# `vcsxwfp128` — Vector128 Convert From Signed Fixed-Point Word to Floating-Point

> **Category:** [VMX128](../categories/vmx128.md) · **Form:** [VX128_3](../forms/VX128_3.md) · **Opcode:** `0x180002b0`

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## Assembler Mnemonics

| Mnemonic | XML entry | Flags | Description |
| --- | --- | --- | --- |
| `vcsxwfp128` | `vcsxwfp128` | — | Vector128 Convert From Signed Fixed-Point Word to Floating-Point |

## Syntax

```asm
vcsxwfp128 [VD], [VB], [UIMM]
```

## Encoding

### `vcsxwfp128` — form `VX128_3`

- **Opcode word:** `0x180002b0`
- **Primary opcode (bits 0–5):** `6`
- **Extended opcode:** `688`
- **Synchronising:** no

| Bits | Field | Meaning |
| --- | --- | --- |
| 0–5 | `OPCD` | primary opcode (6) |
| 6–10 | `VD128l` | destination low 5 bits |
| 11–15 | `IMM` | 5-bit immediate |
| 16–20 | `VB128l` | source B low 5 bits |
| 21–27 | `XO` | extended opcode |
| 28–29 | `VD128h` | destination high 2 bits |
| 30–31 | `VB128h` | source B high 2 bits |

## Operands

| Field | Role | Description |
| --- | --- | --- |
| `VB` | vcsxwfp128: read | Source B vector register. |
| `UIMM` | vcsxwfp128: read | 16-bit unsigned immediate. Zero-extended. |
| `VD` | vcsxwfp128: write | Destination vector register. |

## Register Effects

### `vcsxwfp128`

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

## Status-Register Effects

_No condition-register or status-register effects._

## 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

**`vcsxwfp128`**
- xenia-canary XML: [`tools/ppc-instructions.xml` — search for `mnem="vcsxwfp128"`](../../xenia-canary/tools/ppc-instructions.xml)
- xenia-canary emit: [`src/xenia/cpu/ppc/ppc_emit_altivec.cc:503`](../../xenia-canary/src/xenia/cpu/ppc/ppc_emit_altivec.cc#L503)
- 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:658`](../../xenia-rs/crates/xenia-cpu/src/decoder.rs#L658)
- xenia-rs interpreter: [`crates/xenia-cpu/src/interpreter.rs:4347-4354`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L4347-L4354)
<details><summary>xenia-rs interpreter body (frozen snapshot)</summary>

```rust
        PpcOpcode::vcsxwfp128 => {
            let uimm = (instr.raw >> 16) & 0x1F;
            let b = crate::vmx::as_i32x4(ctx.vr[instr.vb128()]);
            let mut r = [0f32; 4];
            for i in 0..4 { r[i] = crate::vmx::cvt_i32_to_f32(b[i], uimm); }
            ctx.vr[instr.vd128()] = xenia_types::Vec128::from_f32x4_array(r);
            ctx.pc += 4;
        }
```
</details>

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## Special Cases & Edge Conditions

- **Signed fixed-point (int32) → float with explicit scale.** Each lane computes `VD[i] = (float)VB.w[i] * 2^-UIMM` (equivalently `(int32)VB.w[i] / 2^UIMM`). `UIMM` is a 5-bit unsigned bias that specifies a post-scale — the inverse direction of [`vcfpsxws128`](vcfpsxws128.md), so the `UIMM`s should match for a round-trip.
- **IEEE-754 binary32 output, round-to-nearest.** Values outside the exactly-representable range (`|x| > 2^24`) lose low-order bits; no saturation on the float side.
- **No `VSCR[SAT]` effect** — conversion in this direction never saturates.
- **`VSCR[NJ]` does not affect the int → float path.**
- **VMX128 register-fusion** applies (7-bit register IDs).
- **No IBM AIX entry** — Xenon-only. Closest standard Altivec op is [`vcfsx`](../vmx/vcfsx.md).
- **No `Rc`, no XER / FPSCR.**

## Related Instructions

- [`vcfsx`](../vmx/vcfsx.md) — the standard Altivec `int32 → float` with scale.
- [`vcuxwfp128`](vcuxwfp128.md) — unsigned-int variant.
- [`vcfpsxws128`](vcfpsxws128.md), [`vcfpuxws128`](vcfpuxws128.md) — the inverse (float → int with scale).

## IBM Reference

- No IBM AIX entry — Xbox 360 VMX128 extension only.
- Xbox 360 XDK, Altivec-128 (VMX128) extensions; cross-referenced with [IBM AltiVec Technology Programmer's Interface Manual §`vcfsx`](https://www.nxp.com/docs/en/reference-manual/ALTIVECPIM.pdf).