Motorola MC68030 User Manual page 411

cpSAVE should cause the empty/reset format word to be returned when a
cpSAVE instruction is executed before any other coprocessor instructions.
Thus, an empty/reset state frame consists only of the format word and the
following reserved word in memory (refer to Figure 10-14).
10.2.3.2.2 Not Ready Format Word. When the main processor initiates a cpSAVE
instruction by reading the save CIR the coprocessor can delay the save op-
eration by returning a not ready format word. The main processor then
services any pending interrupts and reads the save CIR again. The not ready
format word delays the save operation until the coprocessor is ready to save
its internal state. The cpSAVE instruction can suspend execution of a general
or conditional coprocessor instruction; the coprocessor can resume execu-
tion of the suspended instruction when the appropriate state is restored with
a cpRESTORE. If no further main processor services are required to complete
coprocessor instruction execution, it may be more efficient to complete the
instruction and thus reduce the size of the saved state. The coprocessor
designer should consider the efficiency of completing the instruction or of
suspending and later resuming the instruction. when the main processor
executes a cpSAVE instruction.
When the main processor initiates a cpRESTORE instruction by writing a
format word to the restore CIR, the coprocessor should usually terminate
any current operations and restore the state frame supplied by the main
processor. Thus, the not ready format word should usually not be returned
by the coprocessor during the execution of a cpRESTORE instruction. If the
coprocessor must delay the cpRESTORE operation for any reason, it can
return the not ready format word when the main processor reads the restore
CIR. If the main processor reads the not ready format word from the restore
CIR during the cpRESTORE instruction, it reads the restore CIR again without
servicing any pending interrupts.
10.2.3.2.3 Invalid Format Word. When the format word placed in the restore CIR
to initiate a cpRESTORE instruction does not describe a valid coprocessor
state frame, the coprocessor returns the invalid format word in the restore
CIR. When the main processor reads this format word during the cpRESTORE
instruction, it writes the abort mask to the control CIR and initiates format
error exception processing. The two least significant bits of the abort mask
are 01; the fourteen most significant bits are undefined.
MOTOROLA
MC68030 USER'S MANUAL
10-23
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