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

# `vcfsx` — Vector Convert from Signed Fixed-Point Word

> **Category:** [VMX (Altivec)](../categories/vmx.md) · **Form:** [VX](../forms/VX.md) · **Opcode:** `0x1000034a`

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## Assembler Mnemonics

| Mnemonic | XML entry | Flags | Description |
| --- | --- | --- | --- |
| `vcfs` | `vcfsx` | — | Vector Convert from Signed Fixed-Point Word |

## Syntax

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

## Encoding

### `vcfsx` — form `VX`

- **Opcode word:** `0x1000034a`
- **Primary opcode (bits 0–5):** `4`
- **Extended opcode:** `842`
- **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` | vcfsx: read | Source B vector register. |
| `UIMM` | vcfsx: read | 16-bit unsigned immediate. Zero-extended. |
| `VD` | vcfsx: write | Destination vector register. |

## Register Effects

### `vcfsx`

- **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

**`vcfsx`**
- xenia-canary XML: [`tools/ppc-instructions.xml` — search for `mnem="vcfsx"`](../../xenia-canary/tools/ppc-instructions.xml)
- xenia-canary emit: [`src/xenia/cpu/ppc/ppc_emit_altivec.cc:500`](../../xenia-canary/src/xenia/cpu/ppc/ppc_emit_altivec.cc#L500)
- xenia-rs opcode: [`crates/xenia-cpu/src/opcode.rs:93`](../../xenia-rs/crates/xenia-cpu/src/opcode.rs#L93)
- xenia-rs decoder: [`crates/xenia-cpu/src/decoder.rs:509`](../../xenia-rs/crates/xenia-cpu/src/decoder.rs#L509)
- xenia-rs interpreter: [`crates/xenia-cpu/src/interpreter.rs:4306-4313`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L4306-L4313)
<details><summary>xenia-rs interpreter body (frozen snapshot)</summary>

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

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## Special Cases & Edge Conditions

- **Convert signed-Q `int32` lane to `binary32`.** For each of the four word lanes, `VD[i] = (float)VB[i] / 2^UIMM`, where `UIMM` is the 5-bit immediate at bits 11..15 of the instruction. UIMM ranges 0..31; UIMM=0 is plain integer-to-float.
- **Big-endian word lanes.** Lane 0 (`VD[0..3]` after `stvx`) is the most-significant word.
- **Use case.** Q-format fixed-point (`Qm.n`) → IEEE float in one instruction. UIMM gives the fractional bit count, so `vcfsx vD, vB, 16` interprets each lane as Q15.16.
- **Inexact rounding.** Values whose magnitude exceeds `2^24` lose mantissa precision (only 24 bits in `binary32`'s significand). The default rounding mode is round-to-nearest-even; VMX has no per-instruction rounding control.
- **`VSCR[NJ]` (flush-denormals)** affects the output if the scaled value is sub-normal. Xenia's `crate::vmx::cvt_i32_to_f32` honours this via the architectural `VSCR[NJ]` snapshot.
- **No `VSCR[SAT]` or XER changes**, no exceptions raised.
- **No VMX128 sibling.**
- **Round-trip caveat.** `vctsxs` (the inverse) saturates instead of wrapping, so a `vcfsx`/`vctsxs` round-trip is *not* identity for values outside the signed-int32 representable range — important for fixed-point interpolation kernels.

## Related Instructions

- [`vcfux`](vcfux.md) — same shape, unsigned source.
- [`vctsxs`](vctsxs.md) — inverse: float → signed-Q `int32` with saturation.
- [`vctuxs`](vctuxs.md) — inverse: float → unsigned-Q `uint32` with saturation.
- [`vrfin`](vrfin.md), [`vrfiz`](vrfiz.md) — float-to-integer rounding modes when no Q-format scale is needed.

## IBM Reference

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