7
next instruction. Note that by using this sequence, the MPU can take a
pre-instruction
exception in response to a pending interrupt (if a not-ready, come-again format word is
received) and then return to restart the instruction rather than taking a mid-instruction
exception.
Note that after the state save operation is complete, the FPCP is in the idle state with no
pending exceptions.
7.5.3,2 FRESTORE. This dialog is utilized for the context restore instruction. The dialog for
this instruction is shown in Figure 7-26. There are no primitive responses during this dialog;
instead, the FPCP controls the frame transfer to a limited extent through the use of the
format word encoding.
MC68881/MC68882
-
fi
TO
53 LONG-WORD TRAI~SFERS
Figure 7-26. FRESTORE Instruction Dialog
The mare processor initiates this dialog by evaluating the effective address, fetching a
format word from that address, and writing the format word to the restore CIR. The main
processor then reads the restore ClR to verify that the format word is valid. During this
read cycle, the FPCP returns a format word that indicates if the format word that was
written is valid for the current revision of the device. If the format word is valid, the same
value that was written is read back from the restore CIR, and the main processor proceeds
to transfer the state frame appropriate for the format word. The state frame size is O, 6, or
45 long words for the current implementation of the MC68881. For the MC68882, the
corresponding state frame sizes are 0, 14, and 53 long words. The invalid format word
may also be returned as discussed in 7.5.4.7 FORMAT EXCEPTION, RESTORE
INSTRUC-
TION.
Note that after the state restore operation is complete, the FPCP is in the state of the
instruction that was previously suspended with an FSAVE instruction.
FREESCALE
7-30
MC68881/MC68882 USER'S MANUAL
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