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

# `vaddshs` — Vector Add Signed Half Word Saturate

> **Category:** [VMX (Altivec)](../categories/vmx.md) · **Form:** [VX](../forms/VX.md) · **Opcode:** `0x10000340`

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## Assembler Mnemonics

| Mnemonic | XML entry | Flags | Description |
| --- | --- | --- | --- |
| `vaddshs` | `vaddshs` | — | Vector Add Signed Half Word Saturate |

## Syntax

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

## Encoding

### `vaddshs` — form `VX`

- **Opcode word:** `0x10000340`
- **Primary opcode (bits 0–5):** `4`
- **Extended opcode:** `832`
- **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` | vaddshs: read | Source A vector register. |
| `VB` | vaddshs: read | Source B vector register. |
| `VD` | vaddshs: write | Destination vector register. |
| `VSCR` | vaddshs: write | Vector Status and Control Register (NJ/SAT bits). |

## Register Effects

### `vaddshs`

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

## Status-Register Effects

- `vaddshs`: **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

**`vaddshs`**
- xenia-canary XML: [`tools/ppc-instructions.xml` — search for `mnem="vaddshs"`](../../xenia-canary/tools/ppc-instructions.xml)
- xenia-canary emit: [`src/xenia/cpu/ppc/ppc_emit_altivec.cc:356`](../../xenia-canary/src/xenia/cpu/ppc/ppc_emit_altivec.cc#L356)
- 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:505`](../../xenia-rs/crates/xenia-cpu/src/decoder.rs#L505)
- xenia-rs interpreter: [`crates/xenia-cpu/src/interpreter.rs:3306-3317`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L3306-L3317)
<details><summary>xenia-rs interpreter body (frozen snapshot)</summary>

```rust
        PpcOpcode::vaddshs => {
            let a = crate::vmx::as_i16x8(ctx.vr[instr.ra()]);
            let b = crate::vmx::as_i16x8(ctx.vr[instr.rb()]);
            let mut r = [0i16; 8]; let mut sat = false;
            for i in 0..8 {
                let (v, s) = crate::vmx::sat_add_i16(a[i], b[i]);
                r[i] = v; sat |= s;
            }
            if sat { ctx.set_vscr_sat(true); }
            ctx.vr[instr.rd()] = crate::vmx::from_i16x8(r);
            ctx.pc += 4;
        }
```
</details>

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## Special Cases & Edge Conditions

- **Eight signed-half lanes, saturating.** Each `VD[i] = clamp(VA[i] + VB[i], -32768, +32767)` for `i = 0..7`, with both inputs interpreted as signed `int16`. Lane 0 (`VD[0..1]` after `stvx`) is the most-significant half.
- **`VSCR[SAT]` is sticky-set** if *any* lane clamps. Once set, it stays set until explicit clear via [`mtvscr`](mtvscr.md). Xenia uses `crate::vmx::sat_add_i16` ([`crates/xenia-cpu/src/vmx.rs`](../../xenia-rs/crates/xenia-cpu/src/vmx.rs)) which returns the per-lane saturation flag; the OR is written back via `ctx.set_vscr_sat(true)`.
- **The modulo counterpart is `vadduhm`.** Modulo add for signed and unsigned halves is bit-identical, so [`vadduhm`](vadduhm.md) covers both when wraparound is wanted; switch to `vaddshs` only when clipping with sign awareness is desired.
- **Asymmetric clamp.** `+32767 + 1 = +32767`; `-32768 + (-1) = -32768`.
- **Common 16-bit DSP idiom.** Audio mixing and fixed-point colour blending lean heavily on `vaddshs` to combine signed Q15 / Q1.15 quantities without wraparound artefacts.
- **No XER side effects, no NJ involvement** (this is an integer op).
- **No VMX128 sibling.**

## Related Instructions

- [`vadduhs`](vadduhs.md) — same width, unsigned saturating add (clamps to `0..0xFFFF`).
- [`vadduhm`](vadduhm.md) — same width, modulo add; sign-agnostic.
- [`vaddsbs`](vaddsbs.md), [`vaddsws`](vaddsws.md) — signed saturating add at byte / word width.
- [`vsubshs`](vsubshs.md) — the matching signed saturating subtract.
- [`vmhaddshs`](vmhaddshs.md), [`vmhraddshs`](vmhraddshs.md) — signed-half multiply-add with saturation, common for fixed-point DSP.
- [`mtvscr`](mtvscr.md) / [`mfvscr`](mfvscr.md) — read or clear the `VSCR[SAT]` bit affected here.

## IBM Reference

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