MC68340 datasheet(191/441 Pages) FREESCALE

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5- 54

MC68340 USER’S MANUAL

MOTOROLA

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

The LG bit indicates an original operand size of long word. LG is cleared if the original

operand was a byte or word—SIZ will indicate original (and remaining) size. LG is set if

the original was a long word—SIZ will indicate the remaining size at the time of fault. LG

is ignored during unstacking.

0 = Original operand size was byte or word

1 = Original operand size was long word

The SSW 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

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 stale data from being used by the instruction.

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