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exception is also stacked. This data is placed on the stack in the format shown in Figure
5-13. The return address from the initial exception is stacked for RTE .
5.5.3.2 CORRECTING A FAULT. There are two ways to complete a faulted released write
bus cycle. The first is to use a software handler. The second is to rerun the bus cycle via
RTE.
Type II fault handlers must terminate with RTE, but specific requirements must also be
met before an instruction is restarted.
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.
5.5.3.2.1 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, 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
accesses. 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 the RTE instruction to rerun
the bus cycle. Following rewrite, it is not necessary to adjust the PC (or other stack
contents) before executing RTE.
5.5.3.2.2 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, PC, and SR 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 restoration. 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 2 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 rerun 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.
MOTOROLA
MC68340 USER’S MANUAL
5- 57
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