ldsmax

Atomic Load-Signed Maximum Word

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

Atomic signed max to memory.

Details

Atomic Load-Signed Maximum Word atomically loads a 32-bit signed value from memory, computes the signed maximum of the loaded value and Ws, stores the result back, and returns the original loaded value in Wt. This is an AArch64-only instruction requiring LSE extension support. No condition flags are affected; this instruction provides release semantics for memory ordering.

Pseudocode Operation

address ← Xn
original_value ← [address]
new_value ← SignedMax(original_value, Ws)
[address] ← new_value
Wt ← original_value
Memory ordering: Release semantics applied

Example

LDSMAX w6, w3, [x1]

Encoding

Binary Layout
10
111
0
00
0
0
1
Rs
0
100
00
Rn
Rt
 
Format Atomic
Opcode 0xB8204000
Extension LSE (Atomics)

Operands

  • Ws
    Value
  • Wt
    Transfer 32-bit integer register (load/store)
  • Xn
    First source / base 64-bit integer register

Reference (Arm A64 ISA)

Instruction Forms

Encoding Instruction ISA Bit pattern
0xB8204000 LDSMAX <Ws>, <Wt>, [<Xn|SP>] A64 10 | 111 | 0 | 00 | 0 | 0 | 1 | Rs | 0 | 100 | 00 | Rn | Rt
0xF8204000 LDSMAX <Xs>, <Xt>, [<Xn|SP>] A64 11 | 111 | 0 | 00 | 0 | 0 | 1 | Rs | 0 | 100 | 00 | Rn | Rt

Description

Atomic signed 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 signed numbers. The value initially loaded from memory is returned in the destination register. For more information about memory ordering semantics, see Load-Acquire, Store-Release. For information about memory accesses, see Load/Store addressing modes.

Operation

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

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

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

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

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