7
instructions can be used to read the floating-point data registers without overwriting the
FPIAR contents.
The operation boxes that are marked "R" and "W" indicate an operand read or write cycle,
respectively, by the MPU.
7.5.2 Conditional Instructions
This group of instructions includes the FBcc, FDBcc, FNOP, FScc, and FTRAPcc instructions.
These instructions have two factors in common. First, the execution of each instruction is
inherent in the M68000 Family coprocessor instruction set definition, and the dialog used
for all of them is the same. Second, in each of these instructions, the coprocessor completes
all previous floating-point instructions before it begins to evaluate the result of the received
conditional predicate. This guarantees a sequential execution model. The dialog begins
when the main processor writes the conditional predicate to the FPCP and then reads the
response CIR. If the APU and the CU (in the case of an MC68882) are not idle, a null primitive
(CA= 1, IA=I, PC=0, TF=0) is returned, and the main processor reads the response CIR
again later. This process of rereading the response CIR continues until the coprocessor is
idle. Note that it is possible for a pre-instruction exception to be reported at any time during
this process. If no exception is reported, and when the coprocessor is finally idle, the
coprocessor evaluates the condition and responds with a null primitive (CA=0, TF=x,
where x is the true or false result). The main
processor then proceeds with the appropriate
conditional action. The dialog (assuming that the coprocessor is idle when the conditional
predicate is written) is shown in Figure 7-24.
7.5.3 Context Switch Instructions
This group of instructions includes the FSAVE and FRESTORE instructions. The factor
common to these instructions is that the execution of each one is inherent in the M68000
Family coprocessor instruction set definition, and the coprocessor does not control the
MC68020/MC68030
MC6888|/MC688B2
o
o
7=
Figure 7-24. Conditional Instruction Dialog
FREESCALE
7-28
MC68881/MC68882 USER'S MANUAL
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