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

Debug Events
2. The processor takes the SMC exception, branching to the Monitor vector in Monitor mode. The
SCR.NS bit is set to 1, indicating the SMC originated in the Non-secure state.
3. The Vector Catch debug event is taken. Although SCR.NS is set to 1, the processor is in the Secure
state because it is in Monitor mode.
4. The processor jumps to the Secure Prefetch Abort vector, and sets SCR.NS to 0.
Note
Aborts taken in Secure state cause SCR.NS to be set to 0.
5. The abort handler at the Secure Prefetch Abort handler can tell a Vector Catch debug event occurred,
and can determine the address of the SMC instruction from LR_mon. However, it cannot determine
whether that is a Secure or Non-secure address.
Therefore, ARM recommends that you do not program a Vector Catch debug event on the SMC vector when
Monitor debug-mode is enabled.
Note
This is not a security issue, because the sequence given here can only occur if SPIDEN is HIGH.
Possible effect of the Security Extensions on FIQ vector catch
When the Security Extensions are implemented, a debugger might need to consider the implications of the
SCR on a Vector Catch event set on the FIQ vector, when the SCR is configured with both:
the SCR.FW bit set to 0, so the CPSR.F bit cannot be modified in Non-secure state
the SCR.FIQ bit set to 0, so that FIQs are handled in FIQ mode.
With this configuration, if an FIQ occurs in Non-secure state, the processor does not set CPSR.F to disable
FIQs, and so the processor repeatedly takes the FIQ exception.
It might not be possible to debug this situation using the vector catch on FIQ because the instruction at the
FIQ exception vector is never committed for execution and therefore the debug event never occurs.
C3.2.7 Pseudocode details of Software debug events
The following subsections give pseudocode details of Software debug events:
Debug events
Breakpoints and Vector Catches on page C3-28
Watchpoints on page C3-35.
Debug events
The following functions cause the corresponding debug events to occur:
BKPTInstrDebugEvent()
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