dialog in the flexible manner available with the general and conditional instruction types.
The dialog consists of the save or restore command, followed by the transfer of the ap-
propriate state frame. The only control that the FPCP has over this dialog is for the FSAVE
instruction; in which case, it may request that the main processor delay the save operation
until the FPCP is ready to perform it. These dialogs are discussed in the following para-
graphs.
7.5.3.1 FSAVE. This dialog is utilized for the context save instruction. The dialog for this
instruction is shown in Figure 7-25. 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.
MCRS020/MC68030
MCO8881/MC68882
~ j~tiji~tttT~24 ~-
ONLY IF THE MIC68881/MC68882 tS NOT REAOY TO START THE SAVE 0PERATt0N
I
6 TO 53 LONG-WORD TRANSFERS
Figure 7-25. FSAVE Instruction Dialog
The main processor initiates this dialog by reading from the save CIR. During this read
cycle, the FPCP returns a format word that indicates the current state of the machine. For
most cases of this dialog with the MC68881, the first read of the save CIR returns the idle
format word, and the main processor then proceeds to transfer six long words from the
operand CIR to memory. In this dialog with the MC68882, the idle format word is followed
by 14 long words. Optionally, the first primitive may be a null format word, in which case
no state frame is transferred. Alternatively, the first primitive may be a busy format word,
in which case 45 (53 for the MC68882) long words are transferred. Finally, the save CIR
read may return the not-ready, come-again format word. In this case, the main processor
may process pending interrupts and restart the instruction, or reread the save CIR until a
different format word is received. The invalid format word may also be returned, as dis-
cussed in 7.5.4.6 FORMAT EXCEPTION, FSAVE INSTRUCTION.
After the MPU receives a valid format word, it then evaluates the effective address and
writes the format word to that address. The appropriate state frame is then transferred to
the effective address, and the main processor is free to proceed with the execution of the
MC68881/MC68882 USER'S MANUAL FREESCALE
7-29
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