# `vctsxs` — Vector Convert to Signed Fixed-Point Word Saturate

> **Category:** [VMX (Altivec)](../categories/vmx.md) · **Form:** [VX](../forms/VX.md) · **Opcode:** `0x100003ca`

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## Assembler Mnemonics

| Mnemonic | XML entry | Flags | Description |
| --- | --- | --- | --- |
| `vctsxs` | `vctsxs` | — | Vector Convert to Signed Fixed-Point Word Saturate |

## Syntax

```asm
vctsxs [VD], [VB], [UIMM]
```

## Encoding

### `vctsxs` — form `VX`

- **Opcode word:** `0x100003ca`
- **Primary opcode (bits 0–5):** `4`
- **Extended opcode:** `970`
- **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 |
| --- | --- | --- |
| `VB` | vctsxs: read | Source B vector register. |
| `UIMM` | vctsxs: read | 16-bit unsigned immediate. Zero-extended. |
| `VD` | vctsxs: write | Destination vector register. |
| `VSCR` | vctsxs: write | Vector Status and Control Register (NJ/SAT bits). |

## Register Effects

### `vctsxs`

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

## Status-Register Effects

- `vctsxs`: **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

**`vctsxs`**
- xenia-canary XML: [`tools/ppc-instructions.xml` — search for `mnem="vctsxs"`](../../xenia-canary/tools/ppc-instructions.xml)
- xenia-canary emit: [`src/xenia/cpu/ppc/ppc_emit_altivec.cc:536`](../../xenia-canary/src/xenia/cpu/ppc/ppc_emit_altivec.cc#L536)
- xenia-rs opcode: [`crates/xenia-cpu/src/opcode.rs:98`](../../xenia-rs/crates/xenia-cpu/src/opcode.rs#L98)
- xenia-rs decoder: [`crates/xenia-cpu/src/decoder.rs:517`](../../xenia-rs/crates/xenia-cpu/src/decoder.rs#L517)
- xenia-rs interpreter: [`crates/xenia-cpu/src/interpreter.rs:4281-4292`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L4281-L4292)
<details><summary>xenia-rs interpreter body (frozen snapshot)</summary>

```rust
        PpcOpcode::vctsxs => {
            let uimm = (instr.raw >> 16) & 0x1F;
            let b = ctx.vr[instr.rb()].as_f32x4();
            let mut r = [0i32; 4]; let mut sat = false;
            for i in 0..4 {
                let (v, s) = crate::vmx::cvt_f32_to_i32_sat(b[i], uimm);
                r[i] = v; sat |= s;
            }
            if sat { ctx.set_vscr_sat(true); }
            ctx.vr[instr.rd()] = crate::vmx::from_i32x4(r);
            ctx.pc += 4;
        }
```
</details>

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## Special Cases & Edge Conditions

- **Convert IEEE float lane to signed-Q `int32`, saturating.** For each of the four word lanes, `VD[i] = clamp(round_toward_zero(VB[i] * 2^UIMM), INT32_MIN, INT32_MAX)`. The 5-bit `UIMM` (bits 11..15) gives the Q-format fractional shift, in `0..31`.
- **Saturating, not wrapping.** Out-of-range floats clamp to `INT32_MIN` (negative overflow) or `INT32_MAX` (positive overflow) — *not* the wrap-around behaviour of x86 `cvttps2dq` (which produces `0x80000000` on overflow regardless of sign). Xenia's `crate::vmx::cvt_f32_to_i32_sat` ([`crates/xenia-cpu/src/vmx.rs`](../../xenia-rs/crates/xenia-cpu/src/vmx.rs)) handles the difference.
- **NaN → 0.** A NaN input becomes `0` in the output lane and stickies `VSCR[SAT]`. (Many references state "NaN → INT32_MIN"; verify against [`vmx.rs`](../../xenia-rs/crates/xenia-cpu/src/vmx.rs) for the canonical xenia behaviour, which differs from POWER ISA wording.)
- **`VSCR[SAT]` is sticky-set** if any lane saturates (overflow or NaN). Cleared only by [`mtvscr`](mtvscr.md).
- **Rounding is truncate-toward-zero.** Always; no per-instruction rounding control.
- **`VSCR[NJ]` flushes denormal *inputs* to zero before scaling** (Xenon default).
- **Big-endian word lanes.** Lane 0 is the most-significant word.
- **No XER changes, no traps.**
- **No VMX128 sibling.**
- **Inverse of [`vcfsx`](vcfsx.md)**, but the inverse direction saturates rather than wraps — round-trips lose the magnitude of out-of-range values.

## Related Instructions

- [`vctuxs`](vctuxs.md) — same shape, unsigned destination.
- [`vcfsx`](vcfsx.md), [`vcfux`](vcfux.md) — inverse direction (int → float with Q-shift).
- [`vrfin`](vrfin.md), [`vrfip`](vrfip.md), [`vrfim`](vrfim.md), [`vrfiz`](vrfiz.md) — float-to-float rounding modes (round-to-nearest, up, down, toward-zero) when staying in float.
- [`mtvscr`](mtvscr.md) / [`mfvscr`](mfvscr.md) — read or clear `VSCR[SAT]`.

## IBM Reference

- [AIX 7.3 — `vctsxs` (Vector Convert to Signed Fixed-Point Word Saturate)](https://www.ibm.com/docs/en/aix/7.3.0?topic=set-vctsxs-vector-convert-signed-fixed-point-word-saturate-instruction)
- [IBM AltiVec Technology Programmer's Interface Manual, Chapter 5 — Conversion Instructions](https://www.nxp.com/docs/en/reference-manual/ALTIVECPIM.pdf)