MC68030 Datasheet, PDF (424/602 Pages) Motorola, Inc – ENHANCED 32-BIT MICROPROCESSOR

Coprocessor Interface Description
10.5.1.4 COPROCESSOR SYSTEM-RELATED EXCEPTIONS. System-related
exceptions detected by a DMA coprocessor include those associated with bus activity and
any other exceptions (interrupts, for example) occurring external to the coprocessor. The
actions taken by the coprocessor and the main processor depend on the type of exception
that occurs.
When an address or bus error is detected by a DMA coprocessor, the coprocessor should
store any information necessary for the main processor exception handling routines in
system-accessible registers. The coprocessor should place one of the three take exception
primitives encoded with an appropriate exception vector number in the response CIR. Which
of the three primitives is used depends upon the point in the coprocessor instruction at which
the exception was detected and the point in the instruction execution at which the main
processor should continue after exception processing.
10.5.1.5 FORMAT ERRORS. Format errors are the only coprocessor-detected exceptions
that are not signaled to the main processor with a response primitive. When the main
processor writes a format word to the restore CIR during the execution of a cpRESTORE
instruction, the coprocessor decodes this word to determine if it is valid (refer to 10.2.3.3
Coprocessor Context Save Instruction). If the format word is not valid, the coprocessor
places the invalid format code in the restore CIR. When the main processor reads the invalid
format code, it aborts the coprocessor instruction by writing an abort mask (refer to 10.3.2
Control CIR) to the control CIR. The main processor then performs exception processing
using a four-word pre-instruction stack frame and the format error exception vector number
14. Thus, if the exception handler does not modify the stack frame, the MC68030 restarts
the cpRESTORE instruction when the RTE instruction in the handler is executed. If the
coprocessor returns the invalid format code when the main processor reads the save CIR
to initiate a cpSAVE instruction, the main processor performs format error exception
processing as outlined for the cpRESTORE instruction.
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MC68030 USER’S MANUAL
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