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

# `vaddubm` — Vector Add Unsigned Byte Modulo

> **Category:** [VMX (Altivec)](../categories/vmx.md) · **Form:** [VX](../forms/VX.md) · **Opcode:** `0x10000000`

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## Assembler Mnemonics

| Mnemonic | XML entry | Flags | Description |
| --- | --- | --- | --- |
| `vaddubm` | `vaddubm` | — | Vector Add Unsigned Byte Modulo |

## Syntax

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

## Encoding

### `vaddubm` — form `VX`

- **Opcode word:** `0x10000000`
- **Primary opcode (bits 0–5):** `4`
- **Extended opcode:** `0`
- **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` | vaddubm: read | Source A vector register. |
| `VB` | vaddubm: read | Source B vector register. |
| `VD` | vaddubm: write | Destination vector register. |

## Register Effects

### `vaddubm`

- **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

**`vaddubm`**
- xenia-canary XML: [`tools/ppc-instructions.xml` — search for `mnem="vaddubm"`](../../xenia-canary/tools/ppc-instructions.xml)
- xenia-canary emit: [`src/xenia/cpu/ppc/ppc_emit_altivec.cc:372`](../../xenia-canary/src/xenia/cpu/ppc/ppc_emit_altivec.cc#L372)
- xenia-rs opcode: [`crates/xenia-cpu/src/opcode.rs:90`](../../xenia-rs/crates/xenia-cpu/src/opcode.rs#L90)
- xenia-rs decoder: [`crates/xenia-cpu/src/decoder.rs:434`](../../xenia-rs/crates/xenia-cpu/src/decoder.rs#L434)
- xenia-rs interpreter: [`crates/xenia-cpu/src/interpreter.rs:3198-3205`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L3198-L3205)
<details><summary>xenia-rs interpreter body (frozen snapshot)</summary>

```rust
        PpcOpcode::vaddubm => {
            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].wrapping_add(b[i]); }
            ctx.vr[instr.rd()] = xenia_types::Vec128::from_bytes(r);
            ctx.pc += 4;
        }
```
</details>

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## Special Cases & Edge Conditions

- **Sixteen independent byte lanes.** `VD[i] = (VA[i] + VB[i]) mod 256` for `i = 0..15`. Lane 0 is the most-significant byte (the byte at the lowest address after `stvx`).
- **Modulo wrap, not saturating.** Overflow silently wraps in 8-bit unsigned arithmetic — there is no carry-out and **`VSCR[SAT]` is not touched**. This is the same bit pattern as a signed-byte modulo add, so `vaddubm` is also the de-facto `vaddsbm` (which doesn't exist in the ISA — modulo arithmetic is sign-agnostic).
- **No carry, no flags.** XER is untouched (Altivec never updates `XER[CA]`/`XER[OV]`). The dedicated [`vaddcuw`](vaddcuw.md) instruction exists *only* because there is no SAT/CA byproduct — extracting the carry needs an explicit op.
- **Aliasing is legal.** `vaddubm v3, v3, v4` (in-place accumulate) is a single-cycle issue on Xenon's VMX pipe.
- **VSCR untouched.** Neither `SAT` nor `NJ` is read or written. Schedulable next to floats, compares and saturating ops without dependency stalls.
- **Pairs with a saturating sibling.** When you need 8-bit add with clamping, switch to [`vaddubs`](vaddubs.md) (unsigned saturate, range `0..0xFF`) or [`vaddsbs`](vaddsbs.md) (signed saturate, range `-128..+127`) — both of which *do* sticky-set `VSCR[SAT]`.
- **No VMX128 sibling.** The `vaddubm` opcode is not exposed as a `*128` form; the 32-register encoding is the only one available.

## Related Instructions

- [`vaddubs`](vaddubs.md) — same lane width, unsigned saturating add (`SAT` sticky-set on overflow).
- [`vaddsbs`](vaddsbs.md) — same lane width, signed saturating add.
- [`vadduhm`](vadduhm.md), [`vadduwm`](vadduwm.md) — modulo add with 8-lane half / 4-lane word width.
- [`vaddcuw`](vaddcuw.md) — produces the per-lane carry bits a 32-bit modulo add discards.
- [`vsububm`](vsububm.md) — the matching modulo subtract.
- [`vavgub`](vavgub.md) — unsigned byte average (carry-aware: `(a + b + 1) >> 1`), useful when byte addition needs rounding without overflow.

## IBM Reference

- [AIX 7.3 — `vaddubm` (Vector Add Unsigned Byte Modulo)](https://www.ibm.com/docs/en/aix/7.3.0?topic=set-vaddubm-vector-add-unsigned-byte-modulo-instruction)
- [IBM AltiVec Technology Programmer's Interface Manual, Chapter 6 — Integer Arithmetic](https://www.nxp.com/docs/en/reference-manual/ALTIVECPIM.pdf)