stg

Store Allocation Tag

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

Stores the Allocation Tag to memory.

Details

Stores the Allocation Tag from Xt (bits [59:56]) into the tag storage of a 16-byte granule at address [Xn + simm]. This instruction is only available in AArch64 and requires the MTE feature. The simm9 immediate is scaled by 16. Data in the granule is not modified. No condition flags are affected.

Pseudocode Operation

address ← Xn + (simm << 4)  // simm is a signed 9-bit value, scaled by 16
tag_to_store ← (Xt >> 56) & 0xF
store_tag_to_memory(address, tag_to_store)

Example

STG Xt, [x1, #-8]

Encoding

Binary Layout
11011001
00
1
imm9
01
Xn
Xt
 
Format Load/Store
Opcode 0xD9200400
Extension MTE (Memory Tagging)

Operands

  • Xt
    Tag Src
  • 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;