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

# `vaddubs` — Vector Add Unsigned Byte Saturate

> **Category:** [VMX (Altivec)](../categories/vmx.md) · **Form:** [VX](../forms/VX.md) · **Opcode:** `0x10000200`

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## Assembler Mnemonics

| Mnemonic | XML entry | Flags | Description |
| --- | --- | --- | --- |
| `vaddubs` | `vaddubs` | — | Vector Add Unsigned Byte Saturate |

## Syntax

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

## Encoding

### `vaddubs` — form `VX`

- **Opcode word:** `0x10000200`
- **Primary opcode (bits 0–5):** `4`
- **Extended opcode:** `512`
- **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` | vaddubs: read | Source A vector register. |
| `VB` | vaddubs: read | Source B vector register. |
| `VD` | vaddubs: write | Destination vector register. |
| `VSCR` | vaddubs: write | Vector Status and Control Register (NJ/SAT bits). |

## Register Effects

### `vaddubs`

- **Reads (always):** `VA`, `VB`
- **Reads (conditional):** _none_
- **Writes (always):** `VD`, `VSCR`
- **Writes (conditional):** _none_

## Status-Register Effects

- `vaddubs`: **VSCR[SAT]** may be stickied on saturating vector operations.

## 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

**`vaddubs`**
- xenia-canary XML: [`tools/ppc-instructions.xml` — search for `mnem="vaddubs"`](../../xenia-canary/tools/ppc-instructions.xml)
- xenia-canary emit: [`src/xenia/cpu/ppc/ppc_emit_altivec.cc:379`](../../xenia-canary/src/xenia/cpu/ppc/ppc_emit_altivec.cc#L379)
- 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:475`](../../xenia-rs/crates/xenia-cpu/src/decoder.rs#L475)
- xenia-rs interpreter: [`crates/xenia-cpu/src/interpreter.rs:3233-3245`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L3233-L3245)
<details><summary>xenia-rs interpreter body (frozen snapshot)</summary>

```rust
        PpcOpcode::vaddubs => {
            let a = ctx.vr[instr.ra()].as_bytes();
            let b = ctx.vr[instr.rb()].as_bytes();
            let mut r = [0u8; 16];
            let mut sat = false;
            for i in 0..16 {
                let (v, s) = crate::vmx::sat_add_u8(a[i], b[i]);
                r[i] = v; sat |= s;
            }
            if sat { ctx.set_vscr_sat(true); }
            ctx.vr[instr.rd()] = xenia_types::Vec128::from_bytes(r);
            ctx.pc += 4;
        }
```
</details>

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## Special Cases & Edge Conditions

- **Sixteen unsigned-byte lanes, saturating.** Each `VD[i] = min(VA[i] + VB[i], 0xFF)` for `i = 0..15`. Lane 0 is the most-significant byte after `stvx`.
- **`VSCR[SAT]` is sticky-set** if any lane saturates. Once set, it stays set until [`mtvscr`](mtvscr.md) clears it. Xenia computes this with `crate::vmx::sat_add_u8` ([`crates/xenia-cpu/src/vmx.rs`](../../xenia-rs/crates/xenia-cpu/src/vmx.rs)).
- **One-sided clamp.** Only the upper bound applies (unsigned add cannot underflow). Distinct from [`vaddsbs`](vaddsbs.md), which clips at both `+127` and `-128`.
- **Pixel-blend workhorse.** Common usage is to add two unsigned-byte colour vectors with clamp-to-white at `0xFF`. Saturation behaves the same way as `_mm_adds_epu8` on x86 SSE2 — making it a one-to-one host translation candidate.
- **Versus modulo.** [`vaddubm`](vaddubm.md) wraps silently and never touches `VSCR[SAT]`. Use `vaddubs` when overflow indicates "too bright" / "out of range" and you want to flag it sticky.
- **No XER side effects.**
- **No VMX128 sibling.**

## Related Instructions

- [`vaddubm`](vaddubm.md) — same width, modulo (non-saturating) add.
- [`vaddsbs`](vaddsbs.md) — same width, signed saturating add (range `-128..+127`).
- [`vadduhs`](vadduhs.md), [`vadduws`](vadduws.md) — unsigned saturating add at half / word width.
- [`vsububs`](vsububs.md) — the matching unsigned saturating subtract (clamps to `0`).
- [`vavgub`](vavgub.md) — rounding average; alternative when you want `(a + b + 1) >> 1` without overflow worry.
- [`mtvscr`](mtvscr.md) / [`mfvscr`](mfvscr.md) — read or clear the sticky `VSCR[SAT]` bit.

## IBM Reference

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