# `vsumsws` — Vector Sum Across Signed Word Saturate

> **Category:** [VMX (Altivec)](../categories/vmx.md) · **Form:** [VX](../forms/VX.md) · **Opcode:** `0x10000788`

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## Assembler Mnemonics

| Mnemonic | XML entry | Flags | Description |
| --- | --- | --- | --- |
| `vsumsws` | `vsumsws` | — | Vector Sum Across Signed Word Saturate |

## Syntax

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

## Encoding

### `vsumsws` — form `VX`

- **Opcode word:** `0x10000788`
- **Primary opcode (bits 0–5):** `4`
- **Extended opcode:** `1928`
- **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` | vsumsws: read | Source A vector register. |
| `VB` | vsumsws: read | Source B vector register. |
| `VD` | vsumsws: write | Destination vector register. |
| `VSCR` | vsumsws: write | Vector Status and Control Register (NJ/SAT bits). |

## Register Effects

### `vsumsws`

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

## Status-Register Effects

- `vsumsws`: **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

**`vsumsws`**
- xenia-canary XML: [`tools/ppc-instructions.xml` — search for `mnem="vsumsws"`](../../xenia-canary/tools/ppc-instructions.xml)
- xenia-canary emit: [`src/xenia/cpu/ppc/ppc_emit_altivec.cc:1771`](../../xenia-canary/src/xenia/cpu/ppc/ppc_emit_altivec.cc#L1771)
- xenia-rs opcode: [`crates/xenia-cpu/src/opcode.rs:127`](../../xenia-rs/crates/xenia-cpu/src/opcode.rs#L127)
- xenia-rs decoder: [`crates/xenia-cpu/src/decoder.rs:550`](../../xenia-rs/crates/xenia-cpu/src/decoder.rs#L550)
- xenia-rs interpreter: [`crates/xenia-cpu/src/interpreter.rs:3658-3667`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L3658-L3667)
<details><summary>xenia-rs interpreter body (frozen snapshot)</summary>

```rust
        PpcOpcode::vsumsws => {
            // vD[3] = sat_i32(vC[3] + sum over i in 0..4 of vA[i])
            let a = crate::vmx::as_i32x4(ctx.vr[instr.ra()]);
            let c = crate::vmx::as_i32x4(ctx.vr[instr.rb()]);
            let s = a.iter().map(|&x| x as i64).sum::<i64>() + c[3] as i64;
            let (v, sat) = crate::vmx::sat_i64_to_i32(s);
            if sat { ctx.set_vscr_sat(true); }
            ctx.vr[instr.rd()] = crate::vmx::from_i32x4([0, 0, 0, v]);
            ctx.pc += 4;
        }
```
</details>

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## Special Cases & Edge Conditions

- **Full 4-lane reduction (signed word).** Sum all four signed words of `VA` plus `VB.w[3]`; saturate to `int32`. Output: `VD.w[3] = sat(VA.w[0]+VA.w[1]+VA.w[2]+VA.w[3]+VB.w[3])`; `VD.w[0..2] = 0`.
- **Only the last lane is meaningful.** The specification writes zeros into the first three lanes so software cannot accidentally consume them.
- **Sticky VSCR[SAT]** set on overflow.
- **Equivalent to a horizontal add-reduce**, common terminator for a multi-step dot-product or sum-of-products pipeline.
- **Big-endian word lanes.**
- **No `Rc`, no XER.**
- **No VMX128 sibling.** VMX128 instead uses `vmsum3fp128` / `vmsum4fp128` for float dot-products.

## Related Instructions

- [`vsum2sws`](vsum2sws.md) — two 2-word partial sums.
- [`vsum4sbs`](vsum4sbs.md), [`vsum4shs`](vsum4shs.md), [`vsum4ubs`](vsum4ubs.md) — per-word partial sums at narrower input widths.
- [`vmsumshm`](vmsumshm.md), [`vmsumshs`](vmsumshs.md) — fused multiply-sum variants.
- [`vmsum3fp128`](../vmx128/vmsum3fp128.md), [`vmsum4fp128`](../vmx128/vmsum4fp128.md) — VMX128 float dot-product helpers.

## IBM Reference

- [AIX 7.3 — `vsumsws` (Vector Sum across Saturated Signed Word)](https://www.ibm.com/docs/en/aix/7.3.0?topic=set-vsumsws-vector-sum-across-saturated-signed-word-instruction)
- [IBM AltiVec Technology Programmer's Interface Manual, Chapter 4 — Integer Arithmetic](https://www.nxp.com/docs/en/reference-manual/ALTIVECPIM.pdf)