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exceptions, will be restarted upon return from the exception handler. When a fault occurs
after an operand has transferred, that transfer is not "undone". However, these memory
locations are accessed a second time when the instruction is restarted. If a register used
in an EA calculation is overwritten before a fault occurs, an incorrect EA is calculated upon
instruction restart.
5.5.3.2.6 Type III—Correcting Faults via RTE. The preferred method of MOVEM bus
fault recovery is to correct the cause of the fault and then execute an RTE instruction
without altering the stack contents.
The RTE recognizes that MOVEM was in progress when a fault occurred, restores the
appropriate machine state, refetches the instruction, repeats the faulted transfer, and
continues the instruction.
MOVEM is the only instruction continued upon return from an exception handler. Although
the instruction is refetched, the EA is not recalculated, and the mask is rescanned the
same number of times as before the fault; modifying the code prior to RTE can cause
unexpected results.
5.5.3.2.7 Type IV—Correcting Faults via Software. Bus error exceptions can occur
during exception processing while the processor is fetching an exception vector or while it
is stacking. The same stack frame and SSW are used in both cases, but each has a
distinct fault address. The stacked faulted exception format/vector word identifies the type
of faulted exception and the contents of the remainder of the frame. A fault address
corresponding to the vector specified in the stacked format/vector word indicates that the
processor could not obtain the address of the exception handler.
A bus error exception handler should execute RTE after correcting a fault. RTE restores
the internal machine state, fetches the address of the original exception handler, recreates
the original exception stack frame, and resumes execution at the exception handler
address.
If the fault is intractable, the exception handler should rewrite the faulted exception stack
frame at SP + $14 + $06 and then jump directly to the original exception handler. The
stack frame can be generated from the information in the bus error frame: the pre-
exception SR (SP + $0C), the format/vector word (SP + $0E), and, if the frame being
written is a six-word frame, the PC of the instruction causing the exception (SP + $10).
The return PC value is available at SP + $02.
A stacked fault address equal to the current SP may indicate that, although the first
exception received a bus error while stacking, the bus error exception stacking
successfully completed. This occurrence is extremely improbable, but the CPU32
supports recovery from it. Once the exception handler determines that the fault has been
corrected, recovery can proceed as described previously. If the fault cannot be corrected,
move the supervisor stack to another area of memory, copy all valid stack frames to the
new stack, create a faulted exception frame on top of the stack, and resume execution at
the exception handler address.
MOTOROLA
MC68340 USER’S MANUAL
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