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

# `lvehx` — Load Vector Element Half Word Indexed

> **Category:** [Memory](../categories/memory.md) · **Form:** [X](../forms/X.md) · **Opcode:** `0x7c00004e`

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## Assembler Mnemonics

| Mnemonic | XML entry | Flags | Description |
| --- | --- | --- | --- |
| `lvehx` | `lvehx` | — | Load Vector Element Half Word Indexed |

## Syntax

```asm
lvehx [VD], [RA0], [RB]
```

## Encoding

### `lvehx` — form `X`

- **Opcode word:** `0x7c00004e`
- **Primary opcode (bits 0–5):** `31`
- **Extended opcode:** `39`
- **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 |
| --- | --- | --- |
| `RA0` | lvehx: read | Source GPR; when the encoded register number is 0 the operand is the literal 64-bit zero, **not** `r0`. |
| `RB` | lvehx: read | Source GPR. |
| `VD` | lvehx: write | Destination vector register. |

## Register Effects

### `lvehx`

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

**`lvehx`**
- xenia-canary XML: [`tools/ppc-instructions.xml` — search for `mnem="lvehx"`](../../xenia-canary/tools/ppc-instructions.xml)
- xenia-canary emit: [`src/xenia/cpu/ppc/ppc_emit_altivec.cc:81`](../../xenia-canary/src/xenia/cpu/ppc/ppc_emit_altivec.cc#L81)
- xenia-rs opcode: [`crates/xenia-cpu/src/opcode.rs:44`](../../xenia-rs/crates/xenia-cpu/src/opcode.rs#L44)
- xenia-rs decoder: [`crates/xenia-cpu/src/decoder.rs:763`](../../xenia-rs/crates/xenia-cpu/src/decoder.rs#L763)
- xenia-rs interpreter: [`crates/xenia-cpu/src/interpreter.rs:1884-1897`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L1884-L1897)
<details><summary>xenia-rs interpreter body (frozen snapshot)</summary>

```rust
        PpcOpcode::lvehx => {
            // Load a halfword from (EA & ~1) into vD at halfword slot
            // (EA & 0xF) >> 1. Other halfword lanes preserved (see lvebx).
            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;
            let slot = ((ea_unaligned & 0xF) >> 1) as usize;
            let mut bytes = ctx.vr[instr.rd()].as_bytes();
            let h = mem.read_u16(ea);
            bytes[slot * 2] = (h >> 8) as u8;
            bytes[slot * 2 + 1] = (h & 0xFF) as u8;
            ctx.vr[instr.rd()] = xenia_types::Vec128::from_bytes(bytes);
            ctx.pc += 4;
        }
```
</details>

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## Special Cases & Edge Conditions

- **Single half-word element load.** Architecturally `lvehx` loads exactly **two** bytes from `EA` (which must be 2-byte aligned) and places them in the half-word lane `(EA mod 16) >> 1` of the destination vector; the other 7 half-word lanes are *undefined*.
- **EA must be half-aligned.** The low bit of `EA` is masked by hardware to align to 2 — an odd `EA` rounds down. Xenia's shared snapshot rounds further, masking to 16-byte alignment.
- **Xenia simplification — full-line read.** The xenia snapshot is shared with `lvebx` / `lvewx`: `ea & ~0xF` then a full 16-byte read into `VD`. Architectural undefined lanes are filled in deterministically, which is stronger than hardware guarantees but practically convenient.
- **`RA0` semantics.** When `RA = 0`, base is literal zero; `lvehx VD, 0, RB` reads at `RB` (and, in xenia, the surrounding aligned line).
- **No update form.** No `lvehux` exists.
- **No VMX128 sibling.** No `lvehx128` — Xbox 360 code prefers `lvx128` plus `vperm`.
- **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 or with `vperm` to assemble a vector from sparse memory.

## Related Instructions

- [`lvebx`](lvebx.md), [`lvewx`](lvewx.md) — byte and word element loads.
- [`lvx`](lvx.md), [`lvxl`](lvxl.md) — full 16-byte aligned vector loads.
- [`lvlx`](lvlx.md), [`lvrx`](lvrx.md) — load-left / load-right partial-vector ops.
- [`stvehx`](stvehx.md) — symmetric single-half store.

## IBM Reference

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