# `stwbrx` — Store Word Byte-Reverse Indexed

> **Category:** [Memory](../categories/memory.md) · **Form:** [X](../forms/X.md) · **Opcode:** `0x7c00052c`

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## Assembler Mnemonics

| Mnemonic | XML entry | Flags | Description |
| --- | --- | --- | --- |
| `stwbrx` | `stwbrx` | — | Store Word Byte-Reverse Indexed |

## Syntax

```asm
stwbrx [RS], [RA0], [RB]
```

## Encoding

### `stwbrx` — form `X`

- **Opcode word:** `0x7c00052c`
- **Primary opcode (bits 0–5):** `31`
- **Extended opcode:** `662`
- **Synchronising:** no

| Bits | Field | Meaning |
| --- | --- | --- |
| 0–5 | `OPCD` | primary opcode |
| 6–10 | `RT/FRT/VRT` | destination |
| 11–15 | `RA/FRA/VRA` | source A |
| 16–20 | `RB/FRB/VRB` | source B |
| 21–30 | `XO` | extended opcode (10 bits) |
| 31 | `Rc` | record-form flag |

## Operands

| Field | Role | Description |
| --- | --- | --- |
| `RS` | stwbrx: read | Source GPR (alias for RD in some stores). |
| `RA0` | stwbrx: read | Source GPR; when the encoded register number is 0 the operand is the literal 64-bit zero, **not** `r0`. |
| `RB` | stwbrx: read | Source GPR. |

## Register Effects

### `stwbrx`

- **Reads (always):** `RS`, `RA0`, `RB`
- **Reads (conditional):** _none_
- **Writes (always):** _none_
- **Writes (conditional):** _none_

## Status-Register Effects

_No condition-register or status-register effects._

## Operation (pseudocode)

```
; Pseudocode derives directly from the xenia-rs interpreter
; arm (see Implementation References). Operation semantics:
;   - Read source operands from the fields listed under Operands.
;   - Apply the arithmetic / logical / memory action described
;     in the Description field above.
;   - Write results to the destination register(s); update any
;     status bits enumerated under Status-Register Effects.
; Consult the IBM AIX reference link under IBM Reference for
; canonical PPC-style pseudocode where xenia's expression is
; terse.
```

## C Translation Example

```c
/* C translation: the xenia-rs interpreter arm below in           */
/* Implementation References is the authoritative semantic        */
/* snapshot. Translate it line-by-line:                            */
/*   - ctx.gpr[N]  -> r[N]       (or f[]/v[] for FPRs/VRs)        */
/*   - mem.read_u*/write_u* -> mem_read_u*_be / mem_write_u*_be   */
/*   - ctx.update_cr_signed(fld, v) -> update_cr_signed(fld, v)   */
/*   - ctx.xer_ca / xer_ov / xer_so -> xer.CA / xer.OV / xer.SO   */
/* The Register Effects and Status-Register Effects tables above  */
/* enumerate every side effect a faithful translation must emit.  */
```

## Implementation References

**`stwbrx`**
- xenia-canary XML: [`tools/ppc-instructions.xml` — search for `mnem="stwbrx"`](../../xenia-canary/tools/ppc-instructions.xml)
- xenia-canary emit: [`src/xenia/cpu/ppc/ppc_emit_memory.cc:679`](../../xenia-canary/src/xenia/cpu/ppc/ppc_emit_memory.cc#L679)
- xenia-rs opcode: [`crates/xenia-cpu/src/opcode.rs:81`](../../xenia-rs/crates/xenia-cpu/src/opcode.rs#L81)
- xenia-rs decoder: [`crates/xenia-cpu/src/decoder.rs:831`](../../xenia-rs/crates/xenia-cpu/src/decoder.rs#L831)
- xenia-rs interpreter: [`crates/xenia-cpu/src/interpreter.rs:1813-1821`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L1813-L1821)
<details><summary>xenia-rs interpreter body (frozen snapshot)</summary>

```rust
        PpcOpcode::stwbrx => {
            let ea = if instr.ra() == 0 { 0u64 } else { ctx.gpr[instr.ra()] };
            let ea = ea.wrapping_add(ctx.gpr[instr.rb()]) as u32;
            if let Some(t) = ctx.reservation_table.as_ref().filter(|t| t.is_enabled()) {
                if t.has_active_reservers() { t.invalidate_for_write(ea); }
            }
            mem.write_u32(ea, (ctx.gpr[instr.rs()] as u32).swap_bytes());
            ctx.pc += 4;
        }
```
</details>

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## Special Cases & Edge Conditions

- **Writes little-endian word.** Takes the low 32 bits of `RS`, reverses the four bytes, writes them at `EA`. Byte at `EA` is `RS[56:63]` (low byte); byte at `EA+3` is `RS[32:39]` (high byte). The xenia snapshot does `(ctx.gpr[instr.rs()] as u32).swap_bytes()`.
- **Used to emit little-endian payloads.** Symmetric counterpart of [`lwbrx`](lwbrx.md). Common when writing PC-side file formats, network packets, GPU command buffers in little-endian layout, etc.
- **High bits of `RS` ignored.** Stores only the low 32 bits; the upper half of the 64-bit GPR is not consulted.
- **X-form only — no D-form, no update form.** Only the indexed form exists. `EA = (RA|0) + RB`.
- **`RA0` semantics.** When `RA = 0`, base is literal zero; `stwbrx RS, 0, RB` writes at exact `RB`.
- **Alignment.** Hardware tolerates unaligned 4-byte writes; cache-inhibited storage may raise alignment exceptions on real hardware.
- **No CR / FPSCR effects.**

## Related Instructions

- [`lwbrx`](lwbrx.md) — load-word byte-reverse (matching load).
- [`sthbrx`](sthbrx.md), [`stdbrx`](stdbrx.md) — narrower / wider byte-reverse stores.
- [`stw`](stw.md), [`stwx`](stw.md) — non-reversing word stores.

## IBM Reference

- [AIX 7.3 — `stwbrx` (Store Word Byte-Reverse Indexed)](https://www.ibm.com/docs/en/aix/7.3.0?topic=set-stwbrx-store-word-byte-reverse-indexed-instruction)
- `PowerISA v2.07B Book II` § "Byte-Reverse Storage Access".