12.7.2. Avoiding unrecoverable states

The processor ignores vector catch debug events on the Prefetch or Data Abort vectors while in Monitor debug-mode because these events would otherwise put the processor in an unrecoverable state.

The debuggers must avoid other similar cases by following these rules, that apply only if the processor is in Monitor debug-mode:

The debugger must write DBGBCR[2:1] for the same breakpoint as either b00 or b10, that selects either match in only USR, SYS, or SVC modes or match in only USR mode, respectively. The debugger must not program either b01, that is, match in any Privileged mode, or b11, that is, match in any mode.

You must only request the debugger to write b00 to DBGBCR[2:1] if you know that the abort handler does not switch to one of the USR, SYS, or SVC mode before saving the context that might be corrupted by a later debug event. You must also be careful about requesting the debugger to set a breakpoint or BKPT debug event inside a Prefetch Abort or Data Abort handler, or a watchpoint debug event on a data address that any of these handlers might access.

In general, you must only set breakpoint or BKPT debug events inside an abort handler after it saves the abort context. You can avoid breakpoint debug events in abort handlers by setting DBGBCR[2:1] as previously described.

If the code being debugged is not running in a Privileged mode, you can prevent watchpoint debug events in abort handlers by setting DBGWCR[2:1] to b10 for match only non-privileged accesses.

Failure to follow these guidelines can lead to debug events occurring before the handler is able to save the context of the abort. This causes the corresponding registers to be overwritten, and results in Unpredictable software behavior.

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