MC68340 datasheet(198/441 Pages) FREESCALE

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MOTOROLA

MC68340 USER’S MANUAL

5- 61

Bus operation in progress at the time of a fault is conveyed by the SSW.

9876543210

SIZ

FUNC

The bus error stack frame is 12 words in length. There are three variations of the frame,

each distinguished by different values in the SSW TP and MV fields.

An internal transfer count register appears at location SP + $14 in all bus error stack

frames. The register contains an 8-bit microcode revision number, and, for type III faults,

an 8-bit transfer count. Register format is shown in Figure 5-14.

MICROCODE REVISION NUMBER

TRANSFER COUNT

Figure 5-14. Internal Transfer Count Register

The microcode revision number is checked before a bus error stack frame is restored via

RTE. In a multiprocessor system, this check ensures that a processor using stacked

information is at the same revision level as the processor that created it.

The transfer count is ignored unless the MV bit in the stacked SSW is set. If the MV bit is

set, the least significant byte of the internal register is reloaded into the MOVEM transfer

counter during RTE execution.

For faults occurring during normal instruction execution (both prefetches and non-MOVEM

operand accesses) SSW TP, MV = 00. Stack frame format is shown in Figure 5-15.

Faults that occur during the operand portion of the MOVEM instruction are identified by

SSW TP, MV = 01. Stack frame format is shown in Figure 5-16.

When a bus error occurs during exception processing, SSW TP, MV = 10. The frame

shown in Figure 5-17 is written below the faulting frame. Stacking begins at the address

pointed to by SP – 6 (SP value is the value before initial stacking on the faulted frame).

The frame can have either four or six words, depending on the type of error. Four-word

stack frames do not include the faulted instruction PC (the internal transfer count register

is located at SP

+ $10 and the SSW is located at SP + $12).

The fault address of a dynamically sized bus cycle is the address of the upper byte,

regardless of the byte that caused the error.

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