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Swap byte in memory 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 no memory ordering requirements.

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

For information about memory accesses see Load/Store addressing modes.



SWPAB (A == 1 && R == 0)

SWPAB <Ws>, <Wt>, [<Xn|SP>]

SWPALB (A == 1 && R == 1)

SWPALB <Ws>, <Wt>, [<Xn|SP>]

SWPB (A == 0 && R == 0)

SWPB <Ws>, <Wt>, [<Xn|SP>]

SWPLB (A == 0 && R == 1)

SWPLB <Ws>, <Wt>, [<Xn|SP>]

if !HaveAtomicExt() then UNDEFINED;

integer t = UInt(Rt);
integer n = UInt(Rn);
integer s = UInt(Rs);

AccType ldacctype = if A == '1' && Rt != '11111' then AccType_ORDEREDATOMICRW else AccType_ATOMICRW;
AccType stacctype = if R == '1' then AccType_ORDEREDATOMICRW else AccType_ATOMICRW;
boolean tag_checked = n != 31;

Assembler Symbols


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


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


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


bits(64) address;
bits(8) data;
bits(8) store_value;

if HaveMTEExt() then

if n == 31 then
    address = SP[];
    address = X[n];

store_value = X[s];
data = MemAtomic(address, MemAtomicOp_SWP, store_value, ldacctype, stacctype);
X[t] = ZeroExtend(data, 32);
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