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ColdFire Core
MCF51QE128 MCU Series Reference Manual, Rev. 3
Freescale Semiconductor 159
7.3.3 Processor Exceptions
7.3.3.1 Access Error Exception
The default operation of the V1 ColdFire processor is the generation of an illegal address reset event if an
access error (also known as a bus error) is detected. If CPUCR[ARD] is set, the reset is disabled and a
processor exception is generated as detailed below.
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 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. In this situation, any address register updates attributable to
the auto-addressing modes, (for example, (An)+,-(An)), have already been performed, so the programming
model contains the updated An value. In addition, if an access error occurs during a MOVEM instruction
loading from memory, any registers already updated before the fault occurs contain the operands from
memory.
The V1 ColdFire processor uses an imprecise reporting mechanism for access errors on operand writes.
Because the actual write cycle may be decoupled from the processor’s issuing of the operation, the
signaling of an access error appears to be decoupled from the instruction that generated the write.
Accordingly, the PC contained in the exception stack frame merely represents the location in the program
when the access error was signaled. All programming model updates associated with the write instruction
are completed. The NOP instruction can collect access errors for writes. This instruction delays its
execution until all previous operations, including all pending write operations, are complete. If any
previous write terminates with an access error, it is guaranteed to be reported on the NOP instruction.
7.3.3.2 Address Error Exception
The default operation of the V1 ColdFire processor is the generation of an illegal address reset event if an
address error is detected. If CPUCR[ARD] equals 1, then the reset is disabled and a processor exception
is generated as detailed below.
Any attempted execution transferring control to an odd instruction address (that is, if bit 0 of the target
address is set) results in an address error exception.
Any attempted use of a word-sized index register (Xn.w) or a scale factor of eight on an indexed effective
addressing mode generates an address error, as does an attempted execution of a full-format indexed
addressing mode, which is defined by bit 8 of extension word 1 being set.
If an address error occurs on an RTS instruction, the Version 1 ColdFire processor overwrites the faulting
return PC with the address error stack frame.