# `vmulosb` — Vector Multiply Odd Signed Byte

> **Category:** [VMX (Altivec)](../categories/vmx.md) · **Form:** [VX](../forms/VX.md) · **Opcode:** `0x10000108`

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## Assembler Mnemonics

| Mnemonic | XML entry | Flags | Description |
| --- | --- | --- | --- |
| `vmulosb` | `vmulosb` | — | Vector Multiply Odd Signed Byte |

## Syntax

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

## Encoding

### `vmulosb` — form `VX`

- **Opcode word:** `0x10000108`
- **Primary opcode (bits 0–5):** `4`
- **Extended opcode:** `264`
- **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` | vmulosb: read | Source A vector register. |
| `VB` | vmulosb: read | Source B vector register. |
| `VD` | vmulosb: write | Destination vector register. |

## Register Effects

### `vmulosb`

- **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

**`vmulosb`**
- xenia-canary XML: [`tools/ppc-instructions.xml` — search for `mnem="vmulosb"`](../../xenia-canary/tools/ppc-instructions.xml)
- xenia-canary emit: [`src/xenia/cpu/ppc/ppc_emit_altivec.cc:1106`](../../xenia-canary/src/xenia/cpu/ppc/ppc_emit_altivec.cc#L1106)
- xenia-rs opcode: [`crates/xenia-cpu/src/opcode.rs:109`](../../xenia-rs/crates/xenia-cpu/src/opcode.rs#L109)
- xenia-rs decoder: [`crates/xenia-cpu/src/decoder.rs:456`](../../xenia-rs/crates/xenia-cpu/src/decoder.rs#L456)
- xenia-rs interpreter: [`crates/xenia-cpu/src/interpreter.rs:3477-3484`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L3477-L3484)
<details><summary>xenia-rs interpreter body (frozen snapshot)</summary>

```rust
        PpcOpcode::vmulosb => {
            let a = crate::vmx::as_i8x16(ctx.vr[instr.ra()]);
            let b = crate::vmx::as_i8x16(ctx.vr[instr.rb()]);
            let mut r = [0i16; 8];
            for i in 0..8 { r[i] = a[2 * i + 1] as i16 * b[2 * i + 1] as i16; }
            ctx.vr[instr.rd()] = crate::vmx::from_i16x8(r);
            ctx.pc += 4;
        }
```
</details>

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## Special Cases & Edge Conditions

- **Odd-lane signed-byte multiply.** Only the odd-indexed byte lanes (1, 3, 5, 7, 9, 11, 13, 15 — big-endian numbering) of `VA` and `VB` participate. Each pair is treated as signed 8-bit, multiplied, and sign-extended to a signed 16-bit result in the corresponding half-word of `VD`. Pairing: `VD.h[i] = (int8)VA.b[2*i+1] * (int8)VB.b[2*i+1]` for `i ∈ 0..7`.
- **Lane-count reduction.** 16 byte lanes → 8 half-word lanes.
- **No overflow.** `(-128) * (-128) = 0x4000`, `(127) * (127) = 0x3F01` — both fit in int16. `VSCR[SAT]` is **not** set.
- **Pair with [`vmulesb`](vmulesb.md)** to get every signed byte × byte product; interleave via `vmrghh`/`vmrglh`, or feed into [`vmsummbm`](vmsummbm.md) for a multiply-accumulate.
- **Signed vs. unsigned distinction.** The `s` in `vmulosb` makes the product arithmetic: negative operands sign-extend. Compare with [`vmuloub`](vmuloub.md) which zero-extends.
- **No `Rc`, no XER, no VSCR side-effect.** No VMX128 sibling.

## Related Instructions

- [`vmulesb`](vmulesb.md) — even-lane signed byte multiply.
- [`vmuloub`](vmuloub.md), [`vmuleub`](vmuleub.md) — unsigned byte twins.
- [`vmulosh`](vmulosh.md), [`vmulesh`](vmulesh.md) — signed half-word even/odd.
- [`vmsummbm`](vmsummbm.md) — fused signed-byte multiply-sum modulo.
- [`vmrghh`](vmrghh.md), [`vmrglh`](vmrglh.md) — interleave the even/odd half-word results.

## IBM Reference

- [AIX 7.3 — `vmulosb` (Vector Multiply Odd Signed Byte)](https://www.ibm.com/docs/en/aix/7.3.0?topic=set-vmulosb-vector-multiply-odd-signed-byte-instruction)
- [IBM AltiVec Technology Programmer's Interface Manual, Chapter 4 — Integer Arithmetic](https://www.nxp.com/docs/en/reference-manual/ALTIVECPIM.pdf)