# `stvewx` — Store Vector Element Word Indexed
> **Category:** [Memory](../categories/memory.md) · **Form:** [X](../forms/X.md) · **Opcode:** `0x7c00018e`
## Assembler Mnemonics
| Mnemonic | XML entry | Flags | Description |
| --- | --- | --- | --- |
| `stvewx` | `stvewx` | — | Store Vector Element Word Indexed |
| `stvewx128` | `stvewx128` | — | Store Vector Element Word Indexed 128 |
## Syntax
```asm
stvewx [VS], [RA0], [RB]
stvewx128 [VS], [RA0], [RB]
```
## Encoding
### `stvewx` — form `X`
- **Opcode word:** `0x7c00018e`
- **Primary opcode (bits 0–5):** `31`
- **Extended opcode:** `199`
- **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 |
### `stvewx128` — form `VX128_1`
- **Opcode word:** `0x10000183`
- **Primary opcode (bits 0–5):** `4`
- **Extended opcode:** `387`
- **Synchronising:** no
| Bits | Field | Meaning |
| --- | --- | --- |
| 0–5 | `OPCD` | primary opcode (4) |
| 6–10 | `VD128l` | destination low 5 bits |
| 11–15 | `RA` | address register |
| 16–20 | `RB` | offset register |
| 21–27 | `XO` | extended opcode |
| 28–29 | `VD128h` | destination high 2 bits |
| 30–31 | `—` | reserved |
## Operands
| Field | Role | Description |
| --- | --- | --- |
| `VS` | stvewx: read; stvewx128: read | Source vector register (alias for VD on stores). |
| `RA0` | stvewx: read; stvewx128: read | Source GPR; when the encoded register number is 0 the operand is the literal 64-bit zero, **not** `r0`. |
| `RB` | stvewx: read; stvewx128: read | Source GPR. |
## Register Effects
### `stvewx`
- **Reads (always):** `VS`, `RA0`, `RB`
- **Reads (conditional):** _none_
- **Writes (always):** _none_
- **Writes (conditional):** _none_
### `stvewx128`
- **Reads (always):** `VS`, `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
**`stvewx`**
- xenia-canary XML: [`tools/ppc-instructions.xml` — search for `mnem="stvewx"`](../../xenia-canary/tools/ppc-instructions.xml)
- xenia-canary emit: [`src/xenia/cpu/ppc/ppc_emit_altivec.cc:180`](../../xenia-canary/src/xenia/cpu/ppc/ppc_emit_altivec.cc#L180)
- xenia-rs opcode: [`crates/xenia-cpu/src/opcode.rs:77`](../../xenia-rs/crates/xenia-cpu/src/opcode.rs#L77)
- xenia-rs decoder: [`crates/xenia-cpu/src/decoder.rs:788`](../../xenia-rs/crates/xenia-cpu/src/decoder.rs#L788)
- xenia-rs interpreter: [`crates/xenia-cpu/src/interpreter.rs:1942-1959`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L1942-L1959)
xenia-rs interpreter body (frozen snapshot)
```rust
PpcOpcode::stvewx => {
// Store vS[slot] (1 word) at EA & ~3. slot = (EA & 0xF) >> 2.
let base = if instr.ra() == 0 { 0u64 } else { ctx.gpr[instr.ra()] };
let ea_unaligned = base.wrapping_add(ctx.gpr[instr.rb()]) as u32;
let ea = ea_unaligned & !0x3u32;
// PPCBUG-512: stvewx was missing invalidate_for_write.
if let Some(t) = ctx.reservation_table.as_ref().filter(|t| t.is_enabled()) {
if t.has_active_reservers() { t.invalidate_for_write(ea); }
}
let slot = ((ea_unaligned & 0xF) >> 2) as usize;
let bytes = ctx.vr[instr.rs()].as_bytes();
let w = ((bytes[slot * 4] as u32) << 24)
| ((bytes[slot * 4 + 1] as u32) << 16)
| ((bytes[slot * 4 + 2] as u32) << 8)
| (bytes[slot * 4 + 3] as u32);
mem.write_u32(ea, w);
ctx.pc += 4;
}
```
xenia-rs interpreter body (frozen snapshot)
```rust
PpcOpcode::stvewx128 => {
// Mirror of stvewx: word-align EA, extract one 32-bit lane, write 4 bytes only.
// Previous code used & !0xF (16-byte) and wrote all 16 bytes, corrupting 12
// adjacent bytes on every execution (PPCBUG-510).
let ea_unaligned = ea_indexed(ctx, instr);
let ea = ea_unaligned & !0x3u32;
if let Some(t) = ctx.reservation_table.as_ref().filter(|t| t.is_enabled()) {
if t.has_active_reservers() { t.invalidate_for_write(ea); }
}
let slot = ((ea_unaligned & 0xF) >> 2) as usize;
let bytes = ctx.vr[instr.vs128()].as_bytes();
let w = ((bytes[slot * 4] as u32) << 24)
| ((bytes[slot * 4 + 1] as u32) << 16)
| ((bytes[slot * 4 + 2] as u32) << 8)
| (bytes[slot * 4 + 3] as u32);
mem.write_u32(ea, w);
ctx.pc += 4;
}
```