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

Debug Events
If these guidelines are not followed, a debug event might occur before the handler has saved the context of
the abort, causing the context to be overwritten. This loss of context results in UNPREDICTABLE software
behavior. The context that might be corrupted by such an event includes LR_abt, SPSR_abt, IFAR, DFAR,
and DFSR.
Debug events in the debug monitor
Because debug exceptions generate Data Abort or Prefetch Abort exceptions, the precautions outlined in the
section Debug exceptions in abort handlers on page C3-25 also apply to debug monitors. The suggested
settings for breakpoints and watchpoints that can avoid taking debug exceptions in a Data Abort handler can
be used to avoid taking debug exceptions in the debug monitor.
In addition, particularly on ARMv7 processors that do not implement the Extended CP14 interface, and
particularly those that implement synchronous Watchpoint debug events, when Monitor debug-mode is
enabled debuggers must avoid:
setting Watchpoint debug events on the addresses of debug registers
setting Breakpoint and Vector Catch debug events on the addresses of instructions in the debug
monitor.
In particular, it is unwise to set a watchpoint on the address of the Watchpoint Control Register (DBGWCR)
for that watchpoint, or to set a breakpoint on the address of an instruction that disables the breakpoint.
The section Generation of debug events on page C3-40 identifies two problem cases:
A write to the DBGWCR for a watchpoint set on the address of that DBGWCR, to disable that
watchpoint, triggers the watchpoint.
In this case:
— if watchpoints are asynchronous, the write to the DBGWCR still takes place and the
watchpoint is disabled. The debug software must then deal with the re-entrant debug
exception.
— if watchpoints are synchronous the value in the DBGWCR after the watchpoint is signaled is
unchanged, and the debug event is left enabled.
an instruction that disables a breakpoint on that instruction triggers the breakpoint.
In this case, the debug exception is taken before the debug event is disabled.
In both of these cases it might be impossible to recover.
Monitor debug-mode vector catch on Secure Monitor Call
Debuggers must be cautious about programming a Vector Catch debug event on the Secure Monitor Call
(SMC) vector when Monitor debug-mode is configured. If such an event is programmed, the following
sequence can occur:
1. Non-secure code executes an SMC instruction.
C3-26 Copyright © 1996-1998, 2000, 2004-2008 ARM Limited. All rights reserved. ARM DDI 0406B