(An)+ 11(1/1/1) 14(1/2/1) 12(1/1/1) 16(1/2/1) 21(1/2/1) 12(2/1/1) 19(2/2/1) 22(2/2/1)
–(An) 12(1/1/1) 15(1/2/1) 13(1/1/1) 17(1/2/1) 22(1/2/1) 13(2/1/1) 20(2/2/1) 23(2/2/1)
(d 16 ,An) or
(d 16 ,PC) 13(1/2/1) 16(1/3/1) 14(1/2/1) 18(1/3/1) 23(1/3/1) 14(2/2/1) 21(2/3/1) 24(2/3/1)
(xxx).W 12(1/2/1) 15(1/3/1) 13(1/2/1) 17(1/3/1) 22(1/3/1) 13(2/2/1) 20(2/3/1) 23(2/3/1)
(xxx).L 14(1/2/1) 17(1/3/1) 15(1/2/1) 19(1/3/1) 24(1/3/1) 15(2/2/1) 22(2/3/1) 25(2/3/1)
(d 8,An,Xn) or
(d 8,PC,Xn) 15(1/2/1) 18(1/3/1) 16(1/2/1) 20(1/3/1) 25(1/3/1) 16(2/2/1) 23(2/3/1) 26(2/3/1)
(d 16 ,An,Xn) or
(d 16 ,PC,Xn) 16(1/2/1) 19(1/3/1) 17(1/2/1) 21(1/3/1) 26(1/3/1) 17(2/2/1) 24(2/3/1) 27(2/3/1)
(B) 16(1/2/1) 19(1/3/1) 17(1/2/1) 21(1/3/1) 26(1/3/1) 17(2/2/1) 24(2/3/1) 27(2/3/1)
(d 16 ,B) 19(1/2/1) 22(1/3/1) 20(1/2/1) 24(1/3/1) 29(1/3/1) 20(2/2/1) 27(2/3/1) 30(2/3/1)
(d 32 ,B) 23(1/3/1) 26(1/4/1) 24(1/3/1) 28(1/4/1) 33(1/4/1) 24(2/3/1) 31(2/4/1) 34(2/4/1)
([B],I) 20(2/2/1) 23(2/3/1) 21(2/2/1) 25(2/3/1) 30(2/3/1) 21(3/2/1) 28(3/3/1) 31(3/3/1)
([B],I,d16 )23(2/2/1) 26(2/3/1) 24(2/2/1) 28(2/3/1) 33(2/3/1) 24(3/2/1) 31(3/3/1) 34(3/3/1)
([B],I,d32 )24(2/3/1) 27(2/4/1) 25(2/3/1) 29(2/4/1) 34(2/4/1) 25(3/3/1) 32(3/4/1) 35(3/4/1)
([d16,B],I) 23(2/2/1) 26(2/3/1) 24(2/2/1) 28(2/3/1) 33(2/3/1) 24(3/2/1) 31(3/3/1) 34(3/3/1)
([d16,B],I,d16 )26(2/3/1) 29(2/4/1) 27(2/3/1) 31(2/4/1) 36(2/4/1) 27(3/3/1) 34(3/4/1) 37(3/4/1)
([d16,B],I,d32 )27(2/3/1) 30(2/4/1) 28(2/3/1) 32(2/4/1) 37(2/4/1) 28(3/3/1) 35(3/4/1) 38(3/4/1)
([d32,B],I) 27(2/3/1) 30(2/4/1) 28(2/3/1) 32(2/4/1) 37(2/4/1) 28(3/3/1) 35(3/4/1) 38(3/4/1)
([d32,B],I,d16 )29(2/3/1) 32(2/4/1) 30(2/3/1) 34(2/4/1) 39(2/4/1) 30(3/3/1) 37(3/4/1) 40(3/4/1)
([d32,B],I,d32 )31(2/4/1) 34(2/5/1) 32(2/4/1) 36(2/5/1) 41(2/5/1) 32(3/4/1) 39(3/5/1) 42(3/5/1)
8-28 M68020 USER’S MANUAL MOTOROLA
WORST CASE (Concluded)
Source Destination
Address Mode ([d16,B],I) ([d16,B],I,d16) ([d16,B],I,d32) ([d32,B],I) ([d32,B],I,d16) ([d32,B],I,d32)
Rn 17(1/2/1) 20(1/2/1) 23(1/3/1) 22(1/2/1) 25(1/3/1) 27(1/3/1)
#<data>.B,W 17(1/2/1) 20(1/2/1) 23(1/3/1) 22(1/2/1) 25(1/3/1) 27(1/3/1)
#<data>.L 19(1/2/1) 22(1/2/1) 25(1/3/1) 24(1/2/1) 27(1/3/1) 29(1/3/1)
(An) 19(2/2/1) 22(2/2/1) 25(2/3/1) 24(2/2/1) 27(2/3/1) 29(2/3/1)
(An)+ 19(2/2/1) 22(2/2/1) 25(2/3/1) 24(2/2/1) 27(2/3/1) 29(2/3/1)
–(An) 20(2/2/1) 23(2/2/1) 26(2/3/1) 25(2/2/1) 28(2/3/1) 30(2/3/1)
(d 16 ,An) or (d16,PC) 21(2/3/1) 24(2/3/1) 27(2/4/1) 26(2/3/1) 29(2/4/1) 31(2/4/1)
(xxx).W 20(2/3/1) 23(2/3/1) 26(2/4/1) 27(2/3/1) 28(2/4/1) 30(2/4/1)
(xxx).L 22(2/3/1) 25(2/3/1) 28(2/4/1) 29(2/3/1) 30(2/4/1) 32(2/4/1)
(d 8,An,Xn) or
(d 8,PC,Xn) 23(2/3/1) 26(2/3/1) 29(2/4/1) 30(2/3/1) 31(2/4/1) 33(2/4/1)
(d 16 ,An,Xn) or
(d 16 ,PC,Xn) 24(2/3/1) 27(2/3/1) 30(2/4/1) 31(2/3/1) 32(2/4/1) 34(2/4/1)
(B) 24(2/3/1) 27(2/3/1) 30(2/4/1) 31(2/3/1) 32(2/4/1) 34(2/4/1)
(d 16 ,B) 27(2/3/1) 30(2/3/1) 33(2/4/1) 34(2/3/1) 35(2/4/1) 37(2/4/1)
(d 32 ,B) 31(2/4/1) 34(2/4/1) 37(2/5/1) 38(2/4/1) 39(2/5/1) 41(2/5/1)
([B],I) 28(3/3/1) 31(3/3/1) 34(3/4/1) 35(3/3/1) 36(3/4/1) 38(3/4/1)
([B],I,d16 )31(3/3/1) 34(3/3/1) 37(3/4/1) 38(3/3/1) 39(3/4/1) 41(3/4/1)
([B],I,d32 )32(3/4/1) 35(3/4/1) 38(3/5/1) 39(3/4/1) 40(3/5/1) 42(3/5/1)
([d16,B],I) 31(3/3/1) 34(3/3/1) 37(3/4/1) 38(3/3/1) 39(3/4/1) 41(3/4/1)
([d16,B],I,d16 )34(3/4/1) 37(3/4/1) 40(3/5/1) 41(3/4/1) 42(3/5/1) 44(3/5/1)
([d16,B],I,d32 )35(3/4/1) 38(3/4/1) 41(3/5/1) 42(3/4/1) 43(3/5/1) 45(3/5/1)
([d32,B],I) 35(3/4/1) 38(3/4/1) 41(3/5/1) 42(3/4/1) 43(3/5/1) 45(3/5/1)
([d32,B],I,d16 )37(3/4/1) 40(3/4/1) 43(3/5/1) 44(3/4/1) 45(3/5/1) 47(3/5/1)
([d32,B],I,d32 )39(3/5/1) 42(3/5/1) 45(3/6/1) 46(3/5/1) 47(3/6/1) 49(3/6/1)
MOTOROLA M68020 USER’S MANUAL 8-29
8.2.7 Special-Purpose MOVE Instruction
The special-purpose MOVE instruction table indicates the number of clock periods needed
for the processor to fetch, calculate, and perform the special-purpose MOVE operation on
the control registers or specified effective address. The total number of clock cycles is
outside the parentheses, the number of read, prefetch, and write cycles is given inside the
parentheses as (r/p/w). These cycles are included in the total clock cycle number.
Instruction Best Case Cache Case Worst Case
EXG Ry,Rx 0(0/0/0) 2(0/0/0) 3(0/1/0)
MOVEC Cr,Rn 3(0/0/0) 6(0/0/0) 7(0/1/0)
MOVEC Rn,Cr 9(0/0/0) 12(0/0/0) 13(0/1/0)
MOVE PSW,Rn 1(0/0/0) 4(0/0/0) 5(0/1/0)
† MOVE PSW,Mem 5(0/0/1) 5(0/0/1) 7(0/1/1)
*MOVE EA,CCR 4(0/0/0) 4(0/0/0) 5(0/1/0)
*MOVE EA,SR 8(0/0/0) 8(0/0/0) 11(0/2/0)
‡ MOVEM EA,RL 8 + 4n(n/0/0) 8 + 4n(n/0/0) 9 + 4n(n/1/0)
‡ MOVEM RL,EA 4 + 3n(0/0/n) 4 + 3n(0/0/n) 5 + 3n(0/1/n)
MOVEP.W Dn,(d16 ,An) 8(0/0/2) 11(0/0/2) 11(0/1/2)
MOVEP.L Dn,(d16 ,An) 14(0/0/4) 17(0/0/4) 17(0/1/4)
MOVEP.W (d16 ,An),Dn 10(2/0/0) 12(2/0/0) 12(2/1/0)
MOVEP.L (d16 ,An),Dn 16(4/0/0) 18(4/0/0) 18(4/1/0)
‡ MOVES EA,Rn 7(1/0/0) 7(1/0/0) 8(1/1/0)
‡ MOVES Rn,EA 5(0/0/1) 5(0/0/1) 7(0/1/1)
MOVE USP 0(0/0/0) 2(0/0/0) 3(0/1/0)
SWAP Rx,Ry 1(0/0/0) 4(0/0/0) 4(0/1/0)
n—Number of Registers to Transfer
RL—Register List
*Add Fetch Effective Address Time
†Add Calculate Effective Address Time
‡Add Calculate Immediate Address Time
8-30 M68020 USER’S MANUAL MOTOROLA
8.2.8 Arithmetic/Logical Instructions
The arithmetic/logical instructions table indicates the number of clock periods needed for
the processor to perform the specified arithmetic/logical operation using the specified
addressing mode. It also includes, in worst case, the amount of time needed to prefetch
the next instruction. Footnotes specify when to add either fetch address or fetch
immediate effective address time. This sum gives the total effective execution time for the
operation using the specified addressing mode. The total number of clock cycles is
outside the parentheses; the number of read, prefetch, and write cycles is given inside the
parentheses as (r/p/w). These cycles are included in the total clock cycle number.
Instruction Best Case Cache Case Worst Case
*ADD EA,Dn 0(0/0/0) 2(0/0/0) 3(0/1/0)
*ADDA EA,An 0(0/0/0) 2(0/0/0) 3(0/1/0)
*ADD Dn,EA 3(0/0/1) 4(0/0/1) 6(0/1/1)
*AND EA,Dn 0(0/0/0) 2(0/0/0) 3(0/1/0)
*AND Dn,EA 3(0/0/1) 4(0/0/1) 6(0/1/1)
*EOR Dn,Dn 0(0/0/0) 2(0/0/0) 3(0/1/0)
*EOR Dn,Mem 3(0/0/1) 4(0/0/1) 6(0/1/1)
*OR EA,Dn 0(0/0/0) 2(0/0/0) 3(0/1/0)
*OR Dn,EA 3(0/0/1) 4(0/0/1) 6(0/1/1)
*SUB EA,Dn 0(0/0/0) 2(0/0/0) 3(0/1/0)
*SUBA EA,An 0(0/0/0) 2(0/0/0) 3(0/1/0)
*SUB Dn,EA 3(0/0/1) 4(0/0/1) 6(0/1/1)
*CMP EA,Dn 0(0/0/0) 2(0/0/0) 3(0/1/0)
*CMPA EA,An 1(0/0/0) 4(0/0/0) 4(0/1/0)
** CMP2 EA,Rn 16(1/0/0) 18(1/0/0) 18(1/1/0)
*MUL.W EA,Dn 25(0/0/0) 27(0/0/0) 28(0/1/0)
** MUL.L EA,Dn 41(0/0/0) 43(0/0/0) 44(0/1/0)
*DIVU.W EA,Dn 42(0/0/0) 44(0/0/0) 44(0/1/0)
** DIVU.L EA,Dn 76(0/0/0) 78(0/0/0) 79(0/1/0)
*DIVS.W EA,Dn 54(0/0/0) 56(0/0/0) 57(0/1/0)
** DIVS.L EA,Dn 88(0/0/0) 90(0/0/0) 91(0/1/0)
*Add Fetch Effective Address Time
**Add Fetch Immediate Address Time
MOTOROLA M68020 USER’S MANUAL 8-31
8.2.9 Immediate Arithmetic/Logical Instructions
The immediate arithmetic/logical instructions table indicates the number of clock periods
needed for the processor to fetch the source immediate data value and perform the
specified arithmetic/logical operation using the specified destination addressing mode.
Footnotes indicate when to add appropriate fetch effective or fetch immediate effective
address time. This computation will give the total execution time needed to perform the
appropriate immediate arithmetic/logical operation. The total number of clock cycles is
outside the parentheses; the number of read, prefetch, and write cycles is given inside the
parentheses as (r/p/w). These cycles are included in the total clock cycle number.
Instruction Best Case Cache Case Worst Case
MOVEQ #<data>,Dn 0(0/0/0) 2(0/0/0) 3(0/1/0)
ADDQ #<data>,Rn 0(0/0/0) 2(0/0/0) 3(0/1/0)
*ADDQ #<data>,Mem 3(0/0/1) 4(0/0/1) 6(0/1/1)
SUBQ #<data>,Rn 0(0/0/0) 2(0/0/0) 3(0/1/0)
*SUBQ #<data>,Mem 3(0/0/1) 4(0/0/1) 6(0/1/1)
** ADDI #<data>,Dn 0(0/0/0) 2(0/0/0) 3(0/1/0)
** ADDI #<data>,Mem 3(0/0/1) 4(0/0/1) 6(0/1/1)
** ANDI #<data>,Dn 0(0/0/0) 2(0/0/0) 3(0/1/0)
** ANDI #<data>,Mem 3(0/0/1) 4(0/0/1) 6(0/1/1)
** EORI #<data>,Dn 0(0/0/0) 2(0/0/0) 3(0/1/0)
** EORI #<data>,Mem 3(0/0/1) 4(0/0/1) 6(0/1/1)
** ORI #<data>,Dn 0(0/0/0) 2(0/0/0) 3(0/1/0)
** ORI #<data>,Mem 3(0/0/1) 4(0/0/1) 6(0/1/1)
** SUBI #<data>,Dn 0(0/0/0) 2(0/0/0) 3(0/1/0)
** SUBI #<data>,Mem 3(0/0/1) 4(0/0/1) 6(0/1/1)
** CMPI #<data>,EA 0(0/0/0) 2(0/0/0) 3(0/1/0)
*Add Fetch Effective Address Time
**Add Fetch Immediate Address Time
8-32 M68020 USER’S MANUAL MOTOROLA
8.2.10 Binary-Coded Decimal Operations
The binary-coded decimal operations table indicates the number of clock periods needed
for the processor to perform the specified operation using the given addressing modes,
with complete execution times given. No additional tables are needed to calculate total
effective execution time for these instructions. The total number of clock cycles is outside
the parentheses; the number of read, prefetch, and write cycles is given inside the
parentheses as (r/p/w). These cycles are included in the total clock cycle number.
Instruction Best Case Cache Case Worst Case
ABCD Dn,Dn 4(0/0/0) 4(0/0/0) 5(0/1/0)
ABCD –(An),–(An) 14(2/0/1) 16(2/0/1) 17(2/1/1)
SBCD Dn,Dn 4(0/0/0) 4(0/0/0) 5(0/1/0)
SBCD –(An),–(An) 14(2/0/1) 16(2/0/1) 17(2/1/1)
ADDX Dn,Dn 2(0/0/0) 2(0/0/0) 3(0/1/0)
ADDX –(An),–(An) 10(2/0/1) 12(2/0/1) 13(2/1/1)
SUBX Dn,Dn 2(0/0/0) 2(0/0/0) 3(0/1/0)
SUBX –(An),–(An) 10(2/0/1) 12(2/0/1) 13(2/1/1)
CMPM (An)+,(An)+ 8(2/0/0) 9(2/0/0) 10(2/1/0)
PACK Dn,Dn,#<data>3(0/0/0) 6(0/0/0) 7(0/1/0)
PACK –(An),–(An),#<data>11(1/0/1) 13(1/0/1) 13(1/1/1)
UNPK Dn,Dn,#<data>5(0/0/0) 8(0/0/0) 9(0/1/0)
UNPK –(An),–(An),#<data>11(1/0/1) 13(1/0/1) 13(1/1/1)
MOTOROLA M68020 USER’S MANUAL 8-33
8.2.11 Single-Operand Instructions
The single-operand instructions table indicates the number of clock periods needed for the
processor to perform the specified operation on the given addressing mode. Footnotes
indicate when it is necessary to add another table entry to calculate the total effective
execution time for the instruction. The total number of clock cycles is outside the
parentheses; the number of read, prefetch, and write cycles is given inside the
parentheses as (r/p/w). These cycles are included in the total clock cycle number.
Instruction Best Case Cache Case Worst Case
CLR Dn 0(0/0/0) 2(0/0/0) 3(0/1/0)
† CLR Mem 3(0/0/1) 4(0/0/1) 6(0/1/1)
NEG Dn 0(0/0/0) 2(0/0/0) 3(0/1/0)
*NEG Mem 3(0/0/1) 4(0/0/1) 6(0/1/1)
NEGX Dn 0(0/0/0) 2(0/0/0) 3(0/1/0)
*NEGX Mem 3(0/0/1) 4(0/0/1) 6(0/1/1)
NOT Dn 0(0/0/0) 2(0/0/0) 3(0/1/0)
*NOT Mem 3(0/0/1) 4(0/0/1) 6(0/1/1)
EXT Dn 1(0/0/0) 4(0/0/0) 4(0/1/0)
NBCD Dn 6(0/0/0) 6(0/0/0) 6(0/1/0)
Scc Dn 1(0/0/0) 4(0/0/0) 4(0/1/0)
† Scc Mem 6(0/0/1) 6(0/0/1) 6(0/1/1)
TAS Dn 1(0/0/0) 4(0/0/0) 4(0/1/0)
† TAS Mem 12(1/0/1) 12(1/0/1) 13(1/1/1)
*TST EA 0(0/0/0) 2(0/0/0) 3(0/1/0)
*Add Fetch Effective Address Time
†Add Calculate Effective Address Time
8-34 M68020 USER’S MANUAL MOTOROLA
8.2.12 Shift/Rotate Instructions
The shift/rotate instructions table indicates the number of clock periods needed for the
processor to perform the specified operation on the given addressing mode. Footnotes
indicate when it is necessary to add another table entry to calculate the total effective
execution time for the instruction. The number of bits shifted does not affect execution
time. The total number of clock cycles is outside the parentheses, the number of read,
prefetch, and write cycles is given inside the parentheses as (r/p/w). These cycles are
included in the total clock cycle number.
Instruction Best Case Cache Case Worst Case
LSL Dn (Static) 1(0/0/0) 4(0/0/0) 4(0/1/0)
LSR Dn (Static) 1(0/0/0) 4(0/0/0) 4(0/1/0)
LSL Dn (Dynamic) 3(0/0/0) 6(0/0/0) 6(0/1/0)
LS R Dn (Dynamic) 3(0/0/0) 6(0/0/0) 6(0/1/0)
*LSL Mem by 1 5(0/0/1) 5(0/0/1) 6(0/1/1)
*LSR Mem by 1 5(0/0/1) 5(0/0/1) 6(0/1/1)
ASL Dn 5(0/0/0) 8(0/0/0) 8(0/1/0)
ASR Dn 3(0/0/0) 6(0/0/0) 6(0/1/0)
*ASL Mem by 1 6(0/0/1) 6(0/0/1) 7(0/1/1)
*ASR Mem by 1 5(0/0/1) 5(0/0/1) 6(0/1/1)
ROL Dn 5(0/0/0) 8(0/0/0) 8(0/1/0)
ROR Dn 5(0/0/0) 8(0/0/0) 8(0/1/0)
*ROL Mem by 1 7(0/0/1) 7(0/0/1) 7(0/1/1)
*ROR Mem by 1 7(0/0/1) 7(0/0/1) 7(0/1/1)
ROXL Dn 9(0/0/0) 12(0/0/0) 12(0/1/0)
ROXR Dn 9(0/0/0) 12(0/0/0) 12(0/1/0)
*ROXd Mem by 1 5(0/0/1) 5(0/0/1) 6(0/1/1)
*Add Fetch Effective Address Time
d—Direction of Shift/Rotate, L or R
MOTOROLA M68020 USER’S MANUAL 8-35
8.2.13 Bit Manipulation Instructions
The bit manipulation instructions table indicates the number of clock periods needed for
the processor to perform the specified bit operation on the given addressing mode.
Footnotes indicate when it is necessary to add another table entry to calculate the total
effective execution time for the instruction. The total number of clock cycles is outside the
parentheses; the number of read, prefetch, and write cycles is given inside the
parentheses as (r/p/w). These cycles are included in the total clock cycle number.
Instruction Best Case Cache Case Worst Case
BTST #<data>,Dn 1(0/0/0) 4(0/0/0) 5(0/1/0)
BTST Dn,Dn 1(0/0/0) 4(0/0/0) 5(0/1/0)
** BTST #<data>,Mem 4(0/0/0) 4(0/0/0) 5(0/1/0)
*BTST Dn,Mem 4(0/0/0) 4(0/0/0) 5(0/1/0)
BCHG #<data>,Dn 1(0/0/0) 4(0/0/0) 5(0/1/0)
BCHG Dn,Dn 1(0/0/0) 4(0/0/0) 5(0/1/0)
** BCHG #<data>,Mem 4(0/0/1) 4(0/0/1) 5(0/1/1)
*BCHG Dn,Mem 4(0/0/1) 4(0/0/1) 5(0/1/1)
BCLR #<data>,Dn 1(0/0/0) 4(0/0/0) 5(0/1/0)
BCLR Dn,Dn 1(0/0/0) 4(0/0/0) 5(0/1/0)
** BCLR #<data>,Mem 4(0/0/1) 4(0/0/1) 5(0/1/1)
*BCLR Dn,Mem 4(0/0/1) 4(0/0/1) 5(0/1/1)
BSET #<data>,Dn 1(0/0/0) 4(0/0/0) 5(0/1/0)
BSET Dn,Dn 1(0/0/0) 4(0/0/0) 5(0/1/0)
** BSET #<data>,Mem 4(0/0/1) 4(0/0/1) 5(0/1/1)
*BSET Dn,Mem 4(0/0/1) 4(0/0/1) 5(0/1/1)
*Add Fetch Effective Address Time
**Add Fetch Immediate Address Time
8-36 M68020 USER’S MANUAL MOTOROLA
8.2.14 Bit Field Manipulation Instructions
The bit field manipulation instructions table indicates the number of clock periods needed
for the processor to perform the specified bit field operation using the given addressing
mode. Footnotes indicate when it is necessary to add another table entry to calculate the
total effective execution time for the instruction. The total number of clock cycles is outside
the parentheses; the number of read, prefetch, and write cycles is given inside the
parentheses as (r/p/w). These cycles are included in the total clock cycle number.
Instruction Best Case Cache Case Worst Case
BFTST Dn 3(0/0/0) 6(0/0/0) 7(0/1/0)
‡ BFTST Mem (< 5 Bytes) 11(1/0/0) 11(1/0/0) 12(1/1/0)
‡ BFTST Mem (5 Bytes) 15(2/0/0) 15(2/0/0) 16(2/1/0)
BFCHG Dn 9(0/0/0) 12(0/0/0) 12(0/1/0)
‡ BFCHG Mem (< 5 Bytes) 16(1/0/1) 16(1/0/1) 16(1/1/1)
‡ BFCHG Mem (5 Bytes) 24(2/0/2) 24(2/0/2) 24(2/1/2)
BFCLR Dn 9(0/0/0) 12(0/0/0) 12(0/1/0)
‡ BFCLR Mem (< 5 Bytes) 16(1/0/1) 16(1/0/1) 16(1/1/1)
‡ BFCLR Mem (5 Bytes) 24(2/0/2) 24(2/0/2) 24(2/1/2)
BFSET Dn 9(0/0/0) 12(0/0/0) 12(0/1/0)
‡ BFSET Mem (< 5 Bytes) 16(1/0/1) 16(1/0/1) 16(1/1/1)
‡ BFSET Mem (5 Bytes) 24(2/0/2) 24(2/0/2) 24(2/1/2)
BFEXTS Dn 5(0/0/0) 8(0/0/0) 8(0/1/0)
‡ BFEXTS Mem (< 5 Bytes) 13(1/0/0) 13(1/0/0) 13(1/1/0)
‡ BFEXTS Mem (5 Bytes) 18(2/0/0) 18(2/0/0) 18(2/1/0)
BFEXTU Dn 5(0/0/0) 8(0/0/0) 8(0/1/0)
‡ BFEXTU Mem (< 5 Bytes) 13(1/0/0) 13(1/0/0) 13(1/1/0)
‡ BFEXTU Mem (5 Bytes) 18(2/0/0) 18(2/0/0) 18(2/1/0)
BFINS Dn 7(0/0/0) 10(0/0/0) 10(0/1/0)
‡ BFINS Mem (< 5 Bytes) 14(1/0/1) 14(1/0/1) 15(1/1/1)
‡ BFINS Mem (5 Bytes) 20(2/0/2) 20(2/0/2) 21(2/1/2)
BFFFO Dn 15(0/0/0) 18(0/0/0) 18(0/1/0)
‡ BFFFO Mem (< 5 Bytes) 24(1/0/0) 24(1/0/0) 24(1/1/0)
‡ BFFFO Mem (5 Bytes) 32(2/0/0) 32(2/0/0) 32(2/1/0)
‡Add Calculate Immediate Address Time
NOTE: A bit field of 32 bits may span five bytes that require two operand cycles to access or may span four bytes that
require only one operand cycle to access.
MOTOROLA M68020 USER’S MANUAL 8-37
8.2.15 Conditional Branch Instructions
The conditional branch instructions table indicates the number of clock periods needed for
the processor to perform the specified branch on the given branch size, with complete
execution times given. No additional tables are needed to calculate total effective
execution time for these instructions. The total number of clock cycles is outside the
parentheses; the number of read, prefetch, and write cycles is given inside the
parentheses as (r/p/w). These cycles are included in the total clock cycle number.
Instruction Best Case Cache Case Worst Case
Bcc (Taken) 3(0/0/0) 6(0/0/0) 9(0/2/0)
Bcc.B (Not Taken) 1(0/0/0) 4(0/0/0) 5(0/1/0)
Bcc.W (Not Taken) 3(0/0/0) 6(0/0/0) 7(0/1/0)
Bcc.L (Not Taken) 3(0/0/0) 6(0/0/0) 9(0/2/0)
DBcc (cc = False, Count Not Expired) 3(0/0/0) 6(0/0/0) 9(0/2/0)
DBcc (cc = False, Count Expired) 7(0/0/0) 10(0/0/0) 10(0/3/0)
DBcc (cc = True) 3(0/0/0) 6(0/0/0) 7(0/1/0)
8-38 M68020 USER’S MANUAL MOTOROLA
8.2.16 Control Instructions
The control instructions table indicates the number of clock periods needed for the
processor to perform the specified operation. Footnotes specify when it is necessary to
add an entry from another table to calculate the total effective execution time for the given
instruction. The total number of clock cycles is outside the parentheses; the number of
read, prefetch, and write cycles is given inside the parentheses as (r/p/w). These cycles
are included in the total clock cycle number.
Instruction Best Case Cache Case Worst Case
ANDI to SR 9(0/0/0) 12(0/0/0) 15(0/2/0)
EORI to SR 9(0/0/0) 12(0/0/0) 15(0/2/0)
ORI to SR 9(0/0/0) 12(0/0/0) 15(0/2/0)
ANDI to CCR 9(0/0/0) 12(0/0/0) 15(0/2/0)
EORI to CCR 9(0/0/0) 12(0/0/0) 15(0/2/0)
ORI to CCR 9(0/0/0) 12(0/0/0) 15(0/2/0)
BSR 5(0/0/1) 7(0/0/1) 13(0/2/1)
** CALLM (Type 0) 28(2/0/6) 30(2/0/6) 36(2/2/6)
** CALLM (Type 1)—No Stack Copy 48(5/0/8) 50(5/0/8) 56(5/2/8)
** CALLM (Type 1)—No Stack Copy 55(6/0/8) 57(6/0/8) 64(6/2/8)
** CALLM (Type 1)—Stack Copy 63 + 6n(7 + n/0/8 + n) 65 + 6n(7 + n/0/8 + n) 71 + 6n(7 + n/2/8 + n)
‡ CAS (Successful Compare) 15(1/0/1) 15(1/0/1) 16(1/1/1)
‡ CAS (Unsuccessful Compare) 12(1/0/0) 12(1/0/0) 13(1/1/0)
CAS2 (Successful Compare) 23(2/0/2) 25(2/0/2) 28(2/2/2)
CAS2 (Unsuccessful Compare) 19(2/0/0) 22(2/0/0) 25(2/2/0)
*CHK 8(0/0/0) 8(0/0/0) 8(0/1/0)
** CHK2 EA,Rn 16(2/0/0) 18(2/0/0) 18(2/1/0)
% JMP 1(0/0/0) 4(0/0/0) 7(0/2/0)
% JSR 3(0/0/1) 5(0/0/1) 11(0/2/1)
† LEA 2(0/0/0) 2(0/0/0) 3(0/1/0)
LINK.W 3(0/0/1) 5(0/0/1) 7(0/1/1)
LINK.L 4(0/0/1) 6(0/0/1) 10(0/2/1)
NOP 2(0/0/0) 2(0/0/0) 3(0/1/0)
† PEA 3(0/0/1) 5(0/0/1) 6(0/1/1)
RTD 9(1/0/0) 10(1/0/0) 12(1/2/0)
RTM (Type 0) 18(4/0/0) 19(4/0/0) 22(4/2/0)
RTM (Type 1) 31(6/0/1) 32(6/0/1) 35(6/2/1)
RTR 13(2/0/0) 14(2/0/0) 15(2/2/0)
RTS 9(1/0/0) 10(1/0/0) 12(1/2/0)
UNLK 5(1/0/0) 6(1/0/0) 7(1/1/0)
n—Number of Operand Transfers Required %—Add Jump Effective Address Time
*Add Fetch Effective Address Time **Add Fetch Immediate Address Time
†Add Calculate Effective Address Time ‡Add Calculate Immediate Address Time
MOTOROLA M68020 USER’S MANUAL 8-39
8.2.17 Exception-Related Instructions
The exception-related instructions table indicates the number of clock periods needed for
the processor to perform the specified exception-related action. Footnotes specify when it
is necessary to add the entry from another table to calculate the total effective execution
time for the given instruction. The total number of clock cycles is outside the parentheses;
the number of read, prefetch, and write cycles is given inside the parentheses as (r/p/w).
These cycles are included in the total clock cycle number.
Instruction Best Case Cache Case Worst Case
BKPT 9(1/0/0) 10(1/0/0) 10(1/0/0)
Interrupt (I-Stack) 26(2/0/4) 26(2/0/4) 33(2/2/4)
Interrupt (M-Stack) 41(2/0/8) 41(2/0/8) 48(2/2/8)
RESET Instruction 518(0/0/0) 518(0/0/0) 519(0/1/0)
STOP 8(0/0/0) 8(0/0/0) 8(0/0/0)
Trace 25(1/0/5) 25(1/0/5) 32(1/2/5)
TRAP #n 20(1/0/4) 20(1/0/4) 27(1/2/4)
Illegal Instruction 20(1/0/4) 20(1/0/4) 27(1/2/4)
A-Line Trap 20(1/0/4) 20(1/0/4) 27(1/2/4)
F-Line Trap 20(1/0/4) 20(1/0/4) 27(1/2/4)
Privilege Violation 20(1/0/4) 20(1/0/4) 27(1/2/4)
TRAPcc (Trap) 23(1/0/5) 25(1/0/5) 32(1/2/5)
TRAPcc (No Trap) 1(0/0/0) 4(0/0/0) 5(0/1/0)
TRAPcc.W (Trap) 23(1/0/5) 25(1/0/5) 33(1/3/5)
TRAPcc.W (No Trap) 3(0/0/0) 6(0/0/0) 7(0/1/0)
TRAPcc.L (Trap) 23(1/0/5) 25(1/0/5) 33(1/3/5)
TRAPcc.L (No Trap) 5(0/0/0) 8(0/0/0) 10(0/2/0)
TRAPV (Trap) 23(1/0/5) 25(1/0/5) 32(1/2/5)
TRAPV (No Trap) 1(0/0/0) 4(0/0/0) 5(0/1/0)
8-40 M68020 USER’S MANUAL MOTOROLA
8.2.18 Save and Restore Operations
The save and restore operations table indicates the number of clock periods needed for
the processor to perform the specified state save or return from exception, with complete
execution times and stack length given. No additional tables are needed to calculate total
effective execution time for these operations. The total number of clock cycles is outside
the parentheses; the number of read, prefetch, and write cycles is given inside the
parentheses as (r/p/w). These cycles are included in the total clock cycle number.
Operation Best Case Cache Case Worst Case
Bus Cycle Fault (Short) 42(1/0/10) 43(1/0/10) 50(1/2/10)
Bus Cycle Fault (Long) 79(1/0/24) 79(1/0/24) 86(1/2/24)
RTE (Normal) 20(4/0/0) 21(4/0/0) 24(4/2/0)
RTE (Six Word) 20(4/0/0) 21(4/0/0) 24(4/2/0)
RTE (Throwaway)*15(4/0/0) 16(4/0/0) 39(4/0/0)
RTE (Coprocessor) 31(7/0/0) 32(7/0/0) 33(7/1/0)
RTE (Short Fault) 42(10/0/0) 43(10/0/0) 45(10/2/0)
RTE (Long Fault) 91(24/0/0) 92(24/0/0) 94(24/2/0)
*Add the time for RTE on second stack frame.
MOTOROLA M68020 USER’S MANUAL 9-1
SECTION 9
APPLICATIONS INFORMATION
This section, which provides guidelines for using the MC68020/EC020, contains
information on floating-point units, byte select logic, power and ground considerations,
clock driver, memory interface, access time calculations, module support, and access
levels.
9.1 FLOATING-POINT UNITS
Floating-point support for the MC68020/EC020 is provided by the MC68881 floating-point
coprocessor or the MC68882 enhanced floating-point coprocessor. Both devices offer a
full implementation of the
IEEE Standard for Binary Floating-Point Arithmetic
(754). The
MC68882 is a pin- and software-compatible upgrade of the MC68881, with an optimized
MPU interface that provides over 1.5 times the performance of the MC68881 at the same
clock frequency.
Both coprocessors provide a logical extension to the integer data processing capabilities
of the main processor. They contain a high-performance floating-point arithmetic unit and
a set of floating-point data registers that are utilized in a manner that is analogous to the
use of the integer data registers of the processor. The MC68881/MC68882 instruction set,
a natural extension of all earlier members of the M68000 family, supports all addressing
modes and data types of the host MC68020/EC020. The programmer perceives the
MC68020/EC020 coprocessor execution model as if both devices are implemented on
one chip. In addition to supporting the full IEEE standard, the MC68881 and MC68882
provide a full set of trigonometric and transcendental functions, on-chip constants, and a
full 80-bit extended-precision real data format.
The interface of the MC68020/EC020 to the MC68881 or MC68882 is easily tailored to
system cost/performance needs. The MC68020/EC020 and the MC68881/MC68882
communicate via standard asynchronous M68000 bus cycles. All data transfers are
performed by the main processor at the request of the MC68881/MC68882; thus, memory
management, bus errors, address errors, and bus arbitration function as if the
MC68881/MC68882 instructions are executed by the main processor. The floating-point
unit and the processor can operate at different clock speeds, and up to seven floating-
point coprocessors can simultaneously reside in an MC68020/EC020 system.
Figure 9-1 illustrates the coprocessor interface connection of an MC68881/MC68882 to an
MC68020/EC020 (uses entire 32-bit data bus). The MC68881/MC68882 is configured to
operate with a 32-bit data bus when both its A0 and SIZE pins are connected to VCC.
Refer to the MC68881UM/AD,
MC68881/MC68882 Floating-Point Coprocessor User's
Manual
, for configuring the MC68881/MC68882 for smaller data bus widths.
9-2 M68020 USER’S MANUAL MOTOROLA
MC68020/EC020
MC68881/MC68882
CHIP
SELECT
D
ECOD
E
FC2–FC0
A31–A2
0
A19–A1
6
A15–A1
3
A12–A
5
A4–A
1
A
0
A
S
D
S
R/W
D31–D2
4
D23–D1
6
D15–D
8
D7–D
0
DSACK
0
DSACK
1
MAIN PROCESSOR
CLOCK
C
S
S
IZE
A
4–A1
A
0
A
S
D
S
R
/W
D
31–D2
4
D
23–D1
6
D
15–D8
D
7–D0
D
SACK
0
D
SACK
1
COPROCESSOR
CLOCK
V
CC
V
CC
*
*
For the MC68EC020, A23–A0.
Figure 9-1. 32-Bit Data Bus Coprocessor Connection
The chip select (CS) decode circuitry is asynchronous logic that detects when a particular
floating-point coprocessor is addressed. The MC68020/EC020 signals used by the logic
include FC2–FC0 and A19–A13. Refer to Section 7 Coprocessor Interface Description
for more information concerning the encoding of these signals. All or just a subset of these
lines may be decoded, depending on the number of coprocessors in the system and the
degree of redundant mapping allowed in the system.
For example, if a system has only one coprocessor, the full decoding of the ten signals
(FC2–FC0 and A19–A13), provided by the PAL equations in Figure 9-3, is not absolutely
necessary. It may be sufficient to use only FC1–FC0 and A17–A16. FC1–FC0 indicate
when a bus cycle is operating in either CPU space ($7) or user-defined space ($3), and
A17–A16 encode the CPU space type as coprocessor space ($2). A15–A13 can be
ignored in this case because they encode the coprocessor identification code (CpID) used
to differentiate between multiple coprocessors in a system. Motorola assemblers always
default to a CpID of $1 for floating-point instructions; this can be controlled with assembler
directives if a different CpID is desired or if multiple coprocessors exist in the system.
MOTOROLA M68020 USER’S MANUAL 9-3
The major concern of a system designer is to design a CS interface that meets the AC
electrical specifications for both the MC68020/EC020 (MPU) and the MC68881/MC68882
(FPCP) without adding unnecessary wait states to FPCP accesses. The following
maximum specifications (relative to CLK low) meet these objectives:
tCLK low to AS low ≤ (MPU Spec 1 – MPU Spec 47A – FPCP Spec 19) (9-1)
tCLK low to CS low ≤ (MPU Spec 1 – MPU Spec 47A – FPCP Spec 19) (9-2)
Even though requirement (9-1) is not met under worst-case conditions, if the MPU AS is
loaded within specifications and the AS input to the FPCP is unbuffered, the requirement
is met under typical conditions. Designing the CS generation circuit to meet requirement
(9-2) provides the highest probability that accesses to the FPCP occur without
unnecessary wait states. A PAL 16L8 (see Figure 9-2) with a maximum propagation delay
of 10 ns, programmed according to the equations in Figure 9-3, can be used to generate
CS. For a 25-MHz system, tCLK low to CS low is less than or equal to 10 ns when this
design is used. Should worst-case conditions cause tCLK low to AS low to exceed
requirement (1), one wait state is inserted in the access to the FPCP; no other adverse
effects occur. Figure 9-4 shows the bus cycle timing for this interface. Refer to
MC68881UM/AD,
MC68881/MC68882 Floating-Point Coprocessor User's Manual
, for
FPCP specifications.
The circuit that generates CS must meet another requirement. When a nonfloating-point
access immediately follows a floating-point access, CS (for the floating-point access) must
be negated before AS and DS (for the subsequent access) are asserted. The PAL circuit
previously described also meets this requirement.
PAL 16L8
10 ns
CLK
AS
FC2
FC1
FC0
A19
A18
A17
A16
GND
V
NC
NC
NC
NC
A13
A14
CLKD
CS
A15
CC
Figure 9-2. Chip Select Generation PAL
9-4 M68020 USER’S MANUAL MOTOROLA
PAL16L8
FPCP CS GENERATION CIRCUITRY FOR 25 MHz OPERATION
MOTOROLA INC., AUSTIN, TEXAS
INPUTS: CLK ~AS FC2 FC1 FC0 A19 A18 A17 A16 A15 A14 A13
OUTPUTS: ~CS CLKD
!~CS = FC2 *FC1 *FC0 ;cpu space = $7
*!A19 *!A18 *A17 *!A16 ;coprocessor access = $2
*!A15 *!A14 *A13 ;coprocessor id = $1
*!CLK ;qualified by MPU clock low
+FC2 *FC1 *FC0 ;cpu space = $7
*!A19 *!A18 *A17 *!A16 ;coprocessor access = $2
*!A15 *!A14 *A13 ;coprocessor id = $1
*!~AS ;qualified by address strobe low
+FC2 *FC1 *FC0 ;cpu space = $7
*!A19 *!A18 *A17 *!A16 ;coprocessor access = $2
*!A15 *!A14 *A13 ;coprocessor id = $1
*CLKD ;qualified by CLKD (delayed CLK)
CLKD = CLK
Description: There are three terms to the CS generation. The first term denotes the earliest time CS can be asserted.
The second term is used to assert CS until the end of the FPCP access. The third term is to ensure that no race
condition occurs in case of a late AS.
Figure 9-3. Chip Select PAL Equations
9
CLK
AS
CS
8
DSACK1/DSACK0
47A
START
19
FPCP SPECIFICATION
MPU SPECIFICATION
Figure 9-4. Bus Cycle Timing Diagram
MOTOROLA M68020 USER’S MANUAL 9-5
9.2 BYTE SELECT LOGIC FOR THE MC68020/EC020
The MC68020/EC020 architecture supports byte, word, and long-word operand transfers
to any 8-, 16-, or 32-bit data port, regardless of alignment. This feature allows the
programmer to write code that is not bus-width specific. When accessed, the peripheral or
memory subsystem reports its actual port size to the controller, and the MC68020/EC020
then dynamically sizes the data transfer accordingly, using multiple bus cycles when
necessary. The following paragraphs describe the generation of byte select control signals
that enable the dynamic bus sizing mechanism, the transfer of differently sized operands,
and the transfer of misaligned operands to operate correctly.
The following signals control the MC68020/EC020 operand transfer mechanism:
A1, A0 — Address signals. The most significant byte of the operand to be
transferred is addressed directly.
SIZ1, SIZ0 — Transfer size signals. Output of the MC68020/EC020. These
indicate the number of bytes of an operand remaining to be
transferred during a given bus cycle.
R/W— Read/Write signal. Output of the MC68020/EC020. For byte
select generation in MC68020/EC020 systems.
DSACK1, DSACK0 — Data transfer and size acknowledge signals. Driven by an
asynchronous port to indicate the actual bus width of
the port.
The MC68020/EC020 assumes that 16-bit ports are situated on data lines D31–D16, and
that 8-bit ports are situated on data lines D31–D24. This ensures that the following logic
works correctly with the MC68020/EC020's on-chip internal-to-external data bus
multiplexer. Refer to Section 5 Bus Operation for more details on the dynamic bus sizing
mechanism.
The need for byte select signals is best illustrated by an example. Consider a long-word
write cycle to an odd address in word-organized memory. The transfer requires three bus
cycles to complete. The first bus cycle transfers the most significant byte of the long word
on D23–D16. The second bus cycle transfers a word on D31–D16, and the last bus cycle
transfers the least significant byte of the original long word on D31–D24. To prevent
overwriting those bytes that are not used in these transfers, a unique byte data strobe
must be generated for each byte when using devices with 16- and 32-bit port widths.
For noncachable read cycles and all write cycles, the required active bytes of the data bus
for any given bus transfer are a function of the SIZ1, SIZ0 and A1, A0 outputs (see Table
9-1). Individual strobes or select signals can be generated by decoding these four signals
for every bus cycle. Devices residing on 8-bit ports can utilize DS or AS since there is only
one valid byte for any transfer.
9-6 M68020 USER’S MANUAL MOTOROLA
Table 9-1. Data Bus Activity for Byte, Word, and Long-Word Ports
Data Bus Active Sections
Byte (B), Word (W), Long-Word (L) Ports
Transfer Size SIZ1 SIZ0 A1 A0 D31–D24 D23–D16 D15–D8 D7–D0
Byte 0
0
0
0
1
1
1
1
0
0
1
1
0
1
0
1
B W L
B
B W
B
—
W L
—
W
—
—
L
—
—
—
—
L
Word 1
1
1
1
0
0
0
0
0
0
1
1
0
1
0
1
B W L
B
B W
B
W L
W L
W
W
—
L
L
—
—
—
L
L
3 Bytes 1
1
1
1
1
1
1
1
0
0
1
1
0
1
0
1
B W L
B
B W
B
W L
W L
W
W
L
L
L
—
—
L
L
L
Long Word 0
0
0
0
0
0
0
0
0
0
1
1
0
1
0
1
B W L
B
B W
B
W L
W L
W
W
L
L
L
—
L
L
L
L
During cachable read cycles, the addressed device must provide valid data over its full
bus width as indicated by DSACK1/DSACK0. While instructions are always prefetched as
long-word-aligned accesses, data fetches can occur with any alignment and size.
Because the MC68020/EC020 assumes that the entire data bus port size contains valid
data, cachable data read bus cycles must provide as much data as signaled by the port
size during a bus cycle. To satisfy this requirement, the R/ W signal must be included in
the byte select logic for the MC68020/EC020.
Figure 9-5 shows a block diagram of an MC68020/EC020 system with a single memory
bank. The PAL provides memory-mapped byte select signals for an asynchronous 32-bit
port and unmapped byte select signals for other memory banks or ports. Figure 9-6
provides sample equations for the PAL.
The PAL equations and circuits presented here cannot be the optimal implementation for
every system. Depending on the CPU clock frequency, memory access times, and system
architecture, different circuits may be required.
MOTOROLA M68020 USER’S MANUAL 9-7
SIZ0
S
IZ
1
A
0
A
1
FC
0
FC
1
A31–A2
A
S
R/W
D31–D0
UUDA
UMD
A
LMD
A
LLD
A
LLDB
LMD
B
UMD
B
UUD
B
MC68020/EC020
PAL16L8
D7–D0
D15–D8
D23–D16
D31–D24
W
E
W
E
W
E
W
E
A31–A2
32-BIT PORT
UUDA
UMDA
LMDA
LLDA
UNMAPPED BYTE
S
ELECTS FOR OTHE
R
3
2-BIT PORTS
CPU
MC74F32
MC74F32
MC74F32
MCM60256A
MCM60256A
MCM60256A
MCM60256A
MC74F00
A21–A18
MC74F32
*
*
For the MC68EC020, A23–A2.
*
Figure 9-5. Example MC68020/EC020 Byte Select PAL System Configuration
9-8 M68020 USER’S MANUAL MOTOROLA
PAL16L8
BYTE_SELECT
MC68020/EC020 BYTE DATA SELECT GENERATION FOR 32-BIT PORTS, MAPPED AND UNMAPPED.
MOTOROLA INC., AUSTIN, TEXAS
INPUTS: A0 A1 SIZ0 SIZ1 RW A18 A19 A20 A21 ~CPU
OUTPUTS: ~UUDA ~UMDA ~LMDA ~LLDA ~UUDA ~UMDB ~LMDB ~LLDB
!~UUDA = RW ;enable upper byte on read of 32-bit port
+!A0 *!A1 ;directly addressed, any size
!~UMDA = RW ;enable upper middle byte on read of 32-bit port
+A0 *!A1 ;directly addressed, any size
+!A1 *!SIZ0 ;even word aligned, size word or long word
+!A1 *SIZ1 ;even word aligned, size is word or three byte
!~LMDA = RW ;enable lower middle byte on read of 32-bit port +!A0 *A1
;directly addressed, any size
+!A1 *!SIZ0 *!SIZ1 ;even word aligned, size is long word
+!A1 *SIZ0 *SIZ1 ;even word aligned, size is three byte
+!A1 *A0 *!SIZ0 ;even word aligned, size is word or long word
!~LLDA = RW ;enable lower byte on read of 32-bit port
+A0 *A1 ;directly addressed, any size
+A0 *SIZ0 *SIZ1 ;odd byte alignment, three byte size
+!SIZ0 *!SIZ1 ;size is long word, any address
+A1 *SIZ1 ;odd word aligned, word or three byte size
!~UUDB = RW *!~CPU * (addressb) ;enable upper byte on read of 32-bit port
+!A0 *!A1 *!~CPU * (addressb) ;directly addressed, any size
!~UMDB = RW *!~CPU * (addressb) ;enable upper middle byte on read of 32-bit port
+ A0 *!A1 *!~CPU * (addressb) ;directly addressed, any size
+!A1 *!SIZ0 *!~CPU * (addressb) ;even word aligned, size word or long word
+!A1 *SIZ1 *!~CPU * (addressb) ;even word aligned, size is word or three byte
!~LMDB =RW *!~CPU * (addressb) ;enable lower middle byte on read of 32-bit port
+!A0 * A1 *!~CPU * (addressb) ;directly addressed, any size
+!A1 *!SIZ0 *!SIZ1 *!~CPU * (addressb) ;even word aligned, size is long word
+!A1 * SIZ0 * SIZ1 *!~CPU * (addressb) ;even word aligned, size is three byte
+!A1 * A0 *!SIZ0 *!~CPU * (addressb) ;even word aligned, size is word or long word
!~LLDB =RW *!~CPU * (addressb) ;enable lower byte on read of 32-bit port
+A0 * A1 *!~CPU * (addressb) ;directly addressed, any size
+ A0 * SIZ0 * SIZ1 *!~CPU * (addressb) ;odd byte alignment, three byte size
+!SIZ0 *!SIZ1 *!~CPU * (addressb) ;size is long word, any address
+A1 * SIZ1 *!~CPU * (addressb) ;odd word aligned, word or three byte size
DESCRIPTION: Byte select signals for writing. On reads, all byte selects are asserted if the respective memory block is addressed.
The input signal CPU prevents byte select assertion during CPU space cycles and is derived from NANDing FC1–FC0 or FC2–FC0.
The label (addressb) is a designer-selectable combination of address lines used to generate the proper address decode for the
system's memory bank. With the address lines given here, the decode block size is 256 Kbytes to 2 Mbytes. A similar address might
be included in the equations for UUDA, UMDA, etc. if the designer wishes them to be memory mapped also.
Figure 9-6. MC68020/EC020 Byte Select PAL Equations
MOTOROLA M68020 USER’S MANUAL 9-9
9.3 POWER AND GROUND CONSIDERATIONS
The MC68020/EC020 is fabricated in Motorola's advanced HCMOS process and is
capable of operating at clock frequencies of up to 25 MHz. While the use of CMOS for a
device containing such a large number of transistors allows significantly reduced power
consumption compared to an equivalent NMOS circuit, the high clock speed makes the
characteristics of power supplied to the device very important. The power supply must be
able to furnish large amounts of instantaneous current when the MC68020/EC020
performs certain operations, and it must remain within the rated specification at all times.
To meet these requirements, more detailed attention must be given to the power supply
connection to the MC68020/EC020 than is required for NMOS devices operating at slower
clock rates.
To reduce the amount of noise in the power supply connected to the MC68020/EC020
and to provide for the instantaneous current requirements, common capacitive decoupling
techniques should be observed. While there is no recommended layout for this capacitive
decoupling, it is essential that the inductance and distance between these devices and the
MC68020/EC020 be minimized to provide sufficiently fast response time to satisfy
momentary current demands and to maintain a constant supply voltage. It is suggested
that high-frequency, high-quality capacitors be placed as close to the MC68020/EC020 as
possible. Table 9-2 lists the VCC and GND pin assignments for the MC68EC020 PPGA
(RP suffix) package. Table 9-3 lists the VCC and GND pin assignments for the
MC68EC020 PQFP (FG suffix) package. Refer to Section 11 Ordering Information and
Mechanical Data for the V CC and GND pin assignments for the MC68020 packages.
When assigning capacitors to the VCC and GND pins, the noisier pins (address and data
buses) should be heavily decoupled from the internal logic pins. Typical decoupling
practices include a high-frequency, high-quality capacitor to decouple every device on the
printed circuit board; however, due to the power requirements and drive capability of the
MC68020/EC020, each VCC pin should be decoupled with an individual capacitor.
Motorola recommends using a capacitor in the range of 0.01 µF to 0.1 µF on each VCC
pin on each device to provide filtering for most frequencies prevalent in a digital system. In
addition to the individual decoupling, several bulk decoupling capacitors should be placed
onto the printed circuit board with typical values in the range of 33 µF to 330 µF. When
power and ground planes are used with an adequate number of high-frequency, high-
quality capacitors, the system noise will be reduced to the required levels, and the
MC68020/EC020 will function properly. Similar decoupling techniques should also be
observed for other VLSI devices in the system.
In addition to the capacitive decoupling of the power supply, care must be taken to ensure
a low-impedance connection between all MC68020/EC020 VCC and GND pins and the
system power supply. A solid power supply connection from the power and ground planes
to the MC68020/EC020 VCC and GND pins, respectively, will meet this requirement.
Failure to provide connections of sufficient quality between the MC68020/EC020 power
pins and the system power supplies will result in increased assertion delays for external
signals, decreased voltage noise margins, increased system noise, and possible errors in
MC68020/EC020 internal logic.
9-10 M68020 USER’S MANUAL MOTOROLA
Table 9-2. VCC and GND Pin Assignments—
MC68EC020 PPGA (RP Suffix)
Pin Group VCC GND
Address Bus B7, C7 A1, A7, C8, D13
Data Bus K12, M9, N9 J13, L8, M1, M8
Internal Logic D1, D2, E12, E13 F11, F12, J1, J2
Clock — B1
Table 9-3. VCC and GND Pin Assignments—
MC68EC020 PQFP (FG Suffix)
Pin Group VCC GND
Address Bus 90 72, 89, 100
Data Bus 44, 57 26, 43, 58, 59
Internal Logic 7, 8, 70, 71 3, 20, 21, 68, 69
Clock — 4
9.4 CLOCK DRIVER
The MC68020/EC020 is designed to sustain high performance while using low-cost
memory subsystems. The MC68020/EC020 requires a stable clock source that is free of
ringing and ground bounce, has sufficient rise and fall times, and meets the minimum and
maximum high and low cycle times. The individual system may require additional clocks
for peripherals with a minimum amount of clock skew. Two possible clock solutions are
provided with the MC88916 and MC74F803. Many other clock solutions can be used.
Some crystal clock drivers are capable of driving the MC68020/EC020 directly. For slower
speed designs, a simple 74F74 flip-flop meets the clocking needs of the MC68020/EC020.
Coupled with the MC88916 or MC74F803 clock generation and distribution circuit, the
MC68020/EC020 provides simple interface to lower speed memory subsystems. The
MC88916 (see Figure 9-7) and MC74F803 (see Figure 9-8) generate the clock signals
required to minimize the skew between different clocks to multiple devices such as
coprocessors, synchronous state machines, DRAM controllers, and memory subsystems.
The MC88916 clock driver can be used in doubling and synchronizing a low-frequency
clock source. The MC74F803 will provide a controlled skew output for clocking other
peripherals.
MOTOROLA M68020 USER’S MANUAL 9-11
MC68020/EC020
25 MHz
12.5-MHz
OSCILLATOR
CONTROLLER
CLOCK (25 MHz
)
BUS CLOCKS
(25 MHz)
MC88916
CLOCK
(50 MHz)
12.5 MHz
2
CLOCK
(25 MHz)
Figure 9-7. High-Resolution Clock Controller
MC68020/EC020
25 MHz
50-MHz
OSCILLATOR
CONTROLLER
CLOCK (25 MHz
)
BUS CLOCKS
(25 MHz)
MC74F803
CLOCK
(
25 MHz
)
2
Figure 9-8. Alternate Clock Solution
9.5 MEMORY INTERFACE
The MC68020/EC020 is capable of running an external bus cycle in a minimum of three
clocks (refer to Section 5 Bus Operation). The MC68020/EC020 runs an asynchronous
bus cycle, terminated by the DSACK1/DSACK0 signals, and has a minimum duration of
three controller clock periods in which up to four bytes (32 bits) are transferred.
During read operations, the MC68020/EC020 latches data on the last falling clock edge of
the bus cycle, one-half clock before the bus cycle ends. Latching data here, instead of the
next rising clock edge, helps to avoid data bus contention with the next bus cycle and
allows the MC68020/EC020 to receive the data into its execution unit sooner for a net
performance increase.
Write operations also use this data bus timing to allow data hold times from the negating
strobes and to avoid any bus contention with the following bus cycle. This
MC68020/EC020 characteristic allows the system to be designed with a minimum of bus
buffers and latches.
One benefit of the MC68020/EC020 on-chip instruction cache is that the effect of external
wait states on performance is lessened because the caches are always accessed in fewer
than “no wait states,” regardless of the external memory configuration.
9-12 M68020 USER’S MANUAL MOTOROLA
9.6 ACCESS TIME CALCULATIONS
The timing paths that are critical in any memory interface are illustrated and defined in
Figure 9-9.
The type of device that is interfaced to the MC68020/EC020 determines exactly which of
the paths is most critical. The address-to-data paths are typically the critical paths for
static devices since there is no penalty for initiating a cycle to these devices and later
validating that access with the appropriate bus control signal. Conversely, the address -
strobe-to-data-valid path is often most critical for dynamic devices since the cycle must be
validated before an access can be initiated. For devices that signal termination of a bus
cycle before data is validated (e.g., error detection and correction hardware and some
external caches), to improve performance, the critical path may be from the address or
strobes to the assertion of BERR (or BERR and HALT). Finally, the address-valid-to -
DSACK1/DSACK0-asserted path is most critical for very fast devices and external
caches, since the time available between the address becoming valid and the
DSACK1/DSACK0 assertion to terminate the bus cycle is minimal. Table 9-4 provides
the equations required to calculate the various memory access times assuming a 50-
percent duty cycle clock.
CLK
A31–A0
S0
S1
S2
S0
AS
DSACK1/DSACK0
a
c
e
b
d
f
BERR, HALT
D31–D0
NOTE: This diagram illustrates access time calculations only
*
*
For the MC68EC020, A23–A0.
Parameter Description System Equation
aAddress Valid to DSACK1/DSACK0 Asserted tAVDL 9-3
bAS Asserted to DSACK1/DSACK0 Asserted tSADL 9-4
cAddress Valid to BERR/HALT Asserted tAVBHL 9-5
dAS Asserted to BERR/HALT Asserted tSABHL 9-6
eAddress Valid to Data Valid tAVDV 9-7
fAS Asserted to Data Valid tSADV 9-8
MOTOROLA M68020 USER’S MANUAL 9-13
Figure 9-9. Access Time Computation Diagram
9-14 M68020 USER’S MANUAL MOTOROLA
Table 9-4. Memory Access Time Equations at 16.67 and 25 MHz
Equation 16.667 MHz N = 3 N = 4 N = 5 N = 6 N = 7 Unit
9-3 tAVDL = (N – 1) • t1 – t2 – t6 – t47A 61 121 181 241 301 ns
9-4 tSADL = (N – 1) • t1 – t9 – t60 25 85 145 205 265 ns
9-5 tAVBHL = N • t1 – t2 – t6 – t27A 22 46 70 94 118 ns
9-6 tSABHL = (N – 1) • t1 – t9 – t27A 40 70 100 130 160 n s
9-7 tAVDV = N • t1 – t2 – t6 – t27 121 181 241 301 361 ns
9-8 tSADV = (N – 1) • t1 – t9 – t27 85 145 205 265 325 n s
Equation 2 5 M Hz N = 3 N = 4 N = 5 N = 6 N = 7 Unit
9-3 tAVDL = (N – 1) • t1 – t2 – t6 – t47A 31 71 111 151 191 ns
9-4 tSADL = (N – 1) • t1 – t9 – t60 17 57 97 137 177 ns
9-5 tAVBHL = N • t1 – t2 – t6 – t27A 22 41 60 79 98 ns
9-6 tSABHL = (N – 1) • t1 – t9 – t27A 26 44 62 80 98 ns
9-7 tAVDV = N • t1 – t2 – t6 – t27 71 111 151 191 231 ns
9-8 tSADV = (N – 1) • t1 – t9 – t27 57 97 137 177 217 ns
Where:
tX = Refers to AC Electrical Specification X
t1 = The Clock Period
t2 = The Clock Low Time
t3 = The Clock High Time
t6 = The Clock High to Address Valid Time
t9 = The Clock Low to AS Low Delay
t27 = The Data-In to Clock Low Setup Time
t27A = The BERR/HALT to Clock Low Setup Time
t47A = The Asynchronous Input Setup Time
N = The Total Number of Clock Periods in the Bus Cycle (N ≥ 3 Cycles)
During asynchronous bus cycles, DSACK1/DSACK0 are used to terminate the current
bus cycle. In true asynchronous operations, such as accesses to peripherals operating at
a different clock frequency, either or both signals may be asserted without regard to the
clock, and then data must be valid a certain amount of time later as defined by
specification 31. With a 25-MHz controller, this time is 32 ns after DSACK1/DSACK0
asserts; with a 16.67-MHz controller, this time is 50 ns after DSACK1/DSACK0 asserts
(both numbers vary with the actual clock frequency).
However, many local memory systems do not operate in a truly asynchronous manner
because either the memory control logic can be related to the MC68020/EC020 clock or
worst-case propagation delays are known; thus, asynchronous setup times for the
DSACK1/DSACK0 signals can be guaranteed. The timing requirements for this pseudo-
synchronous DSACK1/DSACK0 generation is governed by the equation for tAVDL.
MOTOROLA M68020 USER’S MANUAL 9-15
Another way to optimize the CPU-to-memory access times in a system is to use a clock
frequency less than the rated maximum of the specific MC68020/EC020 device. Table 9-5
provides calculated tAVDV (see Equation 9-7 of Table 9-4) results for a 16 MHz
MC68020/EC020 and a 25 MHz MC68020/EC020 operating at various clock frequencies.
If the system uses other clock frequencies, the above equations can be used to calculate
the exact access times.
Table 9-5. Calculated tAVDV Values for Operation at Frequencies
Less Than or Equal to the CPU Maximum Frequency Rating
Equation 9-7 tAVDV 16-MHz MC68020/EC020 25-MHz MC68020/EC020
Clocks Per (N) and Type
Bus Cycle Wait
States Clock at
12.5 MHz Clock at
16.67 MHz Clock at
16.67 MHz Clock at
20 MHz Clock at
25 MHz
3 Clock Asynchronous 0 181 121 131 101 71
4 Clock Asynchronous 1 261 181 191 151 111
5 Clock Asynchronous 2 341 241 251 201 151
6 Clock Asynchronous 3 421 301 311 251 191
9.7 MODULE SUPPORT
The MC68020/EC020 includes support for modules with the CALLM and RTM
instructions. The CALLM instruction references a module descriptor. This descriptor
contains control information for entry into the called module. The CALLM instruction
creates a module stack frame and stores the current module state in that frame and loads
a new module state from the referenced descriptor. The RTM instruction recovers the
previous module state from the stack frame and returns to the calling module.
The module interface facilitates finer resolution of access control by external hardware.
Although the MC68020/EC020 does not interpret the access control information, it
communicates with external hardware when the access control is to be changed and
relies on the external hardware to verify that the changes are legal.
9.7.1 Module Descriptor
Figure 9-10 illustrates the format of the module descriptor. The first long word contains
control information used during execution of the CALLM instruction. The remaining
locations contain data that can be loaded into processor registers by the CALLM
instruction.
9-16 M68020 USER’S MANUAL MOTOROLA
OPT
TYPE
ACCESS LEVEL
(RESERVED, MUST BE ZERO)
31
28
23
15
0
BASE
+$04
MODULE ENTRY WORD POINTER
MODULE DATA AREA POINTER
ADDITIONAL USER-DEFINED INFORMATION
+$08
+$0C
+$10
29
24
16
Figure 9-10. Module Descriptor Format
The opt field specifies how arguments are to be passed to the called module; the
MC68020/EC020 recognizes only the options of 000 and 100; all others cause a format
exception. The 000 option indicates that the called module expects to find arguments from
the calling module on the stack just below the module stack frame. In cases where there is
a change of stack pointer during the call, the MC68020/EC020 will copy the arguments
from the old stack to the new stack. The 100 option indicates that the called module will
access the arguments from the calling module through an indirect pointer in the stack of
the calling module. Hence, the arguments are not copied, but the MC68020/EC020 puts
the value of the stack pointer from the calling module in the module stack frame.
The type field specifies the type of the descriptor; the MC68020/EC020 only recognizes
descriptors of type $00 and $01; all others cause a format exception. The $00 type
descriptor defines a module for which there is no change in access rights, and the called
module builds its stack frame on top of the stack used by the calling module. The $01 type
descriptor defines a module for which there may be a change in access rights; such a
called module may have a separate stack area from that of the calling module.
The access level field is used only with the type $01 descriptor and is passed to external
hardware to change the access control.
The module entry word pointer specifies the entry address of the called module. The first
word at the entry address (see Figure 9-11) specifies the register to be saved in the
module stack frame and then loaded with the module descriptor data area pointer; the first
instruction of the module starts with the next word. The module descriptor data area
pointer field contains the address of the called module data area.
If the access change requires a change of stack pointer, the old value is saved in the
module stack frame, and the new value is taken from the module descriptor stack pointer
field. Any further information in the module descriptor is user defined.
O
PERATION WORD OF FIRST INSTRUCTIO
N
D
/
A
15
14
R
EGISTE
R
12
11
9
0
8
0
7
0
6
5
0
0 0
4
0
3
0
2
0
1
000
10
0
Figure 9-11. Module Entry Word
MOTOROLA M68020 USER’S MANUAL 9-17
All module descriptor types $10–$1F are reserved for user definition and cause a format
error exception. This provides the user with a means of disabling any module by setting a
single bit in its descriptor without loss of any descriptor information.
If the called module does not wish the module data area pointer to be loaded into a
register, the module entry word can select register A7, and the loaded value will be
overwritten with the correction stack pointer value after the module stack frame is created
and filled.
9.7.2 Module Stack Frame
Figure 9-12 illustrates the format of the module stack frame. This frame is constructed by
the CALLM instruction and is removed by the RTM instruction. The first and second long
words contain control information passed by the CALLM instruction to the RTM instruction.
The module descriptor pointer contains the address of the descriptor used during the
module call. All other locations contain information to be restored on return to the calling
module.
The PC is the saved address of the instruction following the CALLM instruction. The opt
and type fields, which specify the argument options and type of module stack frame, are
copied to the frame from the module descriptor by the CALLM instruction; the RTM
instruction will cause a format error if the opt and type fields do not have recognizable
values. The access level is the saved access control information, which is saved from
external hardware by the CALLM instruction and restored by the RTM instruction. The
argument count field is set by the CALLM instruction and is used by the RTM instruction to
remove arguments from the stack of the calling module. The contents of the CCR are
saved by the CALLM instruction and restored by the RTM instruction. The saved stack
pointer field contains the value of the stack pointer when the CALLM instruction started
execution, and that value is restored by RTM. The saved module data area pointer field
contains the saved value of the module data area pointer register from the calling module.
TYPE
SAVED ACCESS LEVEL
15
12
7
0
SP
+$08
SAVED PROGRAM COUNTER
SAVED MODULE DATA AREA POINTER
ARGUMENTS (OPTIONAL)
+$0C
+$10
MODULE DESCRIPTION POINTER
(RESERVED)
CONDITION CODES
ARGUMENT COUNT
OPT
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
+$18
13
8
Figure 9-12. Module Call Stack Frame
9-18 M68020 USER’S MANUAL MOTOROLA
9.8 ACCESS LEVELS
The MC68020/EC020 module mechanism supports a finer level of access control beyond
the distinction between user and supervisor privilege levels. The module mechanism
allows a module with limited access rights to call a module with greater access rights. With
the help of external hardware, the processor can verify that an increase in access rights is
allowable or can detect attempts by a module to gain access rights to which it is not
entitled.
Type $01 module descriptors and module stack frames indicate a request to change
access levels. While processing a type $01 descriptor or frame, the CALLM and RTM
instructions communicate with external access control hardware via accesses in the CPU
space. For these accesses, A19–A16 equal 0001. Figure 9-13 shows the address map for
these CPU space accesses. If the processor receives a bus error on any of these CPU
space accesses during the execution of a CALLM or RTM instruction, the processor will
take a format error exception.
31
0
C
A
L
23
D
A
L
24
A
CCESS STATUS REGISTE
R
I
A
L
$
0
0
$
0
4
$
0
8
$0
C
$
4
0
$
4
4
$
4
8
$4
C
$
5
0
$
5
4
$
5
8
$5
C
(
UNUSED, RESERVED
)
(
UNUSED, RESERVED
)
(
UNUSED, RESERVED
)
(
UNUSED, RESERVED
)
F
UNCTION CODE 5 DESCRIPTOR ADDRESS (SUPERVISOR PROGRAM
)
F
UNCTION CODE 6 DESCRIPTOR ADDRES
S
F
UNCTION CODE 7 DESCRIPTOR ADDRESS (CPU SPACE
)
F
UNCTION CODE 4 DESCRIPTOR ADDRESS (SUPERVISOR DATA
)
F
UNCTION CODE 3 DESCRIPTOR ADDRES
S
F
UNCTION CODE 2 DESCRIPTOR ADDRESS (USER PROGRAM
)
F
UNCTION CODE 1 DESCRIPTOR ADDRESS (USER DATA
)
F
UNCTION CODE 0 DESCRIPTOR ADDRES
S
Figure 9-13. Access Level Control Bus Registers
The current access level register (CAL) contains the access level rights of the currently
executing module. The increase access level register (IAL) is the register through which
the processor requests increased access rights. The decrease access level register (DAL)
is the register through which the processor requests decreased access rights. The formats
of these three registers are undefined to the main processor, but the main processor
assumes that information read from the module descriptor stack frame or the CAL can be
meaningfully written to the IAL or the DAL. The access status register allows the
processor to query the external hardware as to the legality of intended access level
transitions. Table 9-6 lists the valid values of the access status register.
MOTOROLA M68020 USER’S MANUAL 9-19
Table 9-6. Access Status Register Codes
Value Validity Processor Action
$00 Invalid Format Error
$01 Valid No Change in Access Rights
$02–$03 Valid Change Access Rights with No Change of Stack Pointer
$04–$07 Valid Change Access Rights and Change Stack Pointer
Other Undefined Undefined (Take Format Error Exception)
The processor uses the descriptor address registers during the CALLM instruction to
communicate the address of the type $01 descriptor, allowing external hardware to verify
that the address is a valid address for a type $01 descriptor. This validation prevents a
module from creating a type $01 descriptor to increase its access rights.
9.8.1 Module Call
The CALLM instruction is used to make the module call. For the type $00 module
descriptor, the processor creates and fills the module stack frame at the top of the active
system stack. The condition codes of the calling module are saved in the CCR field of the
frame. If opt is equal to 000 (arguments passed on the stack) in the module descriptor, the
MC68020/EC020 does not save the stack pointer or load a new stack pointer value. The
processor uses the module entry word to save and load the module data area pointer
register and then begins execution of the called module.
For the type $01 module descriptor, the processor must first obtain the current access
level from external hardware. It also verifies that the calling module has the right to read
from the area pointed to by the current value of the stack pointer by reading from that
address. It passes the descriptor address and increase access level to external hardware
for validation and then reads the access status. If external hardware determines that the
change in access rights should not be granted, the access status is zero, and the
processor takes a format error exception. No visible processor registers are changed, nor
should the current access level enforced by external hardware be changed. If external
hardware determines that a change should be granted, the external hardware changes its
access level, and the processor proceeds. If the access status register indicates that a
change in the stack pointer is required, the stack pointer is saved internally, a new value is
loaded from the module descriptor, and arguments are copied from the calling stack to the
new stack. Finally, the module stack frame is created and filled on the top of the current
stack. The condition codes of the calling module are saved in the CCR field of the frame.
Execution of the called module then begins as with a type $00 descriptor.
9-20 M68020 USER’S MANUAL MOTOROLA
9.8.2 Module Return
The RTM instruction is used to return from a module. For the type $00 module stack
frame, the processor reloads the condition codes, the PC, and the module data area
pointer register from the frame. The frame is removed from the top of the stack, the
argument count is added to the stack pointer, and execution returns to the calling module.
For the type $01 module stack frame, the processor reads the access level, condition
codes, PC, saved module data area pointer, and saved stack pointer from the module
stack frame. The access level is written to the DAL for validation by external hardware; the
processor then reads the access status to check the validation. If the external hardware
determines that the change in access right should not be granted, the access status is
zero, and the processor takes a format error exception. No visible processor registers are
changed, nor should the current access level enforced by external hardware be changed.
If the external hardware determines that the change in access rights should be granted,
the external hardware changes its access level, the values read from the module stack
frame are loaded into the corresponding processor registers, the argument count is added
to the new stack pointer value, and execution returns to the calling module.
If the called module does not wish the saved module data pointer to be loaded into a
register, the RTM instruction word can select register A7, and the loaded value will be
overwritten with the correct stack pointer value after the module stack frame is
deallocated.
MOTOROLA M68020 USERÕS MANUAL 10-1
SECTION 10
ELECTRICAL CHARACTERISTICS
This section provides the thermal characteristics and electrical specifications for the
MC68020/EC020. Note that the thermal and DC electrical characteristics are listed
separately for the MC68020 and the MC68EC020. All other data applies to both the
MC68020 and the MC68EC020 unless otherwise noted.
10.1 MAXIMUM RATINGS
Rating Symbol Value Unit
Supply Voltage VCC Ð0.3 to +7.0 V
Input Voltage Vin Ð0.5 to +7.0 V
Operating Temperature Range
Minimum Ambient Temperature
Maximum Ambient Temperature
PGA, PPGA, PQFP
Maximum Junction Temperature
CQFP
TA
TA
TJ
0
70
110
°C
°C
°C
Storage Temperature Range Tstg Ð55 to 150 °C
10.2 THERMAL CONSIDERATIONS
The average chip-junction temperature, TJ, in °C can be obtained from:
TJ = TA + (PD ¥ qJA) (10-1)
where:
TA= Ambient Temperature, °C
qJA = Package Thermal Resistance, Junction-to-Ambient, °C/W
PD=P
INT + PI/O
PINT =I
CC X VCC, WattsÑChip Internal Power
PI/O = Power Dissipation on Input and Output PinsÑUser Determined
For most applications, PI/O < PINT and can be neglected.
An approximate relationship between PD and TJ (if PI/O is neglected) is:
PD = K ¸ (TJ + 273°C) (10-2)
The device contains circuitry to
protect the inputs against damage
due to high static voltages or electric
fields; however, normal precautions
should be taken to avoid application
of voltages higher than maximum-
rated voltages to these high-
impedance circuits. Tying unused
inputs to the appropriate logic
voltage level (e.g., either GND or
V
C C ) enhances reliability of
operation.
10-2 M68020 USERÕS MANUAL MOTOROLA
Solving Equations (10-1) and (10-2) for K gives:
K = PD ¥ (TA + 273°C) + qJA¥PD2(10-3)
where K is a constant pertaining to the particular part. K can be determined from equation
(10-3) by measuring PD (at thermal equilibrium) for a known TA. Using this value of K, the
values of PD and TJ can be obtained by solving equations (10-1) and (10-2) iteratively for
any value of TA.
The total thermal resistance of a package (qJA) can be separated into two components,
qJC and qCA. qJC represents the barrier to heat flow from the semiconductor junction to the
package (case) surface, and qCA represents the barrier to heat flow from the case to the
ambient air. These terms are related by the equation:
qJA=qJC + qCA (10-4)
qJC is device related and cannot be influenced by the user. However, qCA is user
dependent and can be minimized by such thermal management techniques as heat sinks,
forced air cooling, and use of thermal convection to increase air flow over the device.
Thus, good thermal design on the part of the user can significantly reduce qCA so that qJA
approximately equals qJC. Substitution of qJC for qJA
in equation (10-1) results in a lower
semiconductor junction temperature.
10.2.1 MC68020 Thermal Characteristics and
DC Electrical Characteristics
MC68020 Thermal Resistance (°C/W)
The following table provides thermal resistance characteristics for junction to ambient and
junction to case for the MC68020 packages with natural convection and no heatsink.
CharacteristicÑNatural Convection and No Heatsink qJA qJC
Thermal Resistance
PGA Package (RC Suffix)
PPGA Package (RP Suffix)
CQFP Package (FE Suffix)
PQFP Package (FC Suffix)
26
32
46
42
3
10
15
20
Resistance is to bottom center (pin side) of case for PGA and PPGA packages, top center of case
for CQFP and PQFP packages.
MC68020 CQFP Package
Table 10-1 provides typical and worst case thermal characteristics for the MC68020
CQFP package both with and without a heatsink. The heatsink used is black anodized
aluminum alloy, 0.72"x0.75"x0.6" high with an omnidirectional 5x6 array of fins.
Attachment was made using Epolite 6400 one part epoxy.
MOTOROLA M68020 USERÕS MANUAL 10-3
Table 10-1. qJA vs. AirflowÑMC68020 CQFP Package
Airflow in Linear Feet/Minute
qJA 0*200 500
Maximum
No Heatsink
With Heatsink
46
35
28
20
24
18
Typical
No Heatsink
With Heatsink
43
32
25
17
21
15
*Natural convection
Table 10-2 shows the relationship between clock speed and power dissipation for any
package type. The worst case operating conditions are used for thermal management
design, while typical values are used for reliability analysis.
Table 10-2. Power vs. Rated Frequency
(at TJ Maximum = 110°C)
Rated Frequency (MHz) PD Maximum (Watts) PD Typical (Watts)
33
25
20
16
1.4
1.2
1.0
0.9
0.84
0.72
0.60
0.54
Table 10-3 shows the relationship between board temperature rise and power dissipation
in the test environment for the CQFP package. Derate qJA based on measurements made
in the application by adding (0.8/PD) * [Tba(application) Ð Tba(table)] to the qJA
values in the
table. Board temperature was measured on the top surface of the board directly under the
device.
Table 10-3. Temperature Rise of Board vs. PD
ÑMC68020 CQFP Package
PD
Natural Convection 0.6W 1.0W 1.75W
Tba (°C)ÑNo Heatsink 18 27 53
Values for thermal resistance presented in this document were derived using the
procedure described in Motorola Reliability Report 7843, ÒThermal Resistance
Measurement Method for MC68XX Microcomponent Devices,Ó and are provided for
design purposes only. Thermal measurements are complex and dependent on procedure
and setup. User-derived values for thermal resistance may differ.
10-4 M68020 USERÕS MANUAL MOTOROLA
MC68020 DC Electrical Characteristics
(VCC = 5.0 Vdc ± 5%; GND = 0 Vdc; Temperature within defined ranges)
Characteristics Symbol Min Max Unit
Input High Voltage VIH 2.0 VCC V
Input Low Voltage VIL GND
Ð0.5
0.8 V
Input Leakage Current
GND £ Vin £ VCC
BERR, BR, BGACK, CLK, IPL2ÐIPL0,
AVEC,DSACK1, DSACK0, CDIS
HALT, RESET
Iin
Ð1.0
Ð20
1.0
20
mA
Hi-Z (Off-State) Leakage Current
@ 2.4 V/0.5 V
A31ÐA0, AS, DBEN, DS, D31ÐD0,
FC2ÐFC0, R/W, RMC, SIZ1ÐSIZ0
ITSI
Ð20 20
mA
Output High Voltage
IOH = 400 mA
A31ÐA0, AS, BG, D31ÐD0, DBEN, DS, R/W,
ECS, IPEND, RMC, SIZ1ÐSIZ0, FC2ÐFC0
VOH
2.4 Ñ
V
Output Low Voltage
IOL = 3.2 mA
IOL = 5.3 mA
IOL = 2.0 mA
IOL = 10.7 mA
A31ÐA0, FC2ÐFC0, SIZ1ÐSIZ0, BG, D31Ð0
AS, DS, R/W, RMC, DBEN, IPEND,
ECS, OCS
HALT, RESET
VOL
Ñ
Ñ
Ñ
Ñ
0.5
0.5
0.5
0.5
V
Power Dissipation (TA = 0°C) PDÑ 2.0 W
Capacitance (see Note)
Vin = 0 V, TA = 25ûC, f = 1
MHz
Cin Ñ20pF
Load Capacitance ECS, OCS
All Other
CLÑ
Ñ
50
130
pF
NOTE: Capacitance is periodically sampled rather than 100% tested.
10.2.2 MC68EC020 Thermal Characteristics and DC Electrical
Characteristics
MC68EC020 Thermal Resistance (°C/W)
The following table provides thermal resistance characteristics for junction to ambient and
junction to case for the MC68EC020 packages with natural convection and no heatsink.
Characteristic Ð Natural Convection and No Heatsink qJA qJC
Thermal Resistance
PPGA Package (RP Suffix)
PQFP Package (FG Suffix)
32
53
10
18
MC68EC020 PQFP Package
Table 10-4 provides typical and worst case thermal characteristics for the MC68EC020
PQFP package without a heatsink.
Table 10-4. qJA vs. AirflowÑMC68EC020 PQFP Package
Airflow in Linear Feet/Minute
qJA 0*50 100 200 300 400
MaximumÑNo Heatsink 53 49 45 41 38 36
TypicalÑNo Heatsink 51 47 43 39 37 35
MOTOROLA M68020 USERÕS MANUAL 10-5
MC68EC020 DC Electrical Characteristics
(VCC = 5.0 Vdc ± 5%; GND = 0 Vdc; Temperature within defined ranges)
Characteristics Symbol Min Max Unit
Input High Voltage VIH 2.0 VCC V
Input Low Voltage VIL GND
Ð0.5
0.8 V
Input Leakage Current
GND £ Vin £ VCC
BERR, BR, CLK, IPL2ÐIPL0,
AVEC,DSACK1, DSACK0, CDIS,
HALT, RESET
Iin
Ð1.0
Ð20
1.0
20
mA
Hi-Z (Off-State) Leakage Current
@ 2.4 V/0.5 V
A23ÐA0, AS, DS, D31ÐD0, FC2ÐFC0,
R/W, RMC, SIZ1ÐSIZ0
ITSI
Ð20 20
mA
Output High Voltage
IOH = 400 mA
A23ÐA0, AS, BG, D31ÐD0, DS, R/W,
RMC, SIZ1ÐSIZ0, FC2ÐFC0
VOH
2.4 Ñ
V
Output Low Voltage
IOL = 3.2 mA
IOL = 5.3 mA
IOL = 10.7 mA
A23ÐA0, FC2ÐFC0, SIZ1ÐSIZ0, BG, D31ÐD0
AS, DS, R/W, RMC,
HALT, RESET
VOL
Ñ
Ñ
Ñ
0.5
0.5
0.5
V
Power Dissipation (TA = 0°C) f = 25 MHz
f = 16 MHz
PINT Ñ
Ñ
1.5
1.2
W
Capacitance (see Note)
Vin = 0 V, TA = 25ûC, f = 1 MH
z
Cin Ñ20pF
Load Capacitance CLÑ 130 pF
NOTE: Capacitance is periodically sampled rather than 100% tested.
10.3 AC ELECTRICAL CHARACTERISTICS
The AC specifications presented consist of output delays, input setup and hold times, and
signal skew times. All signals are specified relative to an appropriate edge of the clock and
possibly to one or more other signals.
The measurement of the AC specifications is defined by the waveforms shown in Figure
10-1. To test the parameters guaranteed by Motorola, inputs must be driven to the voltage
levels specified in Figure 10-1. Outputs are specified with minimum and/or maximum
limits, as appropriate, and are measured as shown in Figure 10-1. Inputs are specified
with minimum setup and hold times, and are measured as shown. Finally, the
measurement for signal-to-signal specifications is also shown.
Note that the testing levels used to verify conformance to the AC specifications do not
affect the guaranteed DC operation of the device as specified in the DC electrical
specifications. The 20 MHz and 33.33 MHz specifications do not apply to the
MC68EC020.
10-6 M68020 USERÕS MANUAL MOTOROLA
0.8 V
2.0 V
BA
DRIVE TO
0.5 V
2.0 V
0.8 V
VALID
OUTPUT n VALID
OUTPUT n + 1
2.0 V
0.8 V
2.0 V
0.8 V
2.0 V
0.8 V
VALID
OUTPUT n VALID
OUTPUT n+1
2.0 V
0.8 V
B
A
VALID
INPUT
2.0 V
0.8 V
2.0 V
0.8 V
DC
DRIVE TO
0.5 V
DRIVE TO
2.4 V
VALID
INPUT
2.0 V
0.8 V
2.0 V
0.8 V
DC
DRIVE
TO 0.5 V
DRIVE
TO 2.4 V
2.0 V
0.8 V
2.0 V
0.8 V
F
E
CLK
OUTPUTS CLK
OUTPUTS CLK
INPUTS CLK
INPUTS CLK
ALL SIGNALS
NOTES:
1. This output timing is applicable to all parameters specified relative to the rising edge of the clock.
2. This output timing is applicable to all parameters specified relative to the falling edge of the clock.
3. This input timing is applicable to all parameters specified relative to the rising edge of the clock.
4. This input timing is applicable to all parameters specified relative to the falling edge of the clock.
5. This timing is applicable to all parameters specified relative to the assertion/negation of another signal.
LEGEND:
A. Maximum output delay specification.
B. Minimum output hold time.
C. Minimum input setup time specification.
D. Minimum input hold time specification.
E. Signal valid to signal valid specification (maximum or minimum).
F. Signal valid to signal invalid specification (maximum or minimum).
FIGURE 10-1
MC68020UM
DRIVE
TO 2.4 V
1
2
3
4
5
Figure 10-1. Drive Levels and Test Points for AC Specifications
MOTOROLA M68020 USERÕS MANUAL 10-7
AC ELECTRICAL CHARACTERISTICSÑCLOCK INPUT (see Figure 10-2)
16.67 MHz 20 MHz 25 MHz*33.33 MHz
Num. Characteristic Min Max Min Max Min Max Min Max Unit
Frequency of Operation 8 16.67 12.5 20 12.5 25 12.5 33.33 MHz
1 Cycle Time 60 125 50 80 40 80 30 80 ns
2,3 Clock Pulse Width
(Measured from 1.5 V to 1.5 V)
24 95 20 54 19 61 14 66 ns
4,5 Clock Rise and Fall Times Ñ 5 Ñ 5 Ñ 4 Ñ 3 ns
*These specifications represent an improvement over previously published specifications for the 25-MHz MC68020 and
are valid only for products bearing date codes of 8827 and later.
0.8 V
2.0 V
FIGURE 10-2
MC68020UM
45
2
1
3
NOTE:Timing measurements are referenced to and from a low voltage of .08 V and a high
voltage of 2.0 V, unless othervise noted. The voltage swing through this r
a
should start outside and pass through the range such that the rise or fall
w
between 0.8 V and 2.0 V.
Figure 10-2. Clock Input Timing Diagram
10-8 M68020 USERÕS MANUAL MOTOROLA
AC ELECTRICAL CHARACTERISTICSÑREAD AND WRITE CYCLES
(VCC = 5.0 Vdc ± 5%; GND = 0 Vdc; Temperature within defined ranges; see Figures 10-3Ð10-5)
16.67 MHz 20 MHz 25 MHz** 33.33 MHz
Num. Characteristics Min Max Min Max Min Max Min Max Unit
6 Clock High to FC, Size, RMC, Address Valid 0 30 0 25 0 25 0 21 ns
*6A Clock High to ECS, OCS Asserted 0 20 0 15 0 12 0 10 ns
7 Clock High to Address, Data, FC, Size, RMC
High Impedance
0 60 0 50 0 40 0 30 ns
8 Clock High to Address, FC, Size,
RMC Invalid
0 Ñ 0 Ñ0Ñ0Ñns
9 Clock Low to AS, DS Asserted 3 30 3 25 3 18 3 15 ns
9A1AS to DS Assertion Skew (Read) Ð15 15 Ð10 10 Ð10 10 Ð10 10 ns
9B11 AS Asserted to DS Asserted (Write) 37 Ñ 32 Ñ 27 Ñ 22 Ñ ns
*10 ECS Width Asserted 20 Ñ 15 Ñ 15 Ñ 10 Ñ ns
*10A OCS Width Asserted 20 Ñ 15 Ñ 15 Ñ 10 Ñ ns
*10B7ECS, OCS Width Negated 15 Ñ 10 Ñ5Ñ5Ñns
11 Address, FC, Size, RMC Valid to AS
(and DS Asserted, Read)
15Ñ10 Ñ6Ñ5Ñns
12 Clock Low to AS, DS Negated 0 30 0 25 0 15 0 15 ns
*12A Clock Low to ECS, OCS Negated 0 30 0 25 0 15 0 15 ns
13 AS, DS Negated to Address, FC, Size,
RMC Invalid
15Ñ10 Ñ10Ñ5Ñns
14 AS (and DS Read) Width Asserted 100 Ñ 85 Ñ 70 Ñ 50 Ñ ns
14A DS Width Asserted (Write) 40 Ñ 38 Ñ 30 Ñ 25 Ñ ns
15 AS, DS Width Negated 40 Ñ 38 Ñ 30 Ñ 23 Ñ ns
15A8DS Negated to AS Asserted 35 Ñ 30 Ñ 25 Ñ 18 Ñ ns
16 Clock High to AS, DS, R/W, DBEN
High Impedance
Ñ 60 Ñ 50 Ñ 40 Ñ 30 ns
17 AS, DS Negated to R/W Invalid 15 Ñ 10 Ñ 10 Ñ5Ñns
18 Clock High to R/W High 0 30 0 25 0 20 0 15 ns
20 Clock High to R/W Low 0 30 0 25 0 20 0 15 ns
21 R/W High to AS Asserted (Read) 15 Ñ 10 Ñ5Ñ5Ñns
22 R/W Low to DS Asserted (Write) 75 Ñ 60 Ñ 50 Ñ 35 Ñ ns
23 Clock High to Data-Out Valid Ñ 30 Ñ 25 Ñ 25 Ñ 18 ns
25 AS, DS Negated to Data-Out Invalid 15 Ñ 10 Ñ5Ñ5Ñns
*25A9DS Negated to DBEN Negated (Write) 15 Ñ 10 Ñ5Ñ5Ñns
26 Data-Out Valid to DS Asserted (Write) 15 Ñ 10 Ñ5Ñ5Ñns
27 Data-In Valid to Clock Low (Setup) (Read) 5 Ñ 5 Ñ5Ñ5Ñns
27A Late BERR/HALT Asserted to Clock Low
(Setup)
20Ñ15 Ñ10Ñ5Ñns
28 AS, DS Negated to DSACK», BERR, HALT,
AVEC Negated
0 80 0 65 0 50 0 40 ns
MOTOROLA M68020 USERÕS MANUAL 10-9
AC ELECTRICAL CHARACTERISTICSÑREAD AND WRITE CYCLES
(Continued)
16.67 MHz 20 MHz 25 MHz** 33.33 MHz
Num. Characteristics Min Max Min Max Min Max Min Max Unit
29 AS, DS Negated to Data-In Invalid
(Data-In Hold Time)
0 Ñ 0 Ñ0Ñ0Ñns
29A AS, DS Negated to Data-In
(High Impedance)
Ñ 60 Ñ 50 Ñ 40 Ñ 30 ns
30 Clock Low to Data-In Invalid
(Data-In Hold Time)
15 Ñ 15 Ñ 10 Ñ 10 Ñ ns
312DSACK» Asserted to Data-In Valid Ñ 50 Ñ 43 Ñ 32 Ñ 17 ns
31A3DSACK» Asserted to DSACK» Valid
(DSACK» Asserted Skew)
Ñ 15 Ñ 10 Ñ 10 Ñ 10 ns
32 RESET Input Transition Time Ñ 1.5 Ñ 1.5 Ñ 1.5 Ñ 1.5 Clks
33 Clock Low to BG Asserted 0 30 0 25 0 20 0 20 ns
34 Clock Low to BG Negated 0 30 0 25 0 20 0 20 ns
35 BR Asserted to BG Asserted (RMC Not
Asserted)
1.5 3.5 1.5 3.5 1.5 3.5 1.5 3.5 Clks
*37 BGACK Asserted to BG Negated 1.5 3.5 1.5 3.5 1.5 3.5 1.5 3.5 Clks
*37A6BGACK Asserted to BR Negated 0 1.5 0 1.5 0 1.5 0 1.5 Clks
39 BG Width Negated 90 Ñ 75 Ñ 60 Ñ 50 Ñ ns
39A BG Width Asserted 90 Ñ 75 Ñ 60 Ñ 50 Ñ ns
*40 Clock High to DBEN Asserted (Read) 0 30 0 25 0 20 0 15 ns
*41 Clock Low to DBEN Negated (Read) 0 30 0 25 0 20 0 15 ns
*42 Clock Low to DBEN Asserted (Write) 0 30 0 25 0 20 0 15 ns
*43 Clock High to DBEN Negated (Write) 0 30 0 25 0 20 0 15 ns
*44 R/W Low to DBEN Asserted (Write) 15 Ñ 10 Ñ 10 Ñ5Ñns
*455DBEN Width Asserted Read
Write
60
120
Ñ
Ñ
50
100
Ñ
Ñ
40
80
Ñ
Ñ
30
60
Ñ
Ñ
ns
46 R/W Width Valid (Write or Read) 150 Ñ 125 Ñ 100 Ñ 75 Ñ ns
47A Asynchronous Input Setup Time 5 Ñ 5 Ñ5Ñ5Ñns
47B Asynchronous Input Hold Time 15 Ñ 15 Ñ 10 Ñ 10 Ñ ns
484DSACK» Asserted to BERR, HALT Asserted Ñ 30 Ñ 20 Ñ 18 Ñ 15 ns
53 Data-Out Hold from Clock High 0 Ñ 0 Ñ0Ñ0Ñns
55 R/W Valid to Data Bus Impedance Change 30 Ñ 25 Ñ 20 Ñ 20 Ñ ns
56 RESET Pulse Width (Reset Instruction) 512 Ñ 512 Ñ 512 Ñ 512 Ñ Clks
57 BERR Negated to HALT Negated (Rerun) 0 Ñ 0 Ñ0Ñ0Ñns
*5810 BGACK Negated to Bus Driven 1 Ñ 1 Ñ1Ñ1ÑClks
5910 BG Negated to Bus Driven 1 Ñ 1 Ñ1Ñ1ÑClks
*This specification does not apply to the MC68EC020.
**These specifications represent an improvement over previously published specifications for the 25-MHz MC68020
and are valid only for product bearing date codes of 8827 and later.
10-10 M68020 USERÕS MANUAL MOTOROLA
AC ELECTRICAL CHARACTERISTICSÑREAD AND WRITE CYCLES
(Concluded)
NOTES:
1. This number can be reduced to 5 ns if strobes have equal loads.
2. If the asynchronous setup time (#47A) requirements are satisfied, the DSACK» low to data setup time (#31) and
DSACK» low to BERR low setup time (#48) can be ignored. The data must only satisfy the data-in clock low
setup time (#27) for the following clock cycle, and BERR must only satisfy the late BERR low to clock low setup
time (#27A) for the following clock cycle.
3. This parameter specifies the maximum allowable skew between DSACK0 to DSACK1 asserted or DSACK1 to
DSACK0 asserted; specification #47A must be met by DSACK0 or DSACK1.
4. This specification applies to the first (DSACK0 or DSACK1) DSACK» signal asserted. In the absence of
DSACK», BERR is an asynchronous input using the asynchronous input setup time (#47A).
5. DBEN may stay asserted on consecutive write cycles.
6. The minimum values must be met to guarantee proper operation. If this maximum value is exceeded, BG may
be reasserted.
7. This specification indicates the minimum high time for ECS and OCS in the event of an internal cache hit
followed immediately by a cache miss or operand cycle.
8. This specification guarantees operation with the MC68881/MC68882, which specifies a minimum time for DS
negated to AS asserted (specification #13A in MC68881UM/AD, MC68881/MC68882 Floating-Point
Coprocessor User's Manual). Without this specification, incorrect interpretation of specifications #9A and #15
would indicate that the MC68020/EC020 does not meet the MC68881/MC68882 requirements.
9. This specification allows a system designer to guarantee data hold times on the output side of data buffers that
have output enable signals generated with DBEN.
10. These specifications allow system designers to guarantee that an alternate bus master has stopped driving the
bus when the MC68020/EC020 regains control of the bus after an arbitration sequence.
11. This specification allows system designers to qualify the CS signal of an MC68881/MC68882 with AS (allowing
7 ns for a gate delay) and still meet the CS to DS setup time requirement (specification 8B of MC68881UM/AD,
MC68881/MC68882 Floating-Point Coprocessor User's Manual).
MOTOROLA M68020 USERÕS MANUAL 10-11
R/W
FIGURE 10-3
MC68020UM
S0 S1 S2 S3 S4 S5
*A31–A0
CLK
7
FC2–FC0
SIZ1–SIZ0
**ECS
OCS
DS
AS
**DBEN
BERR
HALT
DSACK0
DSACK1
D31–D0
29A
2927
40
47A 47B
48
45
31
28
21
18 9
31A
17
12
14
11
16
10
10A
6A
8
6
ALL
ASYNCHRONOUS
INPUTS
12A
13
Timing measurements are referenced to and from a low voltage of 0.8 V
and a high voltage of 2.0 V, unless otherwise noted. The voltage swing
through this range should start outside and pass through the range such
that the rise or fall will be linear between 0.8 V and 2.0 V.
NOTE:
*For the MC68EC020, A23ÐA0.
**This signal does not apply to the MC68EC020.
27A
41
30
46
Figure 10-3. Read Cycle Timing Diagram
10-12 M68020 USERÕS MANUAL MOTOROLA
D31–D0
R/W
FIGURE 10-4
MC68020UM
S0 S1 S2 S3 S4 S5
*A31–A0
CLK
7
FC2–FC0
SIZ1–SIZ0
**ECS
**OCS
DS
AS
**DBEN
BERR
HALT
DSACK0
DSACK1
53
27A
48
45
23
28
20
9
31A
12
14
11
10
10A
6A
8
6
12A
13
15
14A
22
17
55
42
44 25
46
9
Timing measurements are referenced to and from a low voltage of 0.8 V
and a high voltage of 2.0 V, unless otherwise noted. The voltage swing
through this range should start outside and pass through the range such
that the rise or fall will be linear between 0.8 V and 2.0 V.
NOTE:
26
*For the MC68EC020, A23ÐA0.
**This signal does not apply to the MC68EC020.
43
25A
Figure 10-4. Write Cycle Timing Diagram
MOTOROLA M68020 USERÕS MANUAL 10-13
FIGURE 10-5
MC68020UM
S0 S1 S2 S3 S4 S5
*A31–A0
CLK
7
FC2–FC0
SIZ1–SIZ0
**ECS
OCS
DS
R/W
AS
DBEN
BR
DSACK0
DSACK1
D31–D0
33
BG
**BGACK
16
35
37
39A
34
39
Timing measurements are referenced to and from a low voltage of 0.8 V and a high voltage
of 2.0 V, unless otherwise noted. The voltage swing through this range should start outside
and pass through the range such that the rise or fall will be linear between 0.8 V and 2.0 V.
NOTE:
*For the MC68EC020, A23ÐA0.
**This signal does not apply to the MC68EC020.
37A
59
58
Figure 10-5. Bus Arbitration Timing Diagram
10-14 M68020 USERÕS MANUAL MOTOROLA
MOTOROLA MC68838 USER’S MANUAL 13-1
SECTION 11
ORDERING INFORMATION AND MECHANICAL DATA
This section contains the pin assignments and package dimensions of the MC68020 and
the MC68EC020. In addition, detailed information is provided to be used as a guide when
ordering.
11.1 STANDARD ORDERING INFORMATION
11.1.1 Standard MC68020 Ordering Information
Package Type Frequency (MHz) Temperature (°C) Order Number
Ceramic Pin Grid Array
RC Suffix 16.67
20.0
25.0
33.33
0 to 70
0 to 70
0 to 70
0 to 70
MC68020RC16
MC68020RC20
MC68020RC25
MC68020RC33
Plastic Quad Flat Pack
FC Suffix 16.67
20.0
25.0
0 to 70
0 to 70
0 to 70
MC68020FC16
MC68020FC20
MC68020FC25
Plastic Pin Grid Array
RP Suffix 16.67
20.0
25.0
0 to 70
0 to 70
0 to 70
MC68020RP16
MC68020RP20
MC68020RP25
Ceramic Quad Flat Pack
FE Suffix 16.67
20.0
25.0
33.33
0 to 70
0 to 70
0 to 70
0 to 70
MC68020FE16
MC68020FE20
MC68020FE25
MC68020FE33
11.1.2 Standard MC68EC020 Ordering Information
Package Type Frequency (MHz) Temperature (°C) Order Number
Plastic Pin Grid Array
RP Suffix 16.67
25.0 0 to 70
0 to 70 MC68EC020RP16
MC68EC020RP25
Plastic Quad Flat Pack
FG Suffix 16.67
25.0 0 to 70
0 to 70 MC68EC020FG16
MC68EC020FG25
13-2 MC68838 USER’S MANUAL MOTOROLA
11.2 PIN ASSIGNMENTS AND PACKAGE DIMENSIONS
11.2.1 MC68020 RC and RP Suffix—Pin Assignment
GND
ECS
SIZ0
FC0
RESET
GND
BGACK
HALT
FC2
RMC
A1
GND
GND
BR
A31
A30
A28
A27
A26
A24
A23
A20
A22
A18
A19
A17
A16
GND
A12
A15
GND
DSACK1
BERR
AVEC
DSACK0
SIZ1
1
2
3
4
5
6
7
8
9
10
A
B
C
D
E
F
G
H
J
K
BG
L
M
N
11
12
13
D1
D0
AS
R/W
D30
D27
D23
D19
GND
D15
D11
D7
GND
D3
D2
DS
D29
D26
D24
D21
D18
D16
D13
D10
D6
D5
D4
D31
D28
D25
D22
D20
D17
GND
D14
D12
D9
D8
IPL0
IPL1
IPL2
GND
GND
GND
IPEND
A2
OCS
A4
A3
A9
A7
A5
A13
A10
A6
A14
A11
A8
A21
A25
A29
A0
CDIS
DBEN
FC1
V
V
V
V
V
V
V
V
V
CLK
GND
V
CC
CC
CC
CC
CC
CC
CC
CC
CC
CC
MC68020
The VCC and GND pins are separated into four groups to provide individual power supply
connections for the address bus buffers, data bus buffers, and all other output buffers and
internal logic. It is recommended that all pins be connected to power and ground as
indicated.
Group VCC GND
Address Bus A9, D3 A10, B9, C3, F12
Data Bus M8, N8, N13 L7, L11, N7, K3
Logic D1, D2, E3, G11, G13 G12, H13, J3, K1
Clock — B1
MOTOROLA MC68838 USER’S MANUAL 13-3
11.2.2 MC68020 RC Suffix—Package Dimensions
C
K
1
2
3
4
5
6
7
8
9
10
11
12
13
A
B
C
D
E
F
G
H
K
L
M
N
J
G
G
R
C SUFFIX
C
ASE 791-01
M
C68020
T
NOTES:
1. A AND B ARE DATUMS AND T IS A DATUM SURFACE.
2. POSITIONAL TOLERANCE FOR LEADS (114 PLACES).
3. DIMENSIONING AND TOLERANCING PER Y14.5M,1982.
4. CONTROLLING DIMENSION: INCH.
D
0.13 (0.005)
M
T
A
S
B
S
DIM
MILLIMETERS
INCHES
MIN
MAX
MIN
MAX
A
B
C
D
G
1.340
1.380
1.340
1.380
0.100
0.150
34.04
35.05
34.04
35.05
2.54
3.81
0.43
0.55
0.017
0.022
0.100 BSC
2.54 BSC
K
4.32
4.95
0.170
0.195
φ
B
A
13-4 MC68838 USER’S MANUAL MOTOROLA
11.2.3 MC68020 RP Suffix—Package Dimensions
C
K
1
2
3
4
5
6
7
8
9
10
11
12
13
A
B
C
D
E
F
G
H
K
L
M
N
J
G
G
R
P SUFFIX
C
ASE 789E-02
M
C68020
A
B
T
NOTES:
1. DIMENSIONING AND TOLERANCING PER ANSI Y14.5M, 1982.
2. CONTROLLING DIMENSION: INCH.
3. DIMENSION D INCLUDES LEAD FINISH.
4. 789E-01 OBSOLETE. NEW STANDARD 789E-02.
D
114 P
L
DIM
MILLIMETERS
INCHES
MIN
MAX
MIN
MAX
A
B
C
D
G
1.340
1.380
1.340
1.380
0.115
0.135
34.04
35.05
34.04
35.05
2.92
3.18
0.44
0.55
0.017
0.022
0.100 BSC
2.54 BSC
K
2.79
3.81
0.110
0.150
V
V
0.76 (0.030)
M
T
A
S
B
S
φ
T
0.25 (0.010)
φ
X
0.17 (0.007)
M
M
X
L
L
V
1.02
1.52
0.040
0.060
30.48 BSC
1.200 BSC
PIN
A
-
1
MOTOROLA MC68838 USER’S MANUAL 13-5
11.2.4 MC68020 FC and FE Suffix—Pin Assignment
18
50
17
1
34
VCC
67
132
NC*
NC*
NC*
A9
A8
A7
A6
A5
A4
A3
A2
GND
GND
GND
D0
D1
D2
D3
D4
GND
GND
D5
NC*
NC*
VCC
VCC
VCC
VCC
NC*
NC*
GND
BG
GND
GND
CLK
RESET
VCC
VCC
RMC
FC0
FC1
FC2
SIZ0
SIZ1
DBEN
ECS
CDIS
AVEC
DSACK0
DSACK1
BERR
GND
GND
HALT
AS
R/W
NC*
DS
GND
GND
IPEND
OCS
IPL2
IPL1
IPL0
NC*
BGACK
BR
A0
A1
A31
A30
A29
A28
A27
A26
A25
A24
A23
A22
A21
A20
A19
A18
A17
V
V
GND
GND
A16
A15
A14
A13
A12
A11
A10
NC*
NC*
CC
CC
TOP VIEW
The VCC and GND pins are separated into four groups to provide individual power supply
connections for the address bus buffers, data bus buffers, and all other output buffers and
internal logic. It is recommended that all pins be connected to power and ground as
indicated. NC pins are reserved by Motorola for future use and should have no external
connection.
Group VCC GND
Address Bus 13, 38, 39 15, 40, 41, 62
Data Bus 79, 80, 96, 97 77, 78, 98, 99, 119, 120
Logic 7, 8, 65, 66 67, 68, 124, 125
Clock — 11, 12
13-6 MC68838 USER’S MANUAL MOTOROLA
11.2.5 MC68020 FC Suffix—Package Dimensions
Z
PIN 1 INDE
X
0.25 (0.010)
T
X
Y
Z
S
D
132 PL
DIM
A
MILLIMETERS
INCHES
MIN
MAX
MIN
MAX
24.06
24.20
0.947
0.953
24.06
24.20
0.947
0.953
4.07
4.57
0.160
0.180
0.21
0.30
0.008
0.012
0.64 BSC
0.025 BSC
0.51
1.01
0.020
0.040
0.16
0.20
0.006
0.008
0.030
NOTES:
1. DIMENSIONING AND TOLERANCING PER ANSI Y14.5M, 1982.
2. CONTROLLING DIMENSION: INCH
3. DIMENSIONS A, B, N, AND R DO NOT INCLUDE MOLD PROTRUSI
ON
ALLOWABLE MOLD PROTRUSION FOR DIMENSIONS A AND B IS
0.25 (0.010), FOR DIMENSIONS N AND R IS 0.18 (0.007).
4. DATUM PLANE -W- IS LOCATED AT THE UNDERSIDE OF LEADS
WHERE LEADS EXIT PACKAGE BODY.
5. DATUMS -X-, -Y-, AND -Z- TO BE DETERMINED WHERE CENTER L
EA
PACKAGE BODY AT DATUM -W-.
Y
G
P
P
S
S
S
0.05. (0.002)
0.20 (0.008)
T
X
Y
Z
S
S
S
S
0.20 (0.008)
T
X
Y
Z
S
S
S
S
0.25 (0.010)
T
X
Y
Z
S
S
S
S
0.05 (0.002)
N
S
A
H
C
0.20 (0.008)
T
X
Y
Z
S
S
S
S
K
J
SEATING PLANE
.10 (0.004)
T
SECTION P-P
B
C
D
G
H
J
K
F
C SUFFIX
C
ASE 831A-01
M
C68020
V
L
B
R
MOTOROLA MC68838 USER’S MANUAL 13-7
11.2.6 MC68020 FE Suffix—Package Dimensions
FE SUFFIX
C
ASE 831-01
M
C68020
DIM
A
B
C
D
G
H
J
K
L
M
R
MILLIMETERS
INCHES
MIN
MAX
MIN
MAX
21.85
22.86
0.860
0.900
21.85
22.86
0.860
0.900
3.94
4.31
0.155
0.170
0.204
0.292
0.0080
0.0115
0.64 BSC
0.025 BSC
0.64
0.88
0.025
0.035
0.13
0.20
0.005
0.008
0.51
0.76
0.020
0.030
0
°
8
0
8
20.32 REF
0.800 REF
°°°
0.64
0.025
NOTES:
1. DIMENSIONING AND TOLERANCING PER ANSI Y14.5M, 1982.
2. CONTROLLING DIMENSION: INCH
3. DIMENSIONS A AND B DEFINE MAXIMUM CERAMIC BODY DIMENSIONS
INCLUDING GLASS PROTRUSION AND MISMATCH OF CERAMIC BODY
T
OP AND BOTTOM.
4. DATUM PLANE -W- IS LOCATED AT THE UNDERSIDE OF LEADS
WHERE LEADS EXIT PACKAGE BODY.
5. DATUMS -X-, -Y-, AND -Z- TO BE DETERMINED WHERE CENTER LEADS
EXIT PACKAGE BODY AT DATUM -W-.
6. DIMENSIONS S AND V TO BE DETERMINED AT SEATING PLANE, DATUM -T
-.
7. DIMENSIONS A AND B TO BE DETERMINED AT DATUM PLANE -W-.
W
0.10
∩
SEATING PLANE
D
132 PL
J
K
H
C
PIN 1 INDENT
X
0.51 (0.020)
T
X
— Y
0.20 (0.008)
T
X
— Y
Z
M
S
S
S
S
S
Z
S
M
0.51 (0.020)
T
X
— Y
S
S
Z
S
M
0.20 (0.008)
T
X
— Y
Z
M
S
S
S
R
0.20 (0.008)
T
X
— Y
Z
M
S
S
S
(0.004)
T
M
V
B
L
S
A
—
—
13-8 MC68838 USER’S MANUAL MOTOROLA
11.2.7 MC68EC020 RP Suffix—Pin Assignment
FC0
RMC
SIZ0
AVEC
RESET
CLK
GND
A16
A15
A18
A17
A20
A22
A21
A23
A0
BG
BR
A1
1
2
3
4
5
6
7
8
(BOTTOM VIEW)
N
M
L
K
J
H
G
F
E
D
C
B
A
D31
D29
D28
D26
D24
D22
D19
D30
D27
D23
D21
D18
DS
R/W
D17
AS
DSACK1
BERR
D2
D3
IPL0
D0
GND
IPL2
A3
A5
A6
A7
A8
A9
A10
A12
A11
11
12
13
D16
D15
D13
D11
GND
D14
D12
D9
D8
D6
D7
9
10
A14
A4
SIZ1
GND
D5
D4
A2
MC68EC020
V
CC
V
CC
D25
D20
HALT
DSACK0
FC2
A19
D10
GND
CDIS
FC1
GND
GND
IPL1
D1
V
CC
GND
GND
GND
GND
V
CC
V
CC
GND
GND
V
CC
V
CC
V
CC
V
CC
A13
The VCC and GND pins are separated into four groups to provide individual power supply
connections for the address bus buffers, data bus buffers, and all other output buffers and
internal logic. It is recommended that all pins be connected to power and ground as
indicated.
Group VCC GND
Address Bus B7, C7 A1, A7, C8, D13
Data Bus K12, M9, N9 J13, L8, M1, M8
Logic D1, D2, E12, E13 F11, F12, J1, J2
Clock — B1
MOTOROLA MC68838 USER’S MANUAL 13-9
11.2.8 MC68EC020 RP Suffix—Package Dimensions
C
S
1
2
3
4
5
6
7
8
9
10
11
12
13
A
B
C
D
E
F
G
H
K
L
M
N
J
G
G
R
P SUFFIX
C
ASE 789H-01
M
C68EC020
A
B
T
NOTES:
1. DIMENSIONING AND TOLERANCING PER ANSI Y14.5M, 1982.
2. CONTROLLING DIMENSION: INCH
3. DIMENSION D INCLUDES LEAD FINISH.
D
100 P
L
DIM
MILLIMETERS
INCHES
MIN
MAX
MIN
MAX
A
B
C
D
G
1.340
1.380
1.340
1.380
0.115
0.135
34.04
35.05
34.04
35.05
2.92
3.18
0.44
0.55
0.017
0.022
0.100 BSC
2.54 BSC
K
3.05
3.55
0.120
0.140
V
V
0.76 (0.030)
M
T
A
S
B
S
φ
T
0.25 (0.010)
φ
X
0.17 (0.007)
M
M
X
L
L
1.02
1.52
0.040
0.060
S
V
4.32
4.83
0.170
0.190
30.48 BSC
1.200 BSC
K
PIN
A
-
1
13-10 MC68838 USER’S MANUAL MOTOROLA
11.2.9 MC68EC020 FG Suffix—Pin Assignment
A9
1
30
31
50
51
80
81
100
A10
A11
A12
A13
A14
A15
A16
GND
A17
A18
A19
A20
A21
A22
A23
A1
A0
BR
V
CC
BG A8*
CLK
RESET
GND
GND
RMC
FC0
FC1
FC2
SIZ0
SIZ1
CDIS
AVEC
DSACK0
DSACK1
BERR
D7
D8
HALT
AS
DS
R/W
D31
D30
D29
D28 D9
VCC
D27
D26
D25
D24
D23
D22
D21
D20
D19
D18
D17
D16
GND
D14
D13
D12
D11
D10
V
CC
D15
GND
V
CC
D6
D5
GND
D4
D3
D2
D1
D0
IPL0
IPL1
IPL2
GND
GND
GND
A2
A3
A4
A5
A6
A7
V
CC
VCC
MC68EC020
(TOP VIEW
)
NC*
NC
VCC
GND
GND
GND
GND
*NC—Do not connect to this pin.
The VCC and GND pins are separated into four groups to provide individual power supply
connections for the address bus buffers, data bus buffers, and all other output buffers and
internal logic. It is recommended that all pins be connected to power and ground as
indicated. NC pins are reserved by Motorola for future use and should have no external
connection.
Group VCC GND
Address Bus 90 72, 89, 100
Data Bus 44, 57 26, 43, 58, 59
Logic 7, 8, 70, 71 3, 20, 21, 68, 69
Clock — 4
MOTOROLA MC68838 USER’S MANUAL 13-11
11.2.10 MC68EC020 FG Suffix—Package Dimensions
0.25
A
B
AA
0.20 (0.008)
C
A – B
D
M
S
S
0.50 (0.002)
D
1
30
A
Z
50
31
51
B
80
Y
0.20 (0.008) H A – B DMS S
0.20 (0.008) CA – B D
MS S
0.05 (0.002) D
A – B
0.20 (0.008)
H
A – B
D
M
S
S
S
DETAIL "C"
U
R
T
Q
K
X
DIM
A
MILLIMETERS
INCHES
MIN
MAX
MIN
MAX
19.90
20.10
0.783
0.791
NOTES:
1. DIMENSIONING AND TOLERANCING PER ANSI Y14.5M, 1982.
2. CONTROLLING DIMENSION: MILLIMETER.
3
. DATUM PLANE -H- IS LOCATED AT BOTTOM OF LEAD AND IS
C
OINCIDENT WITH THE LEAD WHERE THE LEAD EXITS THE
P
LASTIC BODY AT THE BOTTOM OF THE PARTING LINE.
4. DATUMS -A-, -B-, AND -D- TO BE DETERMINED AT DATUM PLANE -H-.
5
. DIMENSIONS S AND V TO BE DETERMINED AT SEATING PLANE -C
-.
6
. DIMENSIONS A AND B DO NOT INCLUDE MOLD PROTRUSION. ALLO
WA
P
ROTRUSION IS 0.25 (0.010) PER SIDE. DIMENSIONS A AND B DO INCL
UD
M
OLD MISMATCH AND ARE DETERMINED AT DATUM PLANE -H-.
7
. DIMENSION D DOES NOT INCLUDE DAMBAR PROTRUSION. ALLOWA
BL
D
AMBAR PROTRUSION SHALL BE 0.08 (0.003) TOTAL IN EXCESS OF TH
E D
D
IMENSION AT MAXIMUM MATERIAL CONDITION. DAMBAR CANNOT BE
L
OCATED ON THE LOWER RADIUS OR THE FOOT.
FG SUFFIX
C
ASE 842D-01
M
C68EC020
C
M
M
H
G
E
C
SEATING
P
LANE
H
∩
0.10 (0
DATUM
P
LA
NE
DETAIL "C
DETAIL "B"
F
N
BASE METAL
D
J
0.20 (0.008)
C
A – B
M
DETAIL "A"
L
V
DETAIL "A"
P
A, B, D
H
DATUM
P
LANE
B
13.90
14.10
0.547
0.555
C
3.30
0.130
D
0.22
0.38
0.009
0.015
E
2.55
3.05
0.100
0.120
F
0.22
0.33
0.009
0.013
G
0.65 BSC
0.026 BSC
H
0.10
0.36
0.004
0.014
J
0.13
0.23
0.005
0.009
K
0.65
0.95
0.026
0.037
L
12.35 REF
0.486 REF
M
5
16
5
16
N
0.13
0.17
0.005
0.007
P
0.325 BSC
0.013 BSC
Q
0
7
0
7
R
0.35
0.010
0.014
S
23.65
24.15
0.931
0.951
T
0.13
0.005
U
0
0
V 17.65 18.15 0.695 0.715
° °
°°°°
°°
°°
81
100
—
—
—
—
—
—
MOTOROLA M68020 USER’S MANUAL A-1
APPENDIX A
INTERFACING AN MC68EC020 TO A DMA DEVICE
THAT SUPPORTS A THREE-WIRE BUS ARBITRATION
PROTOCOL
The MC68EC020 supports a two-wire bus arbitration protocol; however, it may become
necessary to interface the MC68EC020 to a device that supports a three-wire arbitration
protocol. Figure A-1 shows a method by which this can be achieved.
BG
PR
Q
Q
CLR
CLK
D
+5 V
74F74
4
74LS08
BR
BGACK
BR
74LS04
74LS04
BG
(MC68EC020)
(DMA)
(DMA)
(DMA)
(MC68EC020)
MC68EC020
BG
BR
BGACK
BR
BG
DMA
Figure A-1. Bus Arbitration Circuit—
MC68EC020 (Two-Wire) to DMA (Three-Wire)
MOTOROLA M68020 USER’S MANUAL INDEX-1
INDEX
— A —
A1, A0 Signals, 5-2, 5-7, 5-9, 5-21, 9-5
A15–A13 Signals, 7-6
A19–A16 Signals, 7-6
A31–A24 Signals, 4-1, 5-3
AC Specifications, 10-5
Access Level, 9-17
Access Time Calculations, 9-12
Address Bus, 3-2, 5-3, 5-25
Address Error Exception, 5-14, 6-6
Address Registers, 1-4
Address Space, 2-4, 5-3
Addressing Modes, 1-8
Arithmetic/Logical Instruction, 8-30, 8-31
AS Signal, 3-4, 5-2, 5-3
Autovector, 5-48
Autovector Interrupt Acknowledge Cycle, 5-48
AVEC Signal, 3-5, 5-4, 5-48, 5-53
— B —
BERR Signal, 3-7, 5-4, 5-25, 5-53, 5-55, 6-4
BG Signal, 3-6, 5-63, 5-66, 5-70
BGACK Signal, 3-6, 5-62, 5-63, 5-66
Binary-Coded Decimal, 8-32
Bit Field Manipulation Instructions, 8-36
Bi t Manipulation Instructions, 8-35
BKPT Instruction, 5-50
Flowchart, 6-17
Block Diagram, 1-2
BR Signal, 3-6, 5-63, 5-66, 5-70, 5-71
Breakpoint Acknowledge Cycle, 5-50, 6-17
Flowchart, 5-50
Timing, 5-50
Breakpoint Instruction Exception, 6-17
Bus, 5-24
Arbitration, 5-62
Cycles, 5-1
Master, 5-1
Operation, 5-1, 5-24
Bus Arbitration (MC68020), 5-63
Control Unit, 5-67
Flowchart, 5-63
Read-Modify-Write, 5-68
Timing, 5-63
Bus Arbitration (MC68EC020), 5-70
Control Unit, 5-73
Flowchart, 5-70
Timing, 5-70
Two-Wire, 5-75, A-1
Bus Controller, 5-22, 8-2, 8-5
Bus Cycles, 5-1, 5-25
Bus Error Exception, 6-4, 6-21
Bus Fault, 6-21
Bus Master, 5-1, 5-25, 5-62
Bus Operation, 5-24
Byte Enable Signals, 5-21
Byte Select Control Signals, 9-5
— C —
Cache, 1-13, 4-1, 5-2, 5-22, 5-62, 8-1, 8-7, 9-11
Control, 4-3
Internal Cache Holding Register, 5-21
Reset, 4-3
Cache Address Register (CAAR), 1-7, 4-3, 4-4
Cache Control Register (CACR), 1-7, 4-2, 4-3
CALLM Instruction, 9-14, 9-16, 9-18
CAS Instruction, 5-39
CAS2 Instruction, 5-39
CDIS Signal, 3-7, 4-3
CLK Signal, 3-7
Clock Drivers, 9-10
Condition Codes, 1-7
Conditional Branch Instructions, 8-37
Control Instructions, 8-38
Coprocessor, 6-25, 7-1
Classification, 7-4
Communication Protocol, 7-4
Conditional Instruction Category, 7-10
Coprocessor Context Rest ore Instruction
Category, 7-22
INDEX-2 M68020 USER’S MANUAL MOTOROLA
Coprocessor Context Save and Context
Restore Instruction Categories, 7-16
Coprocessor Context Save Instruction
Category, 7-20
Coprocessor General Instruction Category,
7-8
Coprocessor Instructions, 7-1, 7-7
For m at Words, 7-18
Instruction, 6-25, 7-3
Instruction Execution, 7-6
Coprocessor Detected
Data-Processing-Related Exceptions, 7-51
Exception, 7-49
Format Errors, 7-52
Illegal Coprocessor Command or Condition
Words, 7-51
Protocol Violation Exceptions, 7-50
System-Related Exceptions, 7-51
Coprocessor Instruction, 6-25, 7-3
Coprocessor Interface, 5-53, 7-1, 7-2
Coprocessor Interface Register (CIR), 7-4, 7-5,
7-6, 7-24
Command CIR, 7-25
Condition CIR, 7-26
Control CIR, 7-24
Instruction Address CIR, 7-27
Memory Map, 7-24
Operand Address CIR, 7-27
Operand CIR, 7-26
Operation Word CIR, 7-25
Register Select CIR, 7-27
Response CIR, 7-24
Restore CIR, 7-25
Save CIR, 7-25
Selection, 7-6
CPU Address S pace, 2-4, 5-44, 7-6
CPU Space Type, 5-44, 5-53
Cycle
Asynchronous Cycles, 5-1, 5-5
Autovector Interrupt Acknowledge Cycle,
5-45, 5-48
Breakpoint Acknowledge Cycle, 5-50
Coprocessor Interface Bus Cycles, 7-4
Interrupt Acknowledge Cycle, 5-4, 5-45
Operand Transfer Cycle, 5-5
Synchronous Cycle, 5-24
— D —
Data Accesses, 4-2
Data Bus (D31–D0), 3-2, 5-3, 5-5, 5-21, 5-25
Data Registers, 1-4
Data Types, 1-8
DBEN Signal, 3-5, 5-4
DC Electrical Characteristics
MC68020, 10-4
MC68EC020, 10-5
Destination Function Code Register (DFC), 1-7
Differences between MC68020 and MC68EC020,
1-1, 5-62
Double Bus Fault, 5-60
DS Signal, 3-4, 5-4, 5-21
DSACK1, DSACK0 Signals, 3-5, 5-4, 5-5, 5-24,
5-46, 5-53, 9-5
Dynamic Bus Sizing, 5-5
— E —
ECS Signal, 3-4, 5-3
Effective Address, 8-13, 8-14, 8-16, 8-17, 8-19
Electrical Specifications, 10-1
Exception, 2-5, 7-57
Address Error Exception, 5-14, 6-6, 7-57
Breakpoint Instruction, 6-17
Bus Error Exception, 6-4, 6-21
Coprocessor- Detected Exception, 7-49
cpTRAPcc Instruction Traps, 7-55
Data-Processing-Related Exception, 7-51
F -Line Emulator Exception, 7-54
Format Error Exception, 6-10, 7-57
Illegal Instruction, 6-7
Interrupt Exception, 5-45, 6-11, 7-56
Multiple, 6-17
Privilege Violation Exception, 6-7, 6-8, 7-55
Protocol Violation, 7-50
Reset Exception, 6-4
Stack Frames, 6-25
System-Related Exception, 7-51
Trace Exception, 6-9, 7-55
Trap Exception, 6-6
Unimplemented Instruction, 6-7
Exception Handler, 6-2
Exception Processing, 2-1, 2-5, 6-1
Exception-Related Instructions, 8-39
Exception Stack Frame, 2-6, 6-25
Exception Vector Table, 2-5, 6-2
MOTOROLA M68020 USER’S MANUAL INDEX-3
— F —
FC2–FC0 Signals, 2-2, 2-4, 3-2, 5-2, 5-3,
5-44, 7-6
Features, 1-2
F-Line Operation Words, 7-3
Floating-Point Coprocessor, 7-1, 9-1
Flowchart
Breakpoint Acknowledge Cycle, 5-50
Byte Read Cycle, 5-26
Interrupt Acknowledge Cycle, 5-46
Long-Word Read Cycle, 5-26
MC68EC020 Bus Arbitration, 5-70
MC68020 Bu s Arbitration, 5-63
Read-M odify-Write Cycle, 5-39
Reset Exception, 6-4
Write Cycle, 5-33
Format Error Exception, 6-10
— G —
Ground Connections, 3-7, 9-9
— H —
HALT Signal, 3-7, 5-4, 5-25, 5-53, 5-60
— I —
Idle Clock Cycles, 8-7
Illegal Instruction Exception, 6-7
Instruction
Arithmetic/Logical, 8-30, 8-31
Bcc, 8-37
Bit Field Manipulation, 8-36
Bit Manipulation, 8-35
BKPT, 5-50
CALLM, 9-14, 9-16, 9-18
CAS, 5-39
CAS2, 5-39
Control, 8-38
Coprocessor Conditional Instructions, 7-10
Coprocessor Context Restore Instruction
Category, 7-22
Coprocessor Context Save Instruction
Category, 7-20
Coprocessor Instruction, 6-25, 7-1, 7-3, 7-7
Coprocessor Instruction Execution, 7-6
Coprocessor Context Save and Context
Restore Instruction Categories, 7-16
Coprocessor General Instruction Category,
7-8
cpBcc, 7-12
cpDBcc, 7-14
cpRESTORE, 7-17, 7-22
cpSAVE, 7-17, 7-20
cpScc, 7-13
cpTRAPcc, 7-15, 7-55
Exception-Related, 8-39
Illegal Instruction, 6-7
MOVE, 8-20
MOVE SR, 8-3
MOVEA, 8-20
MOVEC, 4-3
NOP, 5-62, 8-3
Prefetches, 4-1
Primitive Instructions, 7-27
RESET, 5-76, 7-58
RTE, 6-19, 6-24
RTM, 9-14, 9-16, 9-19
Shift/Rotate, 8-34
Single-Operand Instructions, 8-33
Special-Purpose MOVE, 8-29
STOP, 6-10
TAS, 5-39
Unimplemented Instruction, 6-7
Instruction Execution, 5-62, 8-1
Instruction Execution Overlap, 8-4
Instruction Pipe, 1-12, 4-1, 6-21
Instruction Prefetches, 8-1
Instruction Set, 1-10
Instruction Timing, 8-8, 8-9
Internal Cache Holding Register, 5-21
Interrupt, 6-1
Flowchart, 6-14
Interrupt Exception, 6-11
Nonmaskable, 6-12
Interrupt Acknowledge Cycle, 5-4, 5-45, 6-16
Timing, 5-46
Interrupt Exception, 5-45, 6-11
Interrupt Priority Mask, 5-45, 6-11
Interrupt Stack Pointer (ISP), 1-4, 2-2
IPEND Signal, 3-5, 6-14
IPL2–IPL0 Signals 3-5, 5-45, 6-11
INDEX-4 M68020 USER’S MANUAL MOTOROLA
— L —
Long-Word Operand, 5-10, 5-14
Long-Word Read Cycle, 5-26
Long-Word Write Cycle, 5-33
— M —
M-Bit (SR), 1-7, 2-2
Main Processor Detected
Address Error, 7-57
Bus Faults, 7-57
cpTRAPcc Instruction Traps, 7-55
Exceptions, 7-52
F -Line Emulator Exception, 7-54
Format Error, 7-57
Interrupts, 7-56
Privilege Violations, 7-55
Protocol Violation, 7-52
Trace Exception, 7-55
Master Stack Pointer (MSP), 1-4, 2-2
Maximum Ratings, 10-1
MC68881/MC68882 Floating-Point Coprocessors,
9-1
Memory Interface, 9-11
Misaligned
Operand, 5-6, 5-14, 8-2
Transfer, 5-1, 5-5
Module, 9-14
Module Call, 9-18
Module Return, 9-19
Module Stack Frame, 9-16
MOVE Instruction, 8-20
MOVE SR Instruction, 8-3
MOVEA Instruction, 8-20
MOVEC Instruction, 4-3
— N —
Nonmaskable Interrupt, 6-12
N OP Instruction, 5-62, 8-3
Normal Processing S tate, 2-1
— O —
OCS Signal, 3-4, 5-3
Ordering Information
MC68020, 11-1
MC68EC020, 11-1
Overlap, 8-3
— P —
Package Dimensions
MC68020 FC Suffix, 11-6
MC68020 FE Suffix, 11-7
MC68020 RC Suffix, 11-3
MC68020 RP Suffix, 11-4
MC68EC020 FG Suffix, 11-11
MC68EC020 RP Suffix, 11-9
Pin Assignment
MC68020 FC Suffix, 11-5
MC68020 FE Suffix, 11-5
MC68020 RC Suffix, 11-2
MC68020 RP Suffix, 11-2
MC68EC020 FG Suffix, 11-10
MC68EC020 RP Suffix, 11-8
Port Size, 5-1, 5-5, 5-21, 9-5
Power Supply, 3-7, 9-9
Primitive, 7-4, 7-27
Busy Response Primitive, 7-30
CA Bit, 7-29
DR Bit, 7-29
Evaluate and Transfer Effective Address
Primitive, 7-35
Evaluate Effective Address and Transfer Data
Primitive, 7-35
Format, 7-28
Null Coprocessor Response Primitive, 7-31
PC Bit, 7-29
Supervisor Check Primitive, 7-33
Take Address and Transfer Data Primitive,
7-39
Take Midinstruction Exception Primitive, 7-47
Take Postinstruction Exception Primitive,
7-48
Take Preinstruction Exception Primitive, 7-45
Transfer from Instruction Stream Primitive,
7-34
Transfer Main P rocessor Control Register
Primitive, 7-41
Transfer Multiple C oprocessor Registers
Primitive, 7-42
Transfer Multiple M ain Processor Registers
Primitive, 7-42
Transfer Operation Word Primitive, 7-33
Transfer Single Main P rocessor Register
Primitive, 7-40
MOTOROLA M68020 USER’S MANUAL INDEX-5
Transfer Status Register and the scanPC
Primitive, 7-44
Transfer to/from Top of Stack Primitive, 7-40
Write to Previously Evaluated Effective
Address Primitive, 7-37
Privilege Level, 2-2
Changing, 2-3
Supervisor Level, 1-4, 2-2
User Level, 1-4, 2-2
Privilege Violation Exception, 6-7, 6-8
Processing States, 2-1
Program Counter (PC), 1-4
Programming Model, 1-4, 7-1, 7-2
— R —
Read Cycle, 5-3, 5-4, 5-8, 5-14, 5-16, 5-18, 5-22,
5-26
Byte Read Cycle, 5-26
Long-Word Read Cycle, 5-26, 8-2
Timing, 5-26
Read-Modify-Write Cycle, 5-3, 5-39, 5-42
Timing, 5-39
Registers
Address Registers, 1-4
CAAR, 1-7, 4-3, 4-4
CACR, 1-7, 4-2, 4-3
Data Registers, 1-4
DFC, 1-7
Internal Cache Holding Register, 5-21
Program Counter (PC), 1-4
SFC, 1-7
SR, 1-7, 4-1
VBR, 1-7
Reset, 4-3
Flowchart, 6-4
Reset Exception, 6-4
RESET Instruction, 7-58
RESET Signal, 3-6, 5-76, 6-4
Reset Exception, 6-4
RESET Instruction, 5-76
RESET Signal, 3-6, 5-76, 6-4
Retry, 5-56
RMC Signal, 3-4, 5-3, 5-39
RTE Instruction, 6-19, 6-24
RTM Instruction, 9-14, 9-16, 9-19
R/W Signal, 3-4, 5-2, 5-3, 9-5
— S —
S-bit (SR), 1-7, 2-2, 2-3
Save and Restore Operations, 8-40
scanPC, 7-28
Sequencer, 8-2, 8-5
Shift/Rotate Instructions, 8-34
Signal(s), 3-8
A1,A0, 5-2, 5-7, 5-9, 5-21, 9-5
A15–A13, 7-6
A19–A16, 7-6
A31–A24, 4-1, 5-3
Address Bus, 3-2, 5-3
AS, 3-4, 5-2, 5-3
AVEC, 3-5, 5-4, 5-48, 5-53
BERR, 3-7, 5-4, 5-25, 5-53, 5-55, 6-4
BG, 3-6, 5-63, 5-66, 5-70, 5-71
BGACK, 3-6, 5-62, 5-63, 5-66
BR, 3-6, 5-63, 5-66, 5-70
Byte Select Control Signals, 9-5
CDIS, 3-7, 4-3
CLK, 3-7
D31–D0, 3-2, 5-3
DBEN, 3-5, 5-4
DS, 3-4, 5-4, 5-21
DSACK1, DSACK0 , 3-5, 5-4, 5-5, 5-24, 5-46,
5-53, 9-5
ECS, 3-4, 5-3
FC2–FC0, 2-4, 3-2, 5-2, 5-3, 5-44, 7-6
Functional Groups, 3-1
HALT, 3-7, 5-4, 5-25, 5-53, 5-60
Input Signal, 5-2
Internal Signal, 5-2
IPEND, 3-5, 6-14
IPL2–IPL0, 3-5, 6-11
OCS, 3-4, 5-3
RESET, 3-6, 5-76, 6-4
RMC, 3-4, 5-3, 5-39
R/W, 3-4, 5-2, 5-3, 9-5
SIZ1, SIZ0, 3-2, 5-2, 5-3, 5-7, 5-9, 5-21, 9-5
Single-Operand Instruction, 8-33
SIZ1, SIZ0 Signals, 3-2, 5-2, 5-3, 5-7, 5-9,
5-21, 9-5
Source Function Code Register (SFC), 1-7
Special-Purpose MOVE Instruction, 8-29
Special Status Word (SSW), 6-21
Spurious Interrupt, 5-48
INDEX-6 M68020 USER’S MANUAL MOTOROLA
Stack Frame
Midinstruction, 7-47
Postinstruction, 7-48
Preinstruction, 7-46
Status Register (SR), 1-7, 4-1, 5-45, 6-1
STOP Instruction, 6-10
Supervisor Privilege Level, 1-4, 2-2
Supervisor Stack Pointer (SSP), 1-4, 2-2
Synchronous Cycles, 5-24
— T —
T1, T0 Bits (SR), 1-7, 6-9
TAS Instruction, 5-39
Thermal Characteristics, 10-1
MC68020, 10-2
MC68020 CQFP Package, 10-2
MC68EC020, 10-4
MC68EC020 PQFP Package, 10-4
Thermal Resistance, 10-2, 10-4
Timing, 5-26, 5-33
Trace Exception, 6-9
Trace Modes, 1-7
Tracing, 6-9
Transfer, 5-10, 5-14, 5-25
Bus Transfer, 5-1
Direction, 5-3
Misaligned, 5-1, 5-5
Operand Transfer, 5-1, 5-5
Trap Exception, 6-6 — U —
Unimplemented Instruction (F-Line Opcode)
Exception, 6-7
User Privilege Level, 1-4, 2-2
User Stack Pointer (USP), 1-4, 2-2
— V —
VCC Connections, 3-7, 9-9
Vector Base Register (VBR), 1-7, 2-5, 6-2
Virtual Machine, 1-12
Virtual Memory, 1-10
— W —
Write Cycle, 5-3, 5-9, 5-10, 5-12, 5-14, 5-16,
5-18, 5-22, 5-33, 5-38
Long-Word Write Cycle, 5-33
Timing, 5-33
