Exception Priorities; Not All Exceptions Can Occur At Once - Samsung S3C2500B User Manual

PROGRAMMER' ' S MODEL
S3C2500B

2.10.1 EXCEPTION PRIORITIES

When multiple exceptions arise at the same time, a fixed priority system determines the order in which they are
handled:
Highest priority:
1. Reset
2. Data abort
3. FIQ
4. IRQ
5. Prefetch abort
Lowest priority:
6. Undefined Instruction, Software interrupt.

2.10.2 NOT ALL EXCEPTIONS CAN OCCUR AT ONCE:

Undefined Instruction and Software Interrupt are mutually exclusive, since they each correspond to particular
(non-overlapping) decoding of the current instruction.
If a data abort occurs at the same time as a FIQ, and FIQs are enabled (ie the CPSR's F flag is clear),
ARM9TDMI enters the data abort handler and then immediately proceeds to the FIQ vector. A normal return from
FIQ will cause the data abort handler to resume execution. Placing data abort at a higher priority than FIQ is
necessary to ensure that the transfer error does not escape detection. The time for this exception entry should be
added to worst-case FIQ latency calculations.
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