xenia-rs/migration/project-root/ppc-manual/vmx/vmulesb.md

# `vmulesb` — Vector Multiply Even Signed Byte

> **Category:** [VMX (Altivec)](../categories/vmx.md) · **Form:** [VX](../forms/VX.md) · **Opcode:** `0x10000308`

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## Assembler Mnemonics

| Mnemonic | XML entry | Flags | Description |
| --- | --- | --- | --- |
| `vmulesb` | `vmulesb` | — | Vector Multiply Even Signed Byte |

## Syntax

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

## Encoding

### `vmulesb` — form `VX`

- **Opcode word:** `0x10000308`
- **Primary opcode (bits 0–5):** `4`
- **Extended opcode:** `776`
- **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` | vmulesb: read | Source A vector register. |
| `VB` | vmulesb: read | Source B vector register. |
| `VD` | vmulesb: write | Destination vector register. |

## Register Effects

### `vmulesb`

- **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

**`vmulesb`**
- xenia-canary XML: [`tools/ppc-instructions.xml` — search for `mnem="vmulesb"`](../../xenia-canary/tools/ppc-instructions.xml)
- xenia-canary emit: [`src/xenia/cpu/ppc/ppc_emit_altivec.cc:1086`](../../xenia-canary/src/xenia/cpu/ppc/ppc_emit_altivec.cc#L1086)
- xenia-rs opcode: [`crates/xenia-cpu/src/opcode.rs:108`](../../xenia-rs/crates/xenia-cpu/src/opcode.rs#L108)
- xenia-rs decoder: [`crates/xenia-cpu/src/decoder.rs:501`](../../xenia-rs/crates/xenia-cpu/src/decoder.rs#L501)
- xenia-rs interpreter: [`crates/xenia-cpu/src/interpreter.rs:3469-3476`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L3469-L3476)
<details><summary>xenia-rs interpreter body (frozen snapshot)</summary>

```rust
        PpcOpcode::vmulesb => {
            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] as i16 * b[2 * i] as i16; }
            ctx.vr[instr.rd()] = crate::vmx::from_i16x8(r);
            ctx.pc += 4;
        }
```
</details>

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## Special Cases & Edge Conditions

- **Even-byte signed multiply, half-word result.** Per output half lane:
  ```
  VD[i] = int16(int8(VA[2*i]) * int8(VB[2*i]))           ; for i = 0..7
  ```
  Only the eight **even-indexed** byte lanes (lanes 0, 2, 4, …, 14 in big-endian) are read from each source. Each `int8 × int8` product is widened to `int16`, producing eight half-word results that fill all of `VD`.
- **No saturation, no `VSCR[SAT]`.** The full 16-bit product of two signed bytes always fits in `int16` (range `-127*-128 = +16256 .. +127*+127 = +16129` is well within `±32767`), so no clipping is needed — even at the bit-pattern extremes `(-128) * (-128) = +16384` is representable.
- **Pairs with [`vmulosb`](vmulosb.md)** (odd-byte sibling). Together they consume all 16 bytes; two instructions are needed for a "multiply every lane" 16×16-bit byte multiply.
- **Big-endian byte indexing.** Even byte indices `0, 2, 4, …, 14` correspond to the high-order halves of each half-word slot.
- **No XER, no exceptions.**
- **Aliasing legal.**
- **No VMX128 sibling.**
- **Common usage.** Signed-coefficient byte multiply for image filters; first half of a 16-byte signed multiply when paired with `vmulosb`.

## Related Instructions

- [`vmulosb`](vmulosb.md) — odd-byte sibling (lanes 1, 3, …, 15).
- [`vmuleub`](vmuleub.md), [`vmuloub`](vmuloub.md) — same split, unsigned.
- [`vmulesh`](vmulesh.md), [`vmulosh`](vmulosh.md) — same family at half width (→ word results).
- [`vmladduhm`](vmladduhm.md) — per-lane modulo multiply-add (low half only).
- [`vpkshus`](vpkshus.md) — saturating pack down from `int16` halves to `uint8` bytes (combine with `vmule*` for "scale + clamp" pipelines).

## IBM Reference

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