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3-4 SA-1100
Developer’s Manual
ARM™ Implementation Options
3.2.4 Vector Summary
Table 3-1 lists byte addresses, and they normally contain branch instructions pointing to the
relevant routines. These addresses (except the reset vector) can be changed (to 0xFFFF xxxx)
through the vector adjust facility (bit 13, register 1, coprocessor 15). The vector adjust is cleared at
reset and cannot modify the reset vector.
3.2.5 Exception Priorities
When multiple exceptions arise at the same time, a fixed priority system determines the order in
which they will be handled:
1. Reset (highest priority)
2. Data abort
3. FIQ
4. IRQ
5. Prefetch abort
6. Undefined instruction, software interrupt (lowest priority)
Note that not all exceptions can occur at once. Undefined instructions and software interrupts are
mutually exclusive because they correspond to particular (nonoverlapping) decodings of the
current instruction.
If a data abort occurs at the same time as a FIQ, and FIQs are enabled (that is, the F flag in the
CPSR is clear), the SA-1100 will enter the data abort handler and then immediately proceed 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.
Table 3-1. Vector Summary
Address Exception Mode on Entry
0x00000000 Reset Supervisor
0x00000004 Undefined instruction Undefined
0x00000008 Software interrupt Supervisor
0x0000000C Abort (prefetch) Abort
0x00000010 Abort (data) Abort
0x00000014 Not used —
0x00000018 IRQ IRQ
0x0000001C FIQ FIQ