# `stswx` — Store String Word Indexed

> **Category:** [Memory](../categories/memory.md) · **Form:** [X](../forms/X.md) · **Opcode:** `0x7c00052a`

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## Assembler Mnemonics

| Mnemonic | XML entry | Flags | Description |
| --- | --- | --- | --- |
| `stswx` | `stswx` | — | Store String Word Indexed |

## Syntax

```asm
(no disassembly template)
```

## Encoding

### `stswx` — form `X`

- **Opcode word:** `0x7c00052a`
- **Primary opcode (bits 0–5):** `31`
- **Extended opcode:** `661`
- **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 |
| --- | --- | --- |

## Register Effects

### `stswx`

- **Reads (always):** _none_
- **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

**`stswx`**
- xenia-canary XML: [`tools/ppc-instructions.xml` — search for `mnem="stswx"`](../../xenia-canary/tools/ppc-instructions.xml)
- xenia-canary emit: [`src/xenia/cpu/ppc/ppc_emit_memory.cc:742`](../../xenia-canary/src/xenia/cpu/ppc/ppc_emit_memory.cc#L742)
- xenia-rs opcode: [`crates/xenia-cpu/src/opcode.rs:75`](../../xenia-rs/crates/xenia-cpu/src/opcode.rs#L75)
- xenia-rs decoder: [`crates/xenia-cpu/src/decoder.rs:830`](../../xenia-rs/crates/xenia-cpu/src/decoder.rs#L830)
- xenia-rs interpreter: [`crates/xenia-cpu/src/interpreter.rs:4663-4689`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L4663-L4689)
<details><summary>xenia-rs interpreter body (frozen snapshot)</summary>

```rust
        PpcOpcode::stswx => {
            let mut ea = ea_indexed(ctx, instr);
            let nb = ctx.xer() & 0x7F;
            let mut rs = instr.rs();
            let mut bytes_left = nb;
            if nb > 0 {
                if let Some(t) = ctx.reservation_table.as_ref().filter(|t| t.is_enabled()) {
                    if t.has_active_reservers() {
                        let first_line = ea & !RESERVATION_MASK;
                        let last_line = ea.wrapping_add(nb - 1) & !RESERVATION_MASK;
                        t.invalidate_for_write(first_line);
                        if last_line != first_line { t.invalidate_for_write(last_line); }
                    }
                }
            }
            while bytes_left > 0 {
                let val = ctx.gpr[rs] as u32;
                for byte_idx in 0..4 {
                    if bytes_left == 0 { break; }
                    mem.write_u8(ea, (val >> (24 - byte_idx * 8)) as u8);
                    ea = ea.wrapping_add(1);
                    bytes_left -= 1;
                }
                rs = (rs + 1) % 32;
            }
            ctx.pc += 4;
        }
```
</details>

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## Special Cases & Edge Conditions

- **Byte count from `XER[25..31]`.** Unlike `stswi`, the byte count `NB` (0..127) is read from `XER[25..31]`. The xenia snapshot does `let nb = (ctx.xer() & 0x7F) as u32;`. `NB = 0` means literally zero bytes — the instruction becomes a no-op.
- **Register packing identical to `stswi`.** Bytes are pulled from successive GPRs, four bytes per register, big-endian within each register, with wraparound `r31 → r0`. The final partial register's unused trailing bytes are not written.
- **`RA0` semantics.** `RA = 0` selects literal zero. The instruction has no update form — `RA` is not modified.
- **Invalid forms.** AIX flags as invalid the cases where the byte-stream wraps through `RA` or `RB` while reading the source registers; xenia performs writes regardless.
- **Big-endian byte ordering inside each register.** Writes most-significant byte of each source GPR's low word first.
- **Used for non-multiple-of-4 copies.** Together with `lswx`, gives a way to store a runtime-determined byte count without per-byte loops. Compilers don't emit it.
- **Alignment.** Architecture allows arbitrary alignment; cache-inhibited storage may raise alignment exceptions on hardware.
- **No CR / FPSCR effects.**

## Related Instructions

- [`lswx`](lswx.md) — symmetric load.
- [`stswi`](stswi.md) — sibling with byte count encoded as `RB` field (immediate-style).
- [`stmw`](stmw.md) — word-granular bulk store (no byte tail handling).
- [`stw`](stw.md), [`stb`](stb.md) — scalar stores.

## IBM Reference

- [AIX 7.3 — `stswx` (Store String Word Indexed)](https://www.ibm.com/docs/en/aix/7.3.0?topic=set-stswx-store-string-word-indexed-instruction)
- `PowerISA v2.07B Book II` § "Load and Store String" for invalid-form rules and `XER` interaction.