SWPB, SWPAB, SWPALB, SWPLB Swap byte in memory This instruction atomically loads an 8-bit byte 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, SWPAB and SWPALB load from memory with acquire semantics. SWPLB and SWPALB store to memory with release semantics. SWPB 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. 0 0 1 1 1 0 0 0 1 1 0 0 0 0 0 0 0 SWPB <Ws>, <Wt>, [<Xn|SP>] 1 0 SWPAB <Ws>, <Wt>, [<Xn|SP>] 1 1 SWPALB <Ws>, <Wt>, [<Xn|SP>] 0 1 SWPLB <Ws>, <Wt>, [<Xn|SP>] 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; <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. var address : bits(64); var data : bits(8); var store_value : bits(8); 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{8}(s); let comparevalue : bits(8) = ARBITRARY : bits(8); // Irrelevant when not executing CAS data = MemAtomic{8}(address, comparevalue, store_value, accdesc); X{32}(t) = ZeroExtend{32}(data); 2026-03-12 12:23:09 2025-09_rel_asl1