ARM VERSION 1.2 - Manual (Page 132)

ARM Instruction Reference
4-22 Copyright © 2000, 2001 ARM Limited. All rights reserved. ARM DUI 0068B
4.2.6 SWP
Swap data between registers and memory.
Use SWP to implement semaphores.
Syntax
SWP{cond}{B} Rd, Rm, [Rn]
where:
cond is an optional condition code (see Conditional execution on page 4-4).
B is an optional suffix. If B is present, a byte is swapped. Otherwise, a 32-bit
word is swapped.
Rd is an ARM register. Data from memory is loaded into Rd .
Rm is an ARM register. The contents of Rm is saved to memory.
Rm can be the same register as Rd . In this case, the contents of the register
is swapped with the contents of the memory location.
Rn is an ARM register. The contents of Rn specify the address in memory
with which data is to be swapped. Rn must be a different register from
both Rd and Rm .
Non word-aligned addresses
Non word-aligned addresses are handled in exactly the same way as an LDR and an STR
instruction (see Address alignment for word transfers on page 4-10).
Architectures
These instructions are available in ARM architecture versions 2a and 3 and above.