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

Debug Events
When Monitor debug-mode is configured, debuggers must avoid these cases by restricting the programming
of the debug event control registers:
DBGVCR[28,27,12,11,4,3] must be programmed as zero, see Vector Catch Register (DBGVCR) on
page C10-67.
The permitted values of the Privileged Mode control bits, DBGBCR[2:1], must be restricted in the
following cases:
— if DBGBCR[22:20] is set to 0b010, selecting an Unlinked Context ID breakpoint
— If DBGBCR[22:20] is set to 0b100 or 0b101, selecting an IVA mismatch breakpoint.
For these cases, DBGBCR[2:1] must be programmed to one of:
— 0b00, selecting match only in User, Supervisor or System mode
— 0b10, selecting match only in User mode.
See Debug exceptions in abort handlers for additional points that must be considered before using
the 0b00 setting.
For details of programming the DBGBCR see Breakpoint Control Registers (DBGBCR) on
page C10-49.
If these restrictions are not followed, processor behavior on a resulting debug event is UNPREDICTABLE.
When the Security Extensions are implemented Vector Catch debug events on the Secure Monitor Call
vector are not ignored and are not UNPREDICTABLE. However, normally DBGVCR[10] is also programmed
as zero, see Monitor debug-mode vector catch on Secure Monitor Call on page C3-26.
Debug exceptions in abort handlers
The previous section indicated that, in ARMv7, a debugger might set DBGBCR[2:1] to 0b00, match in User,
Supervisor and System modes, to avoid the possibility of reaching an unrecoverable state in the Unlinked
Context ID and IVA mismatch breakpoint cases when Monitor debug-mode is selected. However,
DBGBCR[2:1] must only be programmed to 0b00 if you are confident that the abort handler will not switch
to one of these modes before saving context that might be corrupted by an additional debug event. The
context that might be corrupted by such an event includes LR_abt, SPSR_abt, IFAR, DFAR, and DFSR.
It is unlikely that an abort handler would switch to User mode to process an abort before saving these
registers, so setting DBGBCR[2:1] to 0b10, match only in User mode, is safer.
Also, take care when setting a Breakpoint or BKPT Instruction debug event inside a Prefetch Abort or Data
Abort handler, or when setting a Watchpoint debug event on a data address that might be accessed by any
of these handlers.
In general, a user must only set Breakpoint or BKPT Instruction debug events inside an abort handler at a
point after the context that would be corrupted by a debug event has been saved. Breakpoint debug events
in code that might be run by an abort handler can be avoided by setting DBGBCR[2:1] to 0b00 or 0b01, as
appropriate.
Watchpoint debug events in abort handlers can be avoided by setting DBGWCR[2:1] for the watchpoint to
0b10, match only unprivileged accesses, if the code being debugged is not running in a privileged mode.
ARM DDI 0406B Copyright © 1996-1998, 2000, 2004-2008 ARM Limited. All rights reserved. C3-25