# `vaddsbs` — Vector Add Signed Byte Saturate

> **Category:** [VMX (Altivec)](../categories/vmx.md) · **Form:** [VX](../forms/VX.md) · **Opcode:** `0x10000300`

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## Assembler Mnemonics

| Mnemonic | XML entry | Flags | Description |
| --- | --- | --- | --- |
| `vaddsbs` | `vaddsbs` | — | Vector Add Signed Byte Saturate |

## Syntax

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

## Encoding

### `vaddsbs` — form `VX`

- **Opcode word:** `0x10000300`
- **Primary opcode (bits 0–5):** `4`
- **Extended opcode:** `768`
- **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` | vaddsbs: read | Source A vector register. |
| `VB` | vaddsbs: read | Source B vector register. |
| `VD` | vaddsbs: write | Destination vector register. |
| `VSCR` | vaddsbs: write | Vector Status and Control Register (NJ/SAT bits). |

## Register Effects

### `vaddsbs`

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

## Status-Register Effects

- `vaddsbs`: **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

**`vaddsbs`**
- xenia-canary XML: [`tools/ppc-instructions.xml` — search for `mnem="vaddsbs"`](../../xenia-canary/tools/ppc-instructions.xml)
- xenia-canary emit: [`src/xenia/cpu/ppc/ppc_emit_altivec.cc:348`](../../xenia-canary/src/xenia/cpu/ppc/ppc_emit_altivec.cc#L348)
- xenia-rs opcode: [`crates/xenia-cpu/src/opcode.rs:89`](../../xenia-rs/crates/xenia-cpu/src/opcode.rs#L89)
- xenia-rs decoder: [`crates/xenia-cpu/src/decoder.rs:498`](../../xenia-rs/crates/xenia-cpu/src/decoder.rs#L498)
- xenia-rs interpreter: [`crates/xenia-cpu/src/interpreter.rs:3258-3269`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L3258-L3269)
<details><summary>xenia-rs interpreter body (frozen snapshot)</summary>

```rust
        PpcOpcode::vaddsbs => {
            let a = crate::vmx::as_i8x16(ctx.vr[instr.ra()]);
            let b = crate::vmx::as_i8x16(ctx.vr[instr.rb()]);
            let mut r = [0i8; 16]; let mut sat = false;
            for i in 0..16 {
                let (v, s) = crate::vmx::sat_add_i8(a[i], b[i]);
                r[i] = v; sat |= s;
            }
            if sat { ctx.set_vscr_sat(true); }
            ctx.vr[instr.rd()] = crate::vmx::from_i8x16(r);
            ctx.pc += 4;
        }
```
</details>

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## Special Cases & Edge Conditions

- **Sixteen signed-byte lanes, saturating.** Each `VD[i] = clamp(VA[i] + VB[i], -128, +127)` for `i = 0..15`, with both inputs interpreted as signed `int8`. Lane 0 is the most-significant byte (the byte at the lowest address after `stvx`).
- **`VSCR[SAT]` is sticky-set** when *any* lane saturates — either positively (overflow above `+127`) or negatively (underflow below `-128`). The SAT bit is never cleared by this op; software must use [`mtvscr`](mtvscr.md) to clear it. Xenia routes the OR of per-lane saturation flags into `ctx.set_vscr_sat(true)` exactly when at least one lane clamped (see `crate::vmx::sat_add_i8` in [`crates/xenia-cpu/src/vmx.rs`](../../xenia-rs/crates/xenia-cpu/src/vmx.rs)).
- **Compare with the modulo sibling.** [`vaddubm`](vaddubm.md) is bit-pattern-identical to a hypothetical `vaddsbm` and silently wraps without touching `VSCR[SAT]`. Use `vaddsbs` whenever clipping is desired and you need the sticky overflow flag.
- **Asymmetric clamp.** `+127 + 1 = +127`; `-128 + (-1) = -128`. Tests that look for "any saturation" should mask both saturation directions.
- **No XER side effects.** Altivec never updates `XER[CA]` / `XER[OV]`. The only status bit affected is `VSCR[SAT]`.
- **Aliasing legal.** `vaddsbs v3, v3, v4` is the standard accumulate idiom for a clamping sum.
- **No VMX128 sibling.**

## Related Instructions

- [`vaddubs`](vaddubs.md) — same width, **unsigned** saturating add (clamps to `0..255`).
- [`vaddubm`](vaddubm.md) — same width, modulo (non-saturating) add; sign-agnostic.
- [`vaddshs`](vaddshs.md), [`vaddsws`](vaddsws.md) — signed saturating add at half / word width.
- [`vsubsbs`](vsubsbs.md) — the matching signed saturating subtract.
- [`mtvscr`](mtvscr.md) / [`mfvscr`](mfvscr.md) — read or clear the sticky `VSCR[SAT]` bit observed here.

## IBM Reference

- [AIX 7.3 — `vaddsbs` (Vector Add Signed Byte Saturate)](https://www.ibm.com/docs/en/aix/7.3.0?topic=set-vaddsbs-vector-add-signed-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)