LDUMINB, LDUMINAB, LDUMINALB, LDUMINLB

Atomic unsigned minimum on byte in memory

This instruction atomically loads an 8-bit byte from memory, compares it against the value held in a register, and stores the smaller 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, LDUMINAB and LDUMINALB load from memory with acquire semantics.
  • LDUMINLB and LDUMINALB store to memory with release semantics.
  • LDUMINB 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.

    Encoding: Integer

    Variants: FEAT_LSE (ARMv8.1)

    313029282726252423222120191817161514131211109876543210
    001110001011100
    sizeVRARRso3opcRnRt

    No memory ordering (A == 0 && R == 0)

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

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

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

    Acquire-release (A == 1 && R == 1)

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

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

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

    Decoding algorithm

    if !IsFeatureImplemented(FEAT_LSE) then EndOfDecode(Decode_UNDEF);
constant integer s = UInt(Rs);
constant integer t = UInt(Rt);
constant integer n = UInt(Rn);

constant boolean acquire = A == '1' && Rt != '11111';
constant boolean release = R == '1';
constant boolean tagchecked = n != 31;

    Operation

    bits(64) address;

constant boolean privileged = PSTATE.EL != EL0;
constant AccessDescriptor accdesc = CreateAccDescAtomicOp(MemAtomicOp_UMIN, acquire, release,
                                                          tagchecked, privileged);

if n == 31 then
    CheckSPAlignment();
    address = SP[64];
else
    address = X[n, 64];

constant bits(8) comparevalue = bits(8) UNKNOWN; // Irrelevant when not executing CAS
constant bits(8) value = X[s, 8];
constant bits(8) data = MemAtomic(address, comparevalue, value, accdesc);

if t != 31 then
    X[t, 32] = ZeroExtend(data, 32);

    Explanations

    <Ws>: 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.
    <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.

    Operational Notes

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