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Atomic unsigned maximum on byte in memory atomically loads an 8-bit byte from memory, compares it against the value held in a register, and stores the larger value back to memory, treating the values as unsigned numbers. The value initially loaded from memory is returned in the destination register.

  • If the destination register is not WZR, LDUMAXAB and LDUMAXALB load from memory with acquire semantics.
  • LDUMAXLB and LDUMAXALB store to memory with release semantics.
  • LDUMAXB has neither acquire nor release semantics.

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

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

This instruction is used by the alias STUMAXB, STUMAXLB.



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

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

LDUMAXALB (A == 1 && R == 1)

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

LDUMAXB (A == 0 && R == 0)

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

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

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

if !HaveAtomicExt() then UNDEFINED;

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

integer datasize = 8 << UInt(size);
integer regsize = if datasize == 64 then 64 else 32;
AccType ldacctype = if A == '1' && Rt != '11111' then AccType_ORDEREDATOMICRW else AccType_ATOMICRW;
AccType stacctype = if R == '1' then AccType_ORDEREDATOMICRW else AccType_ATOMICRW;
MemAtomicOp op;
case opc of
    when '000' op = MemAtomicOp_ADD;
    when '001' op = MemAtomicOp_BIC;
    when '010' op = MemAtomicOp_EOR;
    when '011' op = MemAtomicOp_ORR;
    when '100' op = MemAtomicOp_SMAX;
    when '101' op = MemAtomicOp_SMIN;
    when '110' op = MemAtomicOp_UMAX;
    when '111' op = MemAtomicOp_UMIN;
boolean tag_checked = n != 31;

Assembler Symbols


Is the 32-bit name of the general-purpose register holding the data value to be operated on with the contents of the memory location, 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.

Alias Conditions

AliasIs preferred when
STUMAXB, STUMAXLBA == '0' && Rt == '11111'


bits(64) address;
bits(datasize) value;
bits(datasize) data;

if HaveMTEExt() then

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

data = MemAtomic(address, op, value, ldacctype, stacctype);

if t != 31 then
    X[t] = ZeroExtend(data, regsize);

Operational information

If PSTATE.DIT is 1, the timing of this instruction is insensitive to the value of the data being loaded or stored.