LDSETH, LDSETAH, LDSETALH, LDSETLH

Atomic bit set on halfword in memory atomically loads a 16-bit halfword from memory, performs a bitwise OR with 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, LDSETAH and LDSETALH load from memory with acquire semantics.
LDSETLH and LDSETALH store to memory with release semantics.
LDSETH 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 STSETH, STSETLH.

Integer
(FEAT_LSE)

31	30	29	28	27	26	25	24	23	22	21	20	19	18	17	16	15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0
0	1	1	1	1	0	0	0	A	R	1	Rs					0	0	1	1	0	0	Rn					Rt
size																	opc

if !IsFeatureImplemented(FEAT_LSE) then UNDEFINED;

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

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

Alias	Is preferred when
STSETH, STSETLH	A == '0' && Rt == '11111'

Operation

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

AccessDescriptor accdesc = CreateAccDescAtomicOp(MemAtomicOp_ORR, acquire, release, tagchecked);

value = X[s, 16];
if n == 31 then
    CheckSPAlignment();
    address = SP[];
else
    address = X[n, 64];

bits(16) comparevalue = bits(16) UNKNOWN;    // Irrelevant when not executing CAS
data = MemAtomic(address, comparevalue, value, accdesc);

if t != 31 then
    X[t, 32] = 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.