Motorola MC68030 User Manual page 410

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10.2.3.2 COPROCESSOR FORMAT WORDS. The coprocessor communicates sta-
tus information to the main processor during the execution of cpSAVE and
cpRESTORE instructions using coprocessor format words. The format words
defined for the M68000 coprocessor interface are listed in Table 10-2.
Table 10-2. Coprocessor Format Word Encodings
Format Code Length Meaning
00
xx
Empty/Reset
01
xx
Not Ready, Come Again
02
xx
Invalid Format
03-0F
xx
Undefined, Reserved
10-FF
length
Valid Format, Coprocessor Defined
The upper byte of the coprocessor format word contains the code used to
communicate coprocessor status information to the main processor. The
MC68030 recognizes four types of format words: empty/reset, not ready,
invalid format, and valid format. The MC68030 interprets the reserved format
codes ($03-$OF) as invalid format words. The lower byte of the coprocessor
format word specifies the size in bytes (which must be a multiple of four) of
the coprocessor state frame. This value is only relevant when the code byte
contains the valid format code (refer to 10.2.3.2.4 Valid Format Word).
10.2.3.2.1
Empty/Reset Format Word. The coprocessor returns the empty/reset
format code during a cpSAVE instruction to indicate that the coprocessor
contains no user-specific information. That is, no coprocessor instructions
have been executed since either a previous cpRESTORE of an empty/reset
format code or the previous hardware reset. If the main processor reads the
empty/reset format word from the save CIR during the initiation of a cpSAVE
instruction, it stores the format word at the effective address specified in the
cpSAVE instruction and executes the next instruction.
10-22
When the main processor reads the empty/reset format word from memory
during the execution of the cpRESTORE instruction, it writes the format word
to the restore CIR. The main processor then reads the restoreCIR and, if the
coprocessor returns the empty/reset format word, executes the next instruc-
tion. The main processor can initialize the coprocessor by writing the empty/
reset format code to the restore CIR.When the coprocessor receives the
empty/reset format code, it terminates any current operations and waits for
the main processor to initiate the next coprocessor instruction. In particular,
after the cpRESTORE of the empty/reset format word, the execution of a
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