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

Debug Register Interfaces
Example C6-4 OS debug register restore sequence, Extended CP14 interface
; On entry, R0 points to a block of saved debug registers.
RestoreDebugRegisters
; (1) Set OS Lock Access Register (DBGOSLAR) and reset pointer. The lock
; will already be set, but this write is needed to reset the pointer.
LDR R1, =0xC5ACCE55
MCR p14, 0, R1, c1, c0, 4 ; Write DBGOSLAR
ISB
; (2) Clear the Sticky Power-down Status bit.
MRC p14, 0, R1, c1, c5, 4 ; Read DBGPRSR to clear StickyPD
ISB
; (3) Get the number of words saved.
MRC p14, 0, R1, c1, c2, 4 ; Dummy read of DBGOSSRR
LDR R1, [R0], #4 ; Load size from the save stack
; (4) Loop writing words from the DBGOSSRR.
CMP R1, #0 ; Check for zero
RestoreDebugRegisters_Loop
ITTT NE
LDRNE R2, [R0], #4 ; Load a word from the save stack
MCRNE p14, 0, R2, c1, c2, 4 ; Store a word of data
SUBSNE R1, R1, #1
BNE RestoreDebugRegisters_Loop
; (5) Clear the OS Lock Access Register (DBGOSLAR). Writing any non-key value
; clears the lock, so use the zero value in R1.
ISB
MCR p14, 0, R1, c1, c0, 4 ; Write DBGOSLAR
; (6) A final ISB guarantees the restored register values are visible to subsequent
; instructions.
ISB
; (7) Return the pointer to first word not read. This pointer is already in R0, so
; all that is needed is to return from this function.
BX LR
ARM DDI 0406B Copyright © 1996-1998, 2000, 2004-2008 ARM Limited. All rights reserved. C6-15