Motorola MC68020 - Page 165

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