# `vavguh` — Vector Average Unsigned Half Word

> **Category:** [VMX (Altivec)](../categories/vmx.md) · **Form:** [VX](../forms/VX.md) · **Opcode:** `0x10000442`

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## Assembler Mnemonics

| Mnemonic | XML entry | Flags | Description |
| --- | --- | --- | --- |
| `vavguh` | `vavguh` | — | Vector Average Unsigned Half Word |

## Syntax

```asm
vavguh [VD], [VA], [VB]
```

## Encoding

### `vavguh` — form `VX`

- **Opcode word:** `0x10000442`
- **Primary opcode (bits 0–5):** `4`
- **Extended opcode:** `1090`
- **Synchronising:** no

| Bits | Field | Meaning |
| --- | --- | --- |
| 0–5 | `OPCD` | primary opcode (4) |
| 6–10 | `VRT/VD` | destination vector register |
| 11–15 | `VRA/VA` | source A vector register |
| 16–20 | `VRB/VB` | source B vector register |
| 21–31 | `XO` | extended opcode (11 bits) |

## Operands

| Field | Role | Description |
| --- | --- | --- |
| `VA` | vavguh: read | Source A vector register. |
| `VB` | vavguh: read | Source B vector register. |
| `VD` | vavguh: write | Destination vector register. |

## Register Effects

### `vavguh`

- **Reads (always):** `VA`, `VB`
- **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

**`vavguh`**
- xenia-canary XML: [`tools/ppc-instructions.xml` — search for `mnem="vavguh"`](../../xenia-canary/tools/ppc-instructions.xml)
- xenia-canary emit: [`src/xenia/cpu/ppc/ppc_emit_altivec.cc:475`](../../xenia-canary/src/xenia/cpu/ppc/ppc_emit_altivec.cc#L475)
- xenia-rs opcode: [`crates/xenia-cpu/src/opcode.rs:92`](../../xenia-rs/crates/xenia-cpu/src/opcode.rs#L92)
- xenia-rs decoder: [`crates/xenia-cpu/src/decoder.rs:525`](../../xenia-rs/crates/xenia-cpu/src/decoder.rs#L525)
- xenia-rs interpreter: [`crates/xenia-cpu/src/interpreter.rs:3418-3425`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L3418-L3425)
<details><summary>xenia-rs interpreter body (frozen snapshot)</summary>

```rust
        PpcOpcode::vavguh => {
            let a = ctx.vr[instr.ra()].as_u16x8();
            let b = ctx.vr[instr.rb()].as_u16x8();
            let mut r = [0u16; 8];
            for i in 0..8 { r[i] = crate::vmx::avg_u16(a[i], b[i]); }
            ctx.vr[instr.rd()] = xenia_types::Vec128::from_u16x8_array(r);
            ctx.pc += 4;
        }
```
</details>

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## Special Cases & Edge Conditions

- **Eight unsigned-half rounding averages.** Each `VD[i] = (VA[i] + VB[i] + 1) >> 1`, computed in 32-bit arithmetic to avoid the intermediate `+1` overflowing, then truncated to `u16`. Rounding is half-up.
- **Big-endian half lanes.** Lane 0 (`VD[0..1]` after `stvx`) is the most-significant half.
- **No `VSCR[SAT]` impact.** The result always fits in `u16`. Xenia uses `crate::vmx::avg_u16` ([`crates/xenia-cpu/src/vmx.rs`](../../xenia-rs/crates/xenia-cpu/src/vmx.rs)).
- **Equivalent to `_mm_avg_epu16`** on x86 SSE2 — same rounding, same width.
- **Common usage.** Higher-precision pixel blending (e.g. RGB565 sums after widening), Q16 unsigned filters.
- **Aliasing legal.**
- **No VMX128 sibling.**

## Related Instructions

- [`vavgsh`](vavgsh.md) — same width, signed rounding average.
- [`vavgub`](vavgub.md), [`vavguw`](vavguw.md) — unsigned rounding average at byte / word width.
- [`vadduhm`](vadduhm.md), [`vadduhs`](vadduhs.md) — addition variants without the divide step.
- [`vsubuhm`](vsubuhm.md) — modulo subtract; difference before averaging.

## IBM Reference

- [AIX 7.3 — `vavguh` (Vector Average Unsigned Half Word)](https://www.ibm.com/docs/en/aix/7.3.0?topic=set-vavguh-vector-average-unsigned-half-word-instruction)
- [IBM AltiVec Technology Programmer's Interface Manual, Chapter 6 — Average Instructions](https://www.nxp.com/docs/en/reference-manual/ALTIVECPIM.pdf)