ARM Compiler toolchain Assembler Reference: SWP and SWPB

SWP and SWPB

Swap data between registers and memory.

Syntax

SWP{B}{cond} Rt, Rt2, [Rn]

where:

cond: is an optional condition code.
B: is an optional suffix. If B is present, a byte is swapped. Otherwise, a 32-bit word is swapped.
Rt: is the destination register. Rt must not be PC.
Rt2: is the source register. Rt2 can be the same register as Rt. Rt2 must not be PC.
Rn: contains the address in memory. Rn must be a different register from both Rt and Rt2. Rn must not be PC.

Usage

You can use SWP and SWPB to implement semaphores:

Data from memory is loaded into Rt.
The contents of Rt2 is saved to memory.
If Rt2 is the same register as Rt, the contents of the register is swapped with the contents of the memory location.

Note

The use of SWP and SWPB is deprecated in ARMv6 and above. You can use LDREX and STREX instructions to implement more sophisticated semaphores in ARMv6 and above.

Architectures

These ARM instructions are available in all versions of the ARM architecture.

There are no Thumb SWP or SWPB instructions.

Copyright © 2010-2011 ARM. All rights reserved.	ARM DUI 0489E
Non-Confidential	ID071611
PDF version

SWP and SWPB

Syntax

Usage

Note

Architectures

See also