stg

Store Allocation Tag

STG <Xt|SP>, [<Xn|SP>, #<simm>]

Stores the Allocation Tag to memory.

Details

Stores the Allocation Tag from Xt to a single 16-byte memory granule at [Xn + offset]. The offset is scaled by 16 (imm9 is left-shifted by 4). No condition flags are affected. AArch64-only; requires MTE extension; generates an exception if MTE is not enabled or tag check fails.

Pseudocode Operation

address ← Xn + (SignExtend(imm9, 9) << 4)
tag ← GetAllocationTag(Xt)
memory[address] ← memory[address] with tag set to tag

Example

STG Xt, [x1, #-8]

Encoding

Binary Layout
11011001
00
1
imm9
10
Xn
Xt
 
Format Load/Store
Opcode 0xD9200800
Extension MTE (Memory Tagging)

Operands

  • Xt
    Source Tag
  • Xn
    First source / base 64-bit integer register
  • simm
    Signed immediate offset

Reference (Arm A64 ISA)

Instruction Forms

Encoding Instruction ISA Bit pattern
0xD9200400 STG <Xt|SP>, [<Xn|SP>], #<simm> A64 11011001 | 00 | 1 | imm9 | 01 | Xn | Xt
0xD9200C00 STG <Xt|SP>, [<Xn|SP>, #<simm>]! A64 11011001 | 00 | 1 | imm9 | 11 | Xn | Xt
0xD9200800 STG <Xt|SP>, [<Xn|SP>{, #<simm>}] A64 11011001 | 00 | 1 | imm9 | 10 | Xn | Xt

Description

Store Allocation Tag stores an Allocation Tag to memory. The address used for the store is calculated from the base register and an immediate signed offset scaled by the Tag granule. The Allocation Tag is calculated from the Logical Address Tag in the source register. This instruction generates an Unchecked access.

Operation

bits(64) address;

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

AccessDescriptor accdesc = CreateAccDescLDGSTG(MemOp_STORE, FALSE);

if !postindex then
    address = GenerateAddress(address, offset, accdesc);

bits(64) data = if t == 31 then SP[] else X[t, 64];
bits(4) tag = AArch64.AllocationTagFromAddress(data);
AArch64.MemTag[address, accdesc] = tag;

if writeback then
    if postindex then
        address = GenerateAddress(address, offset, accdesc);

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