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

Invasive Debug Authentication
C2.1 About invasive debug authentication
Invasive debug can be enabled or disabled. If it is disabled the processor ignores all debug events except
BKPT Instruction. This means that debug events other than the BKPT Instruction debug event do not cause
the processor to enter Debug state or to take a debug exception.
In addition, if a processor implements the Security Extensions, invasive debug can be permitted or not
permitted. When invasive debug is not permitted, all debug events are not permitted. When a debug event is
not permitted:
if the debug event is not a BKPT Instruction debug event then it is ignored
if the debug event is a BKPT Instruction debug event then it causes a debug exception.
Note
The BKPT Instruction debug event is never ignored.
The difference between enabled and permitted is that whether a debug event is permitted depends on both
the security state and the operating mode of the processor.
For debug events that cause entry to Debug state, Secure User halting debug refers to permitting these events
in Secure User mode when invasive debug is not permitted in Secure privileged modes. The debug events
that cause entry to Debug state are:
Halting debug events
if Halting debug-mode is selected, Software debug events.
Support for Secure User halting debug is required in v6.1 Debug. In v7 Debug it is IMPLEMENTATION
DEFINED whether Secure User halting debug is supported. On an implementation that does not support
Secure User halting debug the DBGDIDR.nSUHD_imp bit is RAO, see Debug ID Register (DBGDIDR) on
page C10-3. ARM deprecates the use of Secure User halting debug.
If the Security Extensions are implemented, when invasive debug is not permitted in Secure privileged
modes it must be possible to permit, in Secure User mode, the debug events that do not cause entry to Debug
state. The debug events that do not cause entry to Debug state are Software debug events when Monitor
debug-mode is selected.
Note
When the Security Extensions are implemented, the Debug architecture distinguishes between permitting
invasive halting debug and permitting invasive non-halting debug. However, in Non-secure state and in
Secure privileged modes whether a debug event is permitted does not depend on whether the event would
cause entry to Debug state. Therefore, the distinction between permitting invasive halting debug and
invasive non-halting debug applies only in Secure User mode.
When Secure User halting debug is supported, the processor can be configured so that both invasive halting
debug and invasive non-halting debug are permitted in Secure User mode when invasive debug is not
permitted in Secure privileged modes. Therefore, the alternatives for when a debug event is permitted are:
in all processor modes, in both Secure and Non-secure security states
only in Non-secure state
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