MC68HC000RC12 Datasheet, PDF (103/189 Pages) Freescale Semiconductor, Inc – 8-/16-/32-Bit Microprocessors User’s Manual

Freescale Semiconductor, Inc.
15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
SP
STATUS REGISTER
PROGRAM COUNTER (HIGH)
PROGRAM COUNTER (LOW)
1000
VECTOR OFFSET
SPECIAL STATUS WORD
FAULT ADDRESS (HIGH)
FAULT ADDRESS (LOW)
UNUSED, RESERVED
DATA OUTPUT BUFFER
UNUSED, RESERVED
DATA INPUT BUFFER
UNUSED, RESERVED
INSTRUCTION INPUT BUFFER
VERSION
NUMBER
INTERNAL INFORMATION, 16 WORDS
NOTE: The stack pointer is decremented by 29 words, although only 26
words of information are actually written to memory. The three
.additional words are reserved for future use by Motorola.
Figure 6-8. Exception Stack Order (Bus and Address Error)
The value of the saved program counter does not necessarily point to the instruction that
was executing when the bus error occurred, but may be advanced by as many as five
words. This incrementing is caused by the prefetch mechanism on the MC68010 that
always fetches a new instruction word as each previously fetched instruction word is used.
However, enough information is placed on the stack for the bus error exception handler to
determine why the bus fault occurred. This additional information includes the address
being accessed, the function codes for the access, whether it was a read or a write
access, and the internal register included in the transfer. The fault address can be used by
an operating system to determine what virtual memory location is needed so that the
requested data can be brought into physical memory. The RTE instruction is used to
reload the internal state of the processor at the time of the fault. The faulted bus cycle is
then rerun, and the suspended instruction is completed. If the faulted bus cycle is a read-
modify-write, the entire cycle is rerun, whether the fault occurred during the read or the
write operation.
An alternate method of handling a bus error is to complete the faulted access in software.
Using this method requires the special status word, the instruction input buffer, the data
input buffer, and the data output buffer image. The format of the special status word is
6-18
M68000 8-/16-/32-BIT MICROPROCESSORS USER'S MANUAL
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