Type I) Completing Released Writes Via Software - Motorola CPU32 Reference Manual

There are three varieties of Type III operand fault recovery. The first is completion of
an instruction in software. The second is conversion to Type II with restart via RTE.
The third is continuation from the fault via RTE.

6.3.2.1 (Type I) Completing Released Writes via Software

To complete a bus cycle in software, a handler must first read the SSW function code
field to determine the appropriate address space, then access the fault address pointer
on the stack, and then transfer data from the stacked image of the output buffer to the
fault address.
Because the CPU32 has a 16-bit internal data bus, long operands require two bus ac-
cesses. A fault during the second access of a long operand causes the LG bit in the
SSW to be set. The SIZ field indicates remaining operand size. If operand coherency
is important, the complete operand must be rewritten. After a long operand is rewritten,
the RR bit must be cleared. Failure to clear the RR bit can cause RTE to rerun the bus
cycle. Following rewrite, it is not necessary to adjust the program counter (or other
stack contents) before executing RTE.
6.3.2.2 (Type I) Completing Released Writes via RTE
An exception handler can use the RTE instruction to complete a faulted bus cycle.
When RTE executes, the fault address, data output buffer, program counter, and sta-
tus register are restored from the stack. Any pending breakpoint or trace exceptions,
as indicated by TR, B1, and B0 in the stacked SSW, are requeued during SSW resto-
ration. The RR bit in the SSW is checked during the unstacking operation — if it is set,
the RW, FUNC, and SIZ fields are restored and the released write cycle is rerun.
To maintain long-word operand coherence, stack contents must be adjusted prior to
RTE execution. The fault address must be decremented by two if LG is set and SIZ
indicates a remaining byte or word. SIZ must be set to long. All other fields should be
left unchanged. The bus controller uses the modified fault address and SIZ field to re-
run the complete released write cycle
Manipulating the stacked SSW can cause unpredictable results because RTE checks
only the RR bit to determine if a bus cycle must be rerun. Inadvertent alteration of the
control bits could cause the bus cycle to be a read instead of a write, or could cause
access to a different address space than the original bus cycle. If the rerun bus cycle
is a read, returned data will be ignored.
6.3.2.3 (Type II) Correcting Faults via RTE
Instructions aborted because of a type II fault are restarted upon return from the ex-
ception handler. A fault handler must establish safe restart conditions. If a fault is
caused by a nonresident page in a demand-paged virtual memory configuration, the
fault address must be read from the stack, and the appropriate page retrieved. An RTE
instruction terminates the exception handler. After unstacking the machine state, the
instruction is refetched and restarted.
CPU32
REFERENCE MANUAL
EXCEPTION PROCESSING MOTOROLA
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