Level Two Interface
ARM DDI 0301H Copyright © 2004-2009 ARM Limited. All rights reserved. 8-7
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8.2.2 Store-exclusive instruction
Store-exclusive performs a conditional store to memory. The store only takes place if the
physical address is tagged as exclusive-access for the requesting processor. This operation
returns a status value. If the store updates memory the return value is 0, otherwise it is 1. In both
cases, the physical address is no longer tagged as exclusive-access for any processor.
8.2.3 Example of LDREX and STREX usage
This is an example of typical usage. Suppose you are trying to claim a lock:
Lock address : LockAddr
Lock free : 0x00
Lock taken : 0xFF
MOV R1, #0xFF ; load the ‘lock taken’ value
try LDREX R0, [LockAddr] ; load the lock value
CMP R0, #0 ; is the lock free?
STREXEQ R0, R1, [LockAddr] ; try and claim the lock
CMPEQ R0, #0 ; did this succeed?
BNE try ; no – try again ....
; yes – we have the lock
The typical case, where the lock is free and you have exclusive-access, is six instructions.
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