ARM Architecture Reference Manual ARMv7-A and ARMv7-R edition

C3.5 Debug event prioritization
Debug events can be synchronous or asynchronous:
Debug Events
Breakpoint, Vector Catch, BKPT Instruction, and synchronous Watchpoint debug events are all
synchronous debug events
asynchronous Watchpoints and all Halting debug events are all asynchronous debug events.
A single instruction can generate a number of synchronous debug events. It can also generate a number of
synchronous exceptions. The principles given in Exception priority order on page B1-33 apply to those
exceptions and debug events, in addition to the following:
An instruction fetch that generates an MMU fault, MPU fault, or external abort does not generate a
Breakpoint or Vector Catch debug event.
Breakpoint and Vector Catch debug events are associated with the instruction and are taken before
the instruction executes. Therefore, when a Breakpoint or Vector Catch debug event occurs no other
synchronous exception or debug event that would have occurred as a result of executing the
instruction is generated.
If a single instruction has more than one of the following debug events associated with it, it is
UNPREDICTABLE which is taken:
— Breakpoint
— Vector Catch.
No instruction is valid if it has a Prefetch Abort exception associated with it. Therefore, if an
instruction causes a Prefetch Abort exception no other synchronous exception or debug event that
would have occurred as a result of executing the instruction is generated.
An instruction that generates an Undefined Instruction exception does not cause any memory access,
and therefore cannot cause a Data Abort exception or a Watchpoint debug event.
A memory access that generates an MMU fault or an MPU fault must not generate a Watchpoint
debug event.
A memory access that generates an MMU fault, an MPU fault, or a synchronous Watchpoint debug
event must not generate an external abort.
All other synchronous exceptions and synchronous debug events are mutually exclusive, and are
derived from a decode of the instruction.
The ARM architecture does not define when asynchronous debug events other than asynchronous
Watchpoint debug events are taken. Therefore the prioritization of asynchronous debug events other than
asynchronous Watchpoint debug events is IMPLEMENTATION DEFINED.
ARM DDI 0406B Copyright © 1996-1998, 2000, 2004-2008 ARM Limited. All rights reserved. C3-43