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ColdFire Core
Freescale Semiconductor
3-18
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The 8-bit vector number, vector[7:0], defines the exception type and is calculated by the processor
for all internal faults and represents the value supplied by the interrupt controller in case of an
interrupt. See
3.3.4
Processor Exceptions
3.3.4.1
Access Error Exception
The exact processor response to an access error depends on the memory reference being performed. For
an instruction fetch, the processor postpones the error reporting until the faulted reference is needed by an
instruction for execution. Therefore, faults during instruction prefetches followed by a change of
instruction flow do not generate an exception. When the processor attempts to execute an instruction with
a faulted opword and/or extension words, the access error is signaled and the instruction is aborted. For
this type of exception, the programming model has not been altered by the instruction generating the access
error.
If the access error occurs on an operand read, the processor immediately aborts the current instruction’s
execution and initiates exception processing. The operand execution pipeline includes logic to fully
recover program-visible register updates in the event of a bus transfer error acknowledge on an operand
memory reference. This allows for a precise instruction restart from this class of exceptions. See
Section 3.3.4.16, “Precise Faults”
, for additional information.
If the MMU is disabled, access errors are reported only with an attempted store to write-protected memory.
Therefore, access errors associated with instruction fetch or operand read accesses are not possible. The
Version 4 ColdFire processor, unlike the Version 2 and 3 ColdFire processors, updates the condition code
register if a write-protect error occurs during a CLR or MOV3Q operation to memory.
Internal memory accesses that fault (terminate with an internal memory transfer error acknowledge)
generate an access error exception. MMU TLB misses and access violations use the same fault. If the
MMU is enabled, all TLB misses and protection violations generate an access error exception. To
determine if a fault is due to a TLB miss or another type of access error, new FS encodings (described in
) signal TLB misses on instruction fetch, instruction extension fetch, and data read and writes.
1010
TLB miss on data write
1011
Reserved
1100
Error on operand read
1101
Attempted read, read-modify-write of protected space
1110
TLB miss on data read, or read-modify-write
1111
OEP access error while executing in emulator mode
1
This refers to taking an I/O interrupt during a debug service routine. If an access error occurs
during a debug service routine, FS is set to 0111 if it is due to an instruction fetch or to 1111
for a data access.
Table 3-7. Fault Status Encodings (continued)
FS[3:0]
Definition
Summary of Contents for MCF54455
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