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LDUMAX, LDUMAXA, LDUMAXAL, LDUMAXL

Atomic unsigned maximum on word or doubleword in memory atomically loads a 32-bit word or 64-bit doubleword 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 one of WZR or XZR, LDUMAXA and LDUMAXAL load from memory with acquire semantics.
  • LDUMAXL and LDUMAXAL store to memory with release semantics.
  • LDUMAX 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.

This instruction is used by the alias STUMAX, STUMAXL.

Integer
(Armv8.1)

313029282726252423222120191817161514131211109876543210
1x111000AR1Rs011000RnRt
sizeopc

32-bit LDUMAX (size == 10 && A == 0 && R == 0)

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

32-bit LDUMAXA (size == 10 && A == 1 && R == 0)

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

32-bit LDUMAXAL (size == 10 && A == 1 && R == 1)

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

32-bit LDUMAXL (size == 10 && A == 0 && R == 1)

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

64-bit LDUMAX (size == 11 && A == 0 && R == 0)

LDUMAX <Xs>, <Xt>, [<Xn|SP>]

64-bit LDUMAXA (size == 11 && A == 1 && R == 0)

LDUMAXA <Xs>, <Xt>, [<Xn|SP>]

64-bit LDUMAXAL (size == 11 && A == 1 && R == 1)

LDUMAXAL <Xs>, <Xt>, [<Xn|SP>]

64-bit LDUMAXL (size == 11 && A == 0 && R == 1)

LDUMAXL <Xs>, <Xt>, [<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;
boolean tag_checked = n != 31;

Assembler Symbols

<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.

<Xs>

Is the 64-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.

<Xt>

Is the 64-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.

Alias Conditions

AliasIs preferred when
STUMAX, STUMAXLA == '0' && Rt == '11111'

Operation

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

if HaveMTEExt() then
    SetNotTagCheckedInstruction(!tag_checked);

value = X[s];
if n == 31 then
    CheckSPAlignment();
    address = SP[];
else
    address = X[n];

data = MemAtomic(address, MemAtomicOp_UMAX, 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.

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