Motorola CPU32 Reference Manual page 268

Table 6-2. Exception Priority Groups
Groupl
Exception and
Priority Relative Priority
Characteristics
0 Reset
Aborts all processing (instruction or
exception); does not save old context
1.1 Address Error Suspends processing (instruction or
1.2 Bus Error exception); saves internal context
BKPT#n, CHK, CHK2,
Exception processing is a part of
2 Division by Zero, RTE,
TRAP#n, TRAPcc, TRAPV
instruction execution
Illegal Instruction, Line A,
Exception processing begins before
3 Unimplemented Line F,
Privelege Violation
instruction execution
4.1
Trace
Exception processing begins when current
4.2
Hardware Breakpoint instruction or previous exception
4.3
Interrupt processing is complete
It is important to be aware of the difference between exception processing mode
and execution of an exception handler. Each exception has an assigned vector
that points to an associated handler routine. Exception processing includes
steps described in 6.1.3 Exception Processing Sequence, but does not
include execution of handler routines, which is done in normal mode.
When the CPU32 completes exception processing, it is ready to begin either
exception processing for a pending exception, or execution of a handler routine.
Priority assignment governs the order in which exception processing occurs, not
the order in which exception handlers are executed
As a general rule, when simultaneous exceptions occur, the handler routines for
III
lower priority exceptions are executed before the handler routines for higher •
priority exceptions. For example, consider the arrival of an interrupt during
execution of a TRAP instruction, while tracing is enabled. Trap exception
processing (2) is done first, followed immediately by exception processing for
the trace (4.1), and then by exception processing for the interrupt (4.3). Each
exception places a new context on the stack. When the processor resumes
normal instruction execution, it is vectored to the interrupt handler, which returns
to the trace handler that returns to the trap handler.
There are special cases to which the general rule does not apply. The reset
exception will always be the first exception handled, since reset clears all other
exceptions. It is also possible for high priority exception processing to begin
before low priority exception processing is complete. For example, if a bus error
occurs during trace exception processing, the bus error will be processed and
handled before trace exception processing is completed.
CPU32 REFERENCE MANUAL EXCEPTION PROCESSING MOTOROLA
6-5