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MC68341 USER’S MANUAL
MOTOROLA
RW—Read/Write of Faulted Bus Cycle
Read and write bus cycles are distinguished by the RW bit. Read bus cycles will set this
bit, and write bus cycles will clear it. RW is reloaded into the bus controller if the RR bit
is set during unstacking.
0 = Faulted cycle was an operand write
1 = Faulted cycle was a prefetch or operand read
SIZ—Remaining Size of Faulted Bus Cycle
The SIZ field shows operand size remaining when a fault was detected. This field does
not indicate the initial size of the operand, nor does it necessarily indicate the proper
status of a dynamically sized bus cycle. Dynamic sizing occurs on the external bus and
is transparent to the CPU. Byte size is shown only when the original operand was a
byte. The field is reloaded into the bus controller if the RR bit is set during unstacking.
The SIZ field is encoded as follows:
00 = Long word
01 = Byte
10 = Word
11 = Unused, reserved
FUNC—Function Code of Faulted Bus Cycle
The function code for the faulted cycle is stacked in the FUNC field of the SSW, which is
a copy of FC2–FC0 for the faulted bus cycle. This field is reloaded into the bus
controller if the RR bit is set during unstacking. All unused bits are stacked as zeros and
are ignored during unstacking. Further discussion of the SSW is included in 5.5.3.1
Types of Faults.
5.5.3.1 TYPES OF FAULTS. An efficient implementation of instruction restart dictates that
faults on some bus cycles be treated differently than faults on other bus cycles. The
CPU32 defines four fault types: released write faults, faults during exception processing,
faults during MOVEM operand transfer, and faults on any other bus cycle.
5.5.3.1.1 Type I—Released Write Faults. CPU32 instruction pipelining can cause a final
instruction write to overlap the execution of a following instruction. A write that is
overlapped is called a released write. A released write fault occurs when a bus error or
some other fault occurs on the released write.
Released write faults are taken at the next instruction boundary. The stacked PC is that of
the next unexecuted instruction. If a subsequent instruction attempts an operand access
while a released write fault is pending, the instruction is aborted and the write fault is
acknowledged. This action prevents the instruction from using stale data.
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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