SWPH, SWPAH, SWPALH, SWPLH -- A64

SWPH, SWPAH, SWPALH, SWPLH

Swap halfword in memory

This instruction atomically loads a 16-bit halfword from a memory location, and stores the value held in a register back to the same memory location. The value initially loaded from memory is returned in the destination register.

If the destination register is not WZR, SWPAH and SWPALH load from memory with acquire semantics.
SWPLH and SWPALH store to memory with release semantics.
SWPH has neither acquire nor release semantics.

For more information about memory ordering semantics, see Load-Acquire, Store-Release.

For information about addressing modes, see Load/Store addressing modes.

Integer
(FEAT_LSE)

31	30	29	28	27	26	25	24	23	22	21	20	19	18	17	16	15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0
0	1	1	1	1	0	0	0	A	R	1	Rs					1	0	0	0	0	0	Rn					Rt
size					VR											o3	opc

Decode for all variants of this encoding

if !IsFeatureImplemented(FEAT_LSE) then EndOfDecode(Decode_UNDEF); end; let s : integer{} = UInt(Rs); let t : integer{} = UInt(Rt); let n : integer{} = UInt(Rn); let acquire : boolean = A == '1' && Rt != '11111'; let release : boolean = R == '1'; let tagchecked : boolean = n != 31;

Assembler Symbols

<Ws>	Is the 32-bit name of the general-purpose register to be stored, encoded in the "Rs" field.

<Wt>	Is the 32-bit name of the general-purpose register to be loaded, encoded in the "Rt" field.

<Xn|SP>

Is the 64-bit name of the general-purpose base register or stack pointer, encoded in the "Rn" field.

Operation

var address : bits(64); var data : bits(16); var store_value : bits(16); let privileged : boolean = PSTATE.EL != EL0; let accdesc : AccessDescriptor = CreateAccDescAtomicOp(MemAtomicOp_SWP, acquire, release, tagchecked, privileged, t, s); if n == 31 then CheckSPAlignment(); address = SP{64}(); else address = X{64}(n); end; store_value = X{16}(s); let comparevalue : bits(16) = ARBITRARY : bits(16); // Irrelevant when not executing CAS data = MemAtomic{16}(address, comparevalue, store_value, accdesc); X{32}(t) = ZeroExtend{32}(data);

2026-03_rel 2026-03-26 20:48:11