# `vmaxub` — Vector Maximum Unsigned Byte

> **Category:** [VMX (Altivec)](../categories/vmx.md) · **Form:** [VX](../forms/VX.md) · **Opcode:** `0x10000002`

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## Assembler Mnemonics

| Mnemonic | XML entry | Flags | Description |
| --- | --- | --- | --- |
| `vmaxub` | `vmaxub` | — | Vector Maximum Unsigned Byte |

## Syntax

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

## Encoding

### `vmaxub` — form `VX`

- **Opcode word:** `0x10000002`
- **Primary opcode (bits 0–5):** `4`
- **Extended opcode:** `2`
- **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` | vmaxub: read | Source A vector register. |
| `VB` | vmaxub: read | Source B vector register. |
| `VD` | vmaxub: write | Destination vector register. |

## Register Effects

### `vmaxub`

- **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

**`vmaxub`**
- xenia-canary XML: [`tools/ppc-instructions.xml` — search for `mnem="vmaxub"`](../../xenia-canary/tools/ppc-instructions.xml)
- xenia-canary emit: [`src/xenia/cpu/ppc/ppc_emit_altivec.cc:859`](../../xenia-canary/src/xenia/cpu/ppc/ppc_emit_altivec.cc#L859)
- xenia-rs opcode: [`crates/xenia-cpu/src/opcode.rs:101`](../../xenia-rs/crates/xenia-cpu/src/opcode.rs#L101)
- xenia-rs decoder: [`crates/xenia-cpu/src/decoder.rs:435`](../../xenia-rs/crates/xenia-cpu/src/decoder.rs#L435)
- xenia-rs interpreter: [`crates/xenia-cpu/src/interpreter.rs:4391-4398`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L4391-L4398)
<details><summary>xenia-rs interpreter body (frozen snapshot)</summary>

```rust
        PpcOpcode::vmaxub => {
            let a = ctx.vr[instr.ra()].as_bytes();
            let b = ctx.vr[instr.rb()].as_bytes();
            let mut r = [0u8; 16];
            for i in 0..16 { r[i] = a[i].max(b[i]); }
            ctx.vr[instr.rd()] = xenia_types::Vec128::from_bytes(r);
            ctx.pc += 4;
        }
```
</details>

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## Special Cases & Edge Conditions

- **Per-byte unsigned max.** Sixteen byte lanes; `VD[i] = max(uint8(VA[i]), uint8(VB[i]))`. Lane 0 is the most-significant byte.
- **Unsigned ordering.** `vmaxub(0xFF, 0x01) = 0xFF`, opposite to [`vmaxsb`](vmaxsb.md).
- **No `VSCR` interaction, no XER, no exceptions.**
- **Common usage.** Pixel "brighter of two" channel selection; alpha mask combining.
- **Aliasing legal.** `vmaxub v3, v3, v4` raises `v3`'s lower bound to `v4`.
- **No VMX128 sibling.**

## Related Instructions

- [`vminub`](vminub.md) — the matching minimum.
- [`vmaxsb`](vmaxsb.md) — same width, signed max.
- [`vmaxuh`](vmaxuh.md), [`vmaxuw`](vmaxuw.md) — unsigned max at half / word width.
- [`vcmpgtub`](vcmpgtub.md) — separate compare-and-mask path.
- [`vsel`](vsel.md) — alternative selection.

## IBM Reference

- [AIX 7.3 — `vmaxub` (Vector Maximum Unsigned Byte)](https://www.ibm.com/docs/en/aix/7.3.0?topic=set-vmaxub-vector-maximum-unsigned-byte-instruction)
- [IBM AltiVec Technology Programmer's Interface Manual, Chapter 6 — Min/Max](https://www.nxp.com/docs/en/reference-manual/ALTIVECPIM.pdf)