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

Debug Register Interfaces
The performance monitor registers described in Chapter C9 Performance Monitors.
The trace registers.
The OS Restore sequence always overwrites the debug registers with the values that were saved. In
particular, the values of the DBGDTRTX and DBGDTRRX Registers, and of the DCC status flags TXfull,
TXfull_l, RXfull, and RXfull_l after the OS Restore sequence are the saved values.
If there were valid values in the DBGDTRTX or DBGDTRRX Registers immediately before the OS Restore
sequence then those values are lost.
Example OS Save and Restore sequences
Example OS Save and Restore sequences are described in:
Example OS Save and Restore sequences using the memory-mapped interface
Example OS Save and Restore sequences using the Extended CP14 interface on page C6-14.
Example OS Save and Restore sequences using the memory-mapped interface
On an implementation that includes the OS Save and Restore mechanism and a memory-mapped interface:
Example C6-1 shows the correct sequence for saving the debug logic state, using the
memory-mapped interface, before powering down
Example C6-2 on page C6-13 shows the correct sequence for restoring the debug logic state, using
the memory-mapped interface, when the system is powered on again.
When the debug logic state is restored, if the OS Unlock Catch bit in the Event Catch Register is set to 1 a
debug event is triggered when the DBGOSLAR is cleared. This event might be used by an external debugger
to restart a debugging session. See Event Catch Register (DBGECR) on page C10-78.
Example C6-1 OS debug register save sequence, memory-mapped interface
; On entry, R0 points to a block to save the debug registers in.
SaveDebugRegisters
PUSH {R4, LR}
MOV R4, R0 ; Save pointer
; (1) Set OS Lock Access Register (DBGOSLAR). The architecture requires that DBGOSLAR
; and the other debug registers have at least the Device memory attribute.
BL GetDebugRegisterBase ; Returns base in R0
LDR R1, =0xC5ACCE55
STR R1, [R0, #0x300] ; Write DBGOSLAR
; (2) Get the number of words to save.
LDR R1, [R0, #0x308] ; DBGOSSRR returns size
STR R1, [R4], #4 ; Push on to the save stack
; (3) Loop reading words from the DBGOSSRR.
C6-12 Copyright © 1996-1998, 2000, 2004-2008 ARM Limited. All rights reserved. ARM DDI 0406B