ColdFire CoreMCF5253 Reference Manual, Rev. 1Freescale Semiconductor 3-9• There is a 4-bit fault status field, FS[3:0], at the top of the system stack. This field is defined foraccess and address errors only and written as zeros for all other types of exceptions. See Table 3-7.• The 8-bit vector number, vector[7:0], defines the exception type and is calculated by the processorfor all internal faults and represents the value supplied by the peripheral in the case of an interrupt.Refer to Table 3-5.3.5 Processor Exceptions3.5.1 Access Error ExceptionThe exact processor response to an access error depends on the type of memory reference being performed.For an instruction fetch, the processor postpones the error reporting until the faulted reference is neededby an instruction for execution. Therefore, faults that occur during instruction prefetches that are thenfollowed by a change of instruction flow do not generate an exception. When the processor attempts toexecute an instruction with a faulted opword and/or extension words, the access error is signaled and theinstruction aborted. For this type of exception, the programming model has not been altered by theinstruction generating the access error.If the access error occurs on an operand read, the processor immediately aborts the current instruction’sexecution and initiates exception processing. In this situation, any address register updates attributable tothe auto-addressing modes, (e.g., (An)+,-(An)), have already been performed, so the programming modelcontains the updated An value. In addition, if an access error occurs during the execution of a MOVEMinstruction loading from memory, any registers already updated before the fault occurs contains theoperands from memory.The 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, thesignaling 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 programTable 3-7. Fault Status EncodingFS[3:0] Definition00xx Reserved0100 Error on instruction fetch0101 Reserved011x Reserved1000 Error on operand write1001 Attempted write to write-protected space101x Reserved1100 Error on operand read1101 Reserved111x Reserved
