xenia-rs/migration/project-root/ppc-manual/memory/stvehx.md

# `stvehx` — Store Vector Element Half Word Indexed

> **Category:** [Memory](../categories/memory.md) · **Form:** [X](../forms/X.md) · **Opcode:** `0x7c00014e`

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## Assembler Mnemonics

| Mnemonic | XML entry | Flags | Description |
| --- | --- | --- | --- |
| `stvehx` | `stvehx` | — | Store Vector Element Half Word Indexed |

## Syntax

```asm
stvehx [VS], [RA0], [RB]
```

## Encoding

### `stvehx` — form `X`

- **Opcode word:** `0x7c00014e`
- **Primary opcode (bits 0–5):** `31`
- **Extended opcode:** `167`
- **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 |
| --- | --- | --- |
| `VS` | stvehx: read | Source vector register (alias for VD on stores). |
| `RA0` | stvehx: read | Source GPR; when the encoded register number is 0 the operand is the literal 64-bit zero, **not** `r0`. |
| `RB` | stvehx: read | Source GPR. |

## Register Effects

### `stvehx`

- **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

**`stvehx`**
- xenia-canary XML: [`tools/ppc-instructions.xml` — search for `mnem="stvehx"`](../../xenia-canary/tools/ppc-instructions.xml)
- xenia-canary emit: [`src/xenia/cpu/ppc/ppc_emit_altivec.cc:160`](../../xenia-canary/src/xenia/cpu/ppc/ppc_emit_altivec.cc#L160)
- 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:784`](../../xenia-rs/crates/xenia-cpu/src/decoder.rs#L784)
- xenia-rs interpreter: [`crates/xenia-cpu/src/interpreter.rs:1927-1941`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L1927-L1941)
<details><summary>xenia-rs interpreter body (frozen snapshot)</summary>

```rust
        PpcOpcode::stvehx => {
            // Store vS[slot] (1 halfword) at EA & ~1. slot = (EA & 0xF) >> 1.
            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 & !0x1u32;
            // PPCBUG-512: stvehx 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) >> 1) as usize;
            let bytes = ctx.vr[instr.rs()].as_bytes();
            let h = ((bytes[slot * 2] as u16) << 8) | (bytes[slot * 2 + 1] as u16);
            mem.write_u16(ea, h);
            ctx.pc += 4;
        }
```
</details>

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## Special Cases & Edge Conditions

- **Single half-word element store.** Architecturally `stvehx` writes exactly **two** bytes from half-word lane `(EA mod 16) >> 1` of `VS` to address `EA & ~1` (low bit forced to half-aligned). Other lanes are unaffected, and bytes outside the 2-byte window are unaffected.
- **Xenia simplification — full 16-byte write.** The xenia snapshot is shared with `stvebx` / `stvewx`: writes 16 bytes of the source vector at `ea & ~0xF`. This is stronger than the architectural 2-byte store — it overwrites 14 adjacent bytes that hardware would have left alone.
- **EA forced half-aligned.** Hardware drops the low bit; xenia's shared snapshot drops the low four bits.
- **`RA0` semantics.** `RA = 0` selects literal zero.
- **No update form, no VMX128 sibling.** No `stvehux`; no `stvehx128`.
- **Big-endian half within the lane.** The byte at the lower address is the most-significant byte of the half-word lane.
- **Common idiom.** Pair with `vsplth` to broadcast then store one half; rare in compiled code (compilers prefer `sth`).

## Related Instructions

- [`stvebx`](stvebx.md), [`stvewx`](stvewx.md) — single byte / word element stores.
- [`stvx`](stvx.md), [`stvxl`](stvxl.md) — full 16-byte aligned vector stores.
- [`stvlx`](stvlx.md), [`stvrx`](stvrx.md) — store-left / store-right unaligned ops.
- [`lvehx`](lvehx.md) — symmetric single-half load.

## IBM Reference

- [AIX 7.3 — `stvehx` (Store Vector Element Half Word Indexed)](https://www.ibm.com/docs/en/aix/7.3.0?topic=set-stvehx-store-vector-element-half-word-indexed-instruction)
- `PowerISA v2.07B Book I` "Vector Facility" § "Vector Load and Store".