6
number and state size (180 bytes). The busy state is produced if an FSAVE occurs when a
floating-point instruction is in the initial or middle phase. Due to the volatile nature of the
FPCP internal state during calculation, this state frame does not contain any information
useful to applications programs, and the frame should not be modified in any way.
The MC68882 busy state frame contains 2t2 bytes, including 32 bytes of CU internal state
information (refer to Figure 6-5). The format word contains the coprocessor version number
and the state size. Otherwise the MC68882 busy state frame is identical to the busy state
frame of the MC68881.
6.4.3 FSAVE Protocol
Table 6-5 lists five possible phases of the execution of a floating:point instruction that can
apply at the time an FSAVE instruction is executed. For each phase, the table shows the
response time and the state frame type.
Table 6-5. MC68881/MC68882 Responses
to Save Command
Phase Name
Response Time
State Frame Type
t
Reset Immediate Null I
i
!
Idle Immediate Idle
Initial Immediate Busy i
Middle Periodic Busy )
End Delayed Idle t
When the MPU decodes an FSAVE instruction, it attempts to initiate a save operation in
the FPCP by reading from the save CIR. If the FPCP is ready to perform the save, it responds
with a valid state frame format word. The format word informs the MPU that the copro-
cessor is ready to transfer the state frame and also what size frame is to be saved. If the
FPCP is not ready to begin the transfer of the state frame, it returns the come-again format
word, forcing the MPU to wait. When the MPU receives the come-again format word, it
checks for pending interrupts and processes them if necessary. Otherwise, it repeatedly
reads the save CIR until a format word other than come again is returned. When the FPCP
receives a valid format word, it reads the number of bytes indicated by the format word,
four bytes at a time, from the operand CIR and writes them to memory.
The FPCP always returns one of five format words in the save CIR. Table 6-6 shows the
five format word values and their meanings. In this table, the version number of the idle
and busy format words corresponds to the version number of the initial production versions
of the MC68881; future revisions of the device will utilize different version numbers to
identify unique state frame formats. If the format of the idle or busy state frame of e future
version of the FPCP differs from that of versions $1F and S3F for the MC68881 or SIF for
the MC68882, Freescale will provide the new format information when the new version is
available.
The come-again format word is returned bythe FPCP to force the MPU to wait, as previously
described. When the FPCP is ready to complete a save operation, it returns one of the
other valid format words (null, idle, or busy) to the main processor and then transfers the
FREESCALE
6-36
MC68881/MC68882 USER'S MANUAL
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