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MC68341 USER’S MANUAL
MOTOROLA
attempting to read the stack frame. The version number is located in the most significant
byte (bits 15–8) of the internal register word at location SP
+ $14 in the stack frame. The
validity check ensures that stack frame data will be properly interpreted in multiprocessor
systems.
If a frame is invalid, a format error exception is taken. If it is inaccessible, a bus error
exception is taken. Otherwise, the processor reads the entire frame into the proper
internal registers, de-allocates the stack (12 words), and resumes normal processing. Bus
error frames for faults during exception processing require the RTE instruction to rewrite
the faulted stack frame. If an error occurs during any of the bus cycles required by rewrite,
the processor halts.
If a format error occurs during RTE execution, the processor creates a normal four-word
fault stack frame below the frame that it was attempting to use. If a bus error occurs, a
bus-error stack frame will be created. The faulty stack frame remains intact, so that it may
be examined and repaired by an exception handler or used by a different type of
processor (e.g., MC68010, MC68020, or future M68000 processor) in a multiprocessor
system.
5.5.3 Fault Recovery
There are four phases of recovery from a fault: recognizing the fault, saving the processor
state, repairing the fault (if possible), and restoring the processor state. Saving and
restoring the processor state are described in the following paragraphs.
The stack contents are identified by the special status word (SSW). In addition to
identifying the fault type represented by the stack frame, the SSW contains the internal
processor state corresponding to the fault.
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14
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9876543210
TP
MV
0
TR
B1
B0
RR
RM
IN
RW
LG
SIZ
FUNC
TP—BERR Frame Type
The TP field defines the class of the faulted bus operation. Two bus error exception
frame types are defined. One is for faults on prefetch and operand accesses, and the
other is for faults during exception frame stacking.
0 = Operand or prefetch bus fault
1 = Exception processing bus fault
MV—MOVEM in Progress
MV is set when the operand transfer portion of the MOVEM instruction is in progress at
the time of a bus fault. If a prefetch bus fault occurs while prefetching the MOVEM
opcode and extension word, both the MV and IN bits will be set.
0 = MOVEM was not in progress when fault occurred
1 = MOVEM was in progress when fault occurred
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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