# `vavgsh` — Vector Average Signed Half Word

> **Category:** [VMX (Altivec)](../categories/vmx.md) · **Form:** [VX](../forms/VX.md) · **Opcode:** `0x10000542`

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## Assembler Mnemonics

| Mnemonic | XML entry | Flags | Description |
| --- | --- | --- | --- |
| `vavgsh` | `vavgsh` | — | Vector Average Signed Half Word |

## Syntax

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

## Encoding

### `vavgsh` — form `VX`

- **Opcode word:** `0x10000542`
- **Primary opcode (bits 0–5):** `4`
- **Extended opcode:** `1346`
- **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` | vavgsh: read | Source A vector register. |
| `VB` | vavgsh: read | Source B vector register. |
| `VD` | vavgsh: write | Destination vector register. |

## Register Effects

### `vavgsh`

- **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

**`vavgsh`**
- xenia-canary XML: [`tools/ppc-instructions.xml` — search for `mnem="vavgsh"`](../../xenia-canary/tools/ppc-instructions.xml)
- xenia-canary emit: [`src/xenia/cpu/ppc/ppc_emit_altivec.cc:450`](../../xenia-canary/src/xenia/cpu/ppc/ppc_emit_altivec.cc#L450)
- 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:535`](../../xenia-rs/crates/xenia-cpu/src/decoder.rs#L535)
- xenia-rs interpreter: [`crates/xenia-cpu/src/interpreter.rs:3426-3433`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L3426-L3433)
<details><summary>xenia-rs interpreter body (frozen snapshot)</summary>

```rust
        PpcOpcode::vavgsh => {
            let a = crate::vmx::as_i16x8(ctx.vr[instr.ra()]);
            let b = crate::vmx::as_i16x8(ctx.vr[instr.rb()]);
            let mut r = [0i16; 8];
            for i in 0..8 { r[i] = crate::vmx::avg_i16(a[i], b[i]); }
            ctx.vr[instr.rd()] = crate::vmx::from_i16x8(r);
            ctx.pc += 4;
        }
```
</details>

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## Special Cases & Edge Conditions

- **Eight signed-half rounding averages.** Each `VD[i] = (VA[i] + VB[i] + 1) >> 1`, computed in 32-bit arithmetic to avoid overflow on the intermediate sum, then truncated back to `int16`. Rounding is half-up toward +∞.
- **Big-endian half lanes.** Lane 0 (`VD[0..1]` after `stvx`) is the most-significant half.
- **No `VSCR[SAT]` impact.** The result is always representable in `int16`. Xenia's `crate::vmx::avg_i16` ([`crates/xenia-cpu/src/vmx.rs`](../../xenia-rs/crates/xenia-cpu/src/vmx.rs)) widens to `i32` before adding.
- **No XER side effects.**
- **Common usage.** Audio sample interpolation, fixed-point Q15 midpoint filters, video upscaling at 16-bit precision.
- **Aliasing legal.** `vavgsh v3, v3, v4` collapses two half-precision streams into one.
- **No VMX128 sibling.**

## Related Instructions

- [`vavguh`](vavguh.md) — same width, unsigned rounding average.
- [`vavgsb`](vavgsb.md), [`vavgsw`](vavgsw.md) — signed rounding average at byte / word width.
- [`vadduhm`](vadduhm.md), [`vaddshs`](vaddshs.md) — addition variants without the divide step.
- [`vsubuhm`](vsubuhm.md) — modulo subtract; difference computation before averaging.

## IBM Reference

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