# `vavgsb` — Vector Average Signed Byte

> **Category:** [VMX (Altivec)](../categories/vmx.md) · **Form:** [VX](../forms/VX.md) · **Opcode:** `0x10000502`

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## Assembler Mnemonics

| Mnemonic | XML entry | Flags | Description |
| --- | --- | --- | --- |
| `vavgsb` | `vavgsb` | — | Vector Average Signed Byte |

## Syntax

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

## Encoding

### `vavgsb` — form `VX`

- **Opcode word:** `0x10000502`
- **Primary opcode (bits 0–5):** `4`
- **Extended opcode:** `1282`
- **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` | vavgsb: read | Source A vector register. |
| `VB` | vavgsb: read | Source B vector register. |
| `VD` | vavgsb: write | Destination vector register. |

## Register Effects

### `vavgsb`

- **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

**`vavgsb`**
- xenia-canary XML: [`tools/ppc-instructions.xml` — search for `mnem="vavgsb"`](../../xenia-canary/tools/ppc-instructions.xml)
- xenia-canary emit: [`src/xenia/cpu/ppc/ppc_emit_altivec.cc:443`](../../xenia-canary/src/xenia/cpu/ppc/ppc_emit_altivec.cc#L443)
- 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:533`](../../xenia-rs/crates/xenia-cpu/src/decoder.rs#L533)
- xenia-rs interpreter: [`crates/xenia-cpu/src/interpreter.rs:3410-3417`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L3410-L3417)
<details><summary>xenia-rs interpreter body (frozen snapshot)</summary>

```rust
        PpcOpcode::vavgsb => {
            let a = crate::vmx::as_i8x16(ctx.vr[instr.ra()]);
            let b = crate::vmx::as_i8x16(ctx.vr[instr.rb()]);
            let mut r = [0i8; 16];
            for i in 0..16 { r[i] = crate::vmx::avg_i8(a[i], b[i]); }
            ctx.vr[instr.rd()] = crate::vmx::from_i8x16(r);
            ctx.pc += 4;
        }
```
</details>

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## Special Cases & Edge Conditions

- **Sixteen signed-byte rounding averages.** Each `VD[i] = (VA[i] + VB[i] + 1) >> 1`, performed in arithmetic *wider* than 8 bits (so the `+1` cannot overflow). The result is then truncated back to `int8` — saturation never triggers because the average of two `int8` values fits in `int8`. Rounding is "round half up toward +∞".
- **Big-endian byte lanes.** Lane 0 is the most-significant byte after `stvx`.
- **No `VSCR[SAT]` impact.** Mathematical impossibility — `(a + b + 1) / 2` for `a, b ∈ [-128, 127]` always lies in `[-128, 127]`. Xenia's `crate::vmx::avg_i8` ([`crates/xenia-cpu/src/vmx.rs`](../../xenia-rs/crates/xenia-cpu/src/vmx.rs)) widens to `i16` before the add.
- **No XER side effects.**
- **Common usage.** Filtering / decimation passes, motion-compensation half-pel interpolation in older video codecs (the rounding-up bias matches MPEG/H.263 averaging conventions).
- **Aliasing legal.** `vavgsb v3, v3, v4` is a typical lowpass-step idiom.
- **No VMX128 sibling.**

## Related Instructions

- [`vavgub`](vavgub.md) — same width, unsigned rounding average.
- [`vavgsh`](vavgsh.md), [`vavgsw`](vavgsw.md) — signed rounding average at half / word width.
- [`vaddubm`](vaddubm.md), [`vaddsbs`](vaddsbs.md) — addition variants without the rounding-divide step.
- [`vsububm`](vsububm.md) — modulo subtract; needed for differential before averaging.

## IBM Reference

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