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SWP and SWPB

Swap data between registers and memory.

These instruction are not supported in ARMv8.

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 are saved to memory.
If Rt2 is the same register as Rt, the contents of the register are swapped with the contents of the memory location.

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

These instructions are available in A32.

There are no T32 SWP or SWPB instructions.

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