ARM Cortex-M4 - Page 84

The Cortex-M4 Instruction Set
ARM DUI 0553A Copyright © 2010 ARM. All rights reserved. 3-37
ID121610 Non-Confidential
Restrictions
In these instructions:
• do not use PC
• do not use SP for
Rd and Rt
• for STREX , Rd must be different from both Rt and Rn
• the value of offset must be a multiple of four in the range 0-1020.
Condition flags
These instructions do not change the flags.
Examples
MOV R1, #0x1 ; Initialize the ‘lock taken’ value
try
LDREX R0, [LockAddr] ; Load the lock value
CMP R0, #0 ; Is the lock free?
ITT EQ ; IT instruction for STREXEQ and CMPEQ
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.