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Compare and Swap byte in memory reads an 8-bit byte from memory, and compares it against the value held in a first register. If the comparison is equal, the value in a second register is written to memory. If the write is performed, the read and write occur atomically such that no other modification of the memory location can take place between the read and write.

  • CASAB and CASALB load from memory with acquire semantics.
  • CASLB and CASALB store to memory with release semantics.
  • CASB 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.

The architecture permits that the data read clears any exclusive monitors associated with that location, even if the compare subsequently fails.

If the instruction generates a synchronous Data Abort, the register which is compared and loaded, that is <Ws>, is restored to the values held in the register before the instruction was executed.

No offset


CASAB (L == 1 && o0 == 0)

CASAB <Ws>, <Wt>, [<Xn|SP>{,#0}]

CASALB (L == 1 && o0 == 1)

CASALB <Ws>, <Wt>, [<Xn|SP>{,#0}]

CASB (L == 0 && o0 == 0)

CASB <Ws>, <Wt>, [<Xn|SP>{,#0}]

CASLB (L == 0 && o0 == 1)

CASLB <Ws>, <Wt>, [<Xn|SP>{,#0}]

if !HaveAtomicExt() then UNDEFINED;

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

AccType ldacctype = if L == '1' then AccType_ORDEREDATOMICRW else AccType_ATOMICRW;
AccType stacctype = if o0 == '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 compared and loaded, encoded in the "Rs" field.


Is the 32-bit name of the general-purpose register to be conditionally stored, 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) comparevalue;
bits(8) newvalue;
bits(8) data;

if HaveMTEExt() then

comparevalue = X[s];
newvalue = X[t];

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

data = MemAtomicCompareAndSwap(address, comparevalue, newvalue, ldacctype, stacctype);

X[s] = ZeroExtend(data, 32);
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