Exception Classes; Ordered Exceptions; Unordered Exceptions - Motorola MPC533 Reference Manual

To prevent the loss of state information, exception handlers must save the information
stored in SRR0 and SRR1 soon after the exception is taken to prevent this information from
being lost due to another exception being taken.
3.11.1

Exception Classes

The MPC533 exception classes are shown in Table 3-20.
Synchronous (ordered, precise)
3.11.2

Ordered Exceptions

In the MPC533, all exceptions except for reset, debug port non-maskable interrupts, and
machine check exceptions are ordered. Ordered exceptions satisfy the following criteria:
• Only one exception is reported at a time. If, for example, a single instruction
encounters multiple exception conditions, those conditions are encountered
sequentially. After the exception handler handles an exception, instruction execution
continues until the next exception condition is encountered.
• When the exception is taken, no program state is lost.
3.11.3

Unordered Exceptions

Unordered exceptions may be reported at any time and are not guaranteed to preserve
program state information. The processor can never recover from a reset exception. It can
recover from other unordered exceptions in most cases. However, if a debug port
non-maskable interrupt or machine check exception occurs during the servicing of a
previous exception, the machine state information in SRR0 and SRR1 (and, in some cases,
the DAR and DSISR) may not be recoverable; the processor may be in the process of saving
or restoring these registers.
To determine whether the machine state is recoverable, the RI (recoverable exception) bit
in SRR1 can be read. During exception processing, the RI bit in the MSR is copied to SRR1
and then cleared. The operating system should set the RI bit in the MSR at the end of each
exception handler's prologue (after saving the program state) and clear the bit at the start
of each exception handler's epilogue (before restoring the program state). Then, if an
unordered exception occurs during the servicing of an exception handler, the RI bit in SRR1
will contain the correct value.
MOTOROLA
PRELIMINARY—SUBJECT TO CHANGE WITHOUT NOTICE
Table 3-20. MPC533 Exception Classes
Class
Asynchronous, unordered
Asynchronous, ordered
Chapter 3. Central Processing Unit
Exception Type
Machine check
System reset
External interrupt
Decrementer
Instruction-caused exceptions
Exception Model
3-39