Null Coprocessor Response Primitive Encodings - Motorola MC68020 User Manual

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 PF
x 1 x x
1 0 0 x
1 0 1 x
0 0 0 0
0 0 1 0
0 0 x 1
x = Don't Care
c = 1 or 0 Depending on Coprocessor Condition Evaluation
MOTOROLA
TF General Instructions
x Pass Program Counter to Instruction
Address CIR, Clear PC Bit, and Proceed
with Operation Specified by CA, IA, PF,
and TF Bits
x Reread Response CIR, Do Not Service
Pending Interrupts
x Service Pending Interrupts and Reread the
Response CIR
c If (Trace Pending) Reread Response CIR;
Else, Execute Next Instruction
c If (Trace Pending) Service Pending
Interrupts and Reread Response CIR;
Else, Execute Next Instruction
c Coprocessor Instruction Completed;
Service Pending Exceptions or Execute
Next Instruction
M68020 USER'S MANUAL
Conditional Instructions
Same as General Category
Same as General Category
Same as General Category
Main Processor Completes Instruction
Execution Based on TF = c
Main Processor Completes Instruction
Execution Based on TF = c
Main Processor Completes Instruction
Execution Based on TF = c.
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