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LDCLRH, LDCLRAH, LDCLRALH, LDCLRLH

Atomic bit clear on halfword in memory atomically loads a 16-bit halfword from memory, performs a bitwise AND with the complement of the value held in a register on it, and stores the result back to memory. The value initially loaded from memory is returned in the destination register.

  • If the destination register is not WZR, LDCLRAH and LDCLRALH load from memory with acquire semantics.
  • LDCLRLH and LDCLRALH store to memory with release semantics.
  • LDCLRH 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 STCLRH, STCLRLH.

Integer
(Armv8.1)

313029282726252423222120191817161514131211109876543210
01111000AR1Rs000100RnRt
sizeopc

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

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

LDCLRALH (A == 1 && R == 1)

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

LDCLRH (A == 0 && R == 0)

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

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

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

if !HaveAtomicExt() then UNDEFINED;
integer t = UInt(Rt);
integer n = UInt(Rn);
integer s = UInt(Rs);

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.

<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
STCLRH, STCLRLHA == '0' && Rt == '11111'

Operation

bits(64) address;
bits(16) value;
bits(16) data;
bits(16) result;

if HaveMTEExt() then
    SetNotTagCheckedInstruction(!tag_checked);

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

// All observers in the shareability domain observe the
// following load and store atomically.
data = Mem[address, 2, ldacctype];

result = data AND NOT(value);
Mem[address, 2, stacctype] = result;

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

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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