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MPC7400 RISC Microprocessor Users Manual
Exception Recognition and Priorities
Asynchronous, maskable, recoverable
System management, performance monitor, thermal management, external, and
decrementer interruptsBefore handling this type of exception, the next
instruction in program order must complete. If that instruction causes another
type of exception, that exception is taken and the asynchronous, maskable
recoverable exception remains pending, until the instruction completes. Further
instruction completion is halted. The asynchronous, maskable recoverable
exception is taken when a recoverable state is reached.
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Instruction-related exceptions. These exceptions are further organized into the point
in instruction processing in which they generate an exception.
Instruction fetch
ISI exceptionsOnce this type of exception is detected, dispatching stops and
the current instruction stream is allowed to drain out of the machine. If
completing any of the instructions in this stream causes an exception, that
exception is taken and the instruction fetch exception is discarded (but may be
encountered again when instruction processing resumes). Otherwise, once all
pending instructions have executed and a recoverable state is reached, the ISI
exception is taken.
Instruction dispatch/execution
Program, DSI, alignment, oating-point unavailable, system call, instruction
address breakpoint, and data address breakpointThis type of exception is
determined during dispatch or execution of an instruction. The exception
remains pending until all instructions before the exception-causing instruction in
program order complete. The exception is then taken without completing the
exception-causing instruction. If completing these previous instructions causes
an exception, that exception takes priority over the pending instruction
dispatch/execution exception, which is discarded (but may be encountered again
when instruction processing resumes).
Post-instruction execution
TraceTrace exceptions are generated following execution and completion of
an instruction while trace mode is enabled. If executing the instruction produces
conditions for another type of exception, that exception is taken and the
post-instruction exception is forgotten for that instruction.