Chapter 4. Exceptions
4-5
Exception Recognition and Priorities
4.2 Exception Recognition and Priorities
Exceptions are roughly prioritized by exception class, as follows:
1. Nonmaskable, asynchronous exceptions have priority over all other
exceptionssystem reset and machine check exceptions (although the machine
check exception condition can be disabled so the condition causes the processor to
go directly into the checkstop state). These exceptions cannot be delayed and do not
wait for completion of any precise exception handling.
2. Synchronous, precise exceptions are caused by instructions and are taken in strict
program order.
3. Imprecise exceptions (imprecise mode oating-point enabled exceptions) are
caused by instructions and they are delayed until higher priority exceptions are
taken. Note that the MPC7400 does not implement an exception of this type.
4. Maskable asynchronous exceptions (external, decrementer, system management,
thermal management, and performance monitor interrupts) are delayed until higher
priority exceptions are taken.
The following list of exception categories describes how the MPC7400 handles exceptions
up to the point of signaling the appropriate interrupt to occur. Note that a recoverable state
is reached if the completed store queue is empty (drained, not canceled) and any instruction
that is next in program order, and has been signaled to complete, has completed. If
MSR[RI] = 0, the MPC7400 is in a nonrecoverable state. Also, instruction completion is
deTned as updating all architectural registers associated with that instruction, and then
removing that instruction from the completion buffer.
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Exceptions caused by asynchronous events (interrupts). These exceptions are further
distinguished by whether they are maskable and recoverable.
Asynchronous, nonmaskable, nonrecoverable
System reset for assertion of HRESETHas highest priority and is taken
immediately regardless of other pending exceptions or recoverability (includes
power-on reset).
Asynchronous, maskable, nonrecoverable
Machine check exceptionHas priority over any other pending exception
except system reset for assertion of HRESET (or power-on reset). Taken
immediately regardless of recoverability.
Asynchronous, nonmaskable, recoverable
System reset for SRESETHas priority over any other pending exception
except system reset for HRESET (or power-on reset), or machine check. Taken
immediately when a recoverable state is reached.