Motorola MC68020 User Manual page 165

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.
MAIN PROCESSOR
M1 RECOGNIZE COPROCESSOR INSTRUCTION F-LINE
OPERATION WORD
M2 WRITE COPROCESSOR CONDITION SELECTOR TO
CO NDITION 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)
M4 COMPLETE EXECUTION OF INSTRUCTION BASED
ON THE TRUE/FALSE CONDITION INDICATOR
RETURNED IN THE RESPONSE CIR
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).
MOTOROLA
Figure 7-8. Coprocessor Interface Protocol
for Conditional Category Instructions
M68020 USER'S MANUAL
COPROCESSOR
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 COMPLETE CONDITION EVALUATION
C4 REFLECT "NO COME AGAIN" STATUS WITH TRUE/FALSE
CONDITION INDICATOR IN RESPONSE CIR
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