Motorola MC68030 User Manual page 299

8.2.3
Completing the Bus Cycles with RTE
Another method of completing a faulted bus cycle is to allow the processor
to rerun the bus cycles during execution of the RTE instruction that terminates
the exception handler. This method cannot be used to recover from address
errors. The RTE instruction is always executed. Unless the handler routine
has corrected the error and cleared the fault (and cleared the rerun and OF
bits of the SSW), the RTE instruction can complete the bus cycle(s). If the OF
bit is still set at the time of the RTE execution, the faulted data cycle is rerun
by the RTE instruction. If the fault bit for a stage of the pipe is set and the
corresponding rerun bit was not cleared by the software, the RTE reruns the
associated instruction prefetch. The fault occurs again unless the cause of
the fault, such as a non-resident page in a virtual memory system, has been
corrected. If the rerun bit is set for a stage of the pipe and the fault bit is
cleared, the associated prefetch cycle mayor may not be run by the RTE
instruction (depending on whether the stage is required).
If a fault occurs when the RTE instruction attempts to rerun the bus cycle(s),
the processor creates a new stack frame on the supervisor stack after de-
allocating the previous frame, and address error or bus error exception pro-
cessing starts in the normal manner.
The read-modify-write operations of the MC68030 can also be completed by
the RTE instruction that terminates the handler routine. The rerun operation,
executed by the RTE instruction with the OF bit of the SSW set, reruns the
entire instruction. If the cause of the error has been corrected, the handler
does not need to emulate the instruction but can leave the OF bit set and
execute the RTE instruction.
Systems programmers and designers should be aware that the MMU of the
MC68030 treats any bus cycle with RMC asserted as a write operation for
protection checking, regardless of the state of
R/W
signal. Otherwise, the
potential for partially destroying system pointers with CAS and CAS2 instruc-
tions exists since one portion of the write operation could take place and the
remainder be aborted by a bus error.
MOTOROLA MC68030 USER'S MANUAL
8-31