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Atomic unsigned maximum on byte in memory, without return, 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.

  • STUMAXB does not have release semantics.
  • STUMAXLB stores to memory with release semantics, as described in Load-Acquire, Store-Release.

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

This is an alias of LDUMAXB, LDUMAXAB, LDUMAXALB, LDUMAXLB. This means:



No memory ordering (R == 0)

STUMAXB <Ws>, [<Xn|SP>]

is equivalent to

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

and is always the preferred disassembly.

Release (R == 1)

STUMAXLB <Ws>, [<Xn|SP>]

is equivalent to


and is always the preferred disassembly.

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


The description of LDUMAXB, LDUMAXAB, LDUMAXALB, LDUMAXLB gives the operational pseudocode for this instruction.

Operational information

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