Motorola MC68020 User Manual page 164

MAIN PROCESSOR
M1 RECOGNIZE COPROCESSOR INSTRUCTION F-LINE
OPERATION WORD
M2 WRITE COPROCESSOR COMMAND WORD TO
COMMAND CIR
M3 READ COPROCESSOR RESPONSE PRIMITIVE CODE
FROM RESPONSE CIR
1) PERFORM SERVICE REQUESTED BY RESPONSE
PRIMITIVE
2) IF (COPROCESSOR RESPONSE PRIMITIVE
INDICATES "COME AGAIN") GO TO M3
(SEE NOTE 1)
M4 PROCEED WITH EXECUTION OF NEXT INSTRUCTION
(SEE NOTE 2)
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
7.2.2 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.
7-10
Figure 7-7. Coprocessor Interface Protocol
for General Category Instructions
M68020 USER'S MANUAL
C1 DECODE COMMAND WORD AND INITIATE
COMMAND EXECUTION
C2 WHILE (MAIN PROCESSOR SERVICE IS REQUIRED)
DO STEPS 1) AND 2) BELOW
1) REQUEST SERVICE BY PLACING APPROPRIATE
RESPONSE PRIMITIVE CODE IN RESPONSE CIR
2) RECEIVE SERVICE FROM MAIN PROCESSOR
C3 REFLECT "NO COME AGAIN" IN RESPONSE CIR
C4 COMPLETE COMMAND EXECUTION
C5 REFLECT "PROCESSING FINISHED" STATUS IN
RESPONSE CIR
7.4.1 ScanPC.
COPROCESSOR
MOTOROLA