MC68030 Datasheet, PDF (382/602 Pages) Motorola, Inc – ENHANCED 32-BIT MICROPROCESSOR

Coprocessor Interface Description
10.2.3.2 COPROCESSOR FORMAT WORDS. The coprocessor communicates status
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
00
01
02
03-0F
10-FF
Length
xx
xx
xx
xx
Length
Meaning
Empty/Reset
Not Ready, Come Again
Invalid Format
Undefined, Reserved
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-$0F) 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.
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 restore CIR and, if the coprocessor returns the empty/reset
format word, executes the next instruction. 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 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).
10-22
MC68030 USER’S MANUAL
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