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

# `vnmsubfp` — Vector Negative Multiply-Subtract Floating Point

> **Category:** [VMX (Altivec)](../categories/vmx.md) · **Form:** [VA](../forms/VA.md) · **Opcode:** `0x1000002f`

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## Assembler Mnemonics

| Mnemonic | XML entry | Flags | Description |
| --- | --- | --- | --- |
| `vnmsubfp` | `vnmsubfp` | — | Vector Negative Multiply-Subtract Floating Point |
| `vnmsubfp128` | `vnmsubfp128` | — | Vector128 Negative Multiply-Subtract Floating Point |

## Syntax

```asm
vnmsubfp [VD], [VA], [VC], [VB]
vnmsubfp128 [VD], [VA], [VD], [VB]
```

## Encoding

### `vnmsubfp` — form `VA`

- **Opcode word:** `0x1000002f`
- **Primary opcode (bits 0–5):** `4`
- **Extended opcode:** `47`
- **Synchronising:** no

| Bits | Field | Meaning |
| --- | --- | --- |
| 0–5 | `OPCD` | primary opcode (4) |
| 6–10 | `VRT` | destination vector register |
| 11–15 | `VRA` | source A |
| 16–20 | `VRB` | source B |
| 21–25 | `VRC` | source C / shift |
| 26–31 | `XO` | extended opcode (6 bits) |

### `vnmsubfp128` — form `VX128`

- **Opcode word:** `0x14000150`
- **Primary opcode (bits 0–5):** `5`
- **Extended opcode:** `336`
- **Synchronising:** no

| Bits | Field | Meaning |
| --- | --- | --- |
| 0–5 | `OPCD` | primary opcode (4 or 5) |
| 6–10 | `VD128l` | destination low 5 bits |
| 11–15 | `VA128l` | source A low 5 bits |
| 16–20 | `VB128l` | source B low 5 bits |
| 21 | `VA128H` | source A high bit |
| 22 | `—` | reserved |
| 23–25 | `VC` | optional VC / XO sub-field |
| 26 | `VA128h` | source A middle bit |
| 27 | `—` | reserved |
| 28–29 | `VD128h` | destination high 2 bits |
| 30–31 | `VB128h` | source B high 2 bits |

## Operands

| Field | Role | Description |
| --- | --- | --- |
| `VA` | vnmsubfp: read; vnmsubfp128: read | Source A vector register. |
| `VC` | vnmsubfp: read | Source C vector register / 3-bit selector. |
| `VB` | vnmsubfp: read; vnmsubfp128: read | Source B vector register. |
| `VD` | vnmsubfp: write; vnmsubfp128: read; vnmsubfp128: write | Destination vector register. |

## Register Effects

### `vnmsubfp`

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

### `vnmsubfp128`

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

## Status-Register Effects

_No condition-register or status-register effects._

## Operation (pseudocode)

```
for each 32-bit float lane i in 0..3:
    VD[i] <- −((VA[i] * VC[i]) − VB[i])
```

## 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

**`vnmsubfp`**
- xenia-canary XML: [`tools/ppc-instructions.xml` — search for `mnem="vnmsubfp"`](../../xenia-canary/tools/ppc-instructions.xml)
- xenia-canary emit: [`src/xenia/cpu/ppc/ppc_emit_altivec.cc:1154`](../../xenia-canary/src/xenia/cpu/ppc/ppc_emit_altivec.cc#L1154)
- xenia-rs opcode: [`crates/xenia-cpu/src/opcode.rs:110`](../../xenia-rs/crates/xenia-cpu/src/opcode.rs#L110)
- xenia-rs decoder: [`crates/xenia-cpu/src/decoder.rs:589`](../../xenia-rs/crates/xenia-cpu/src/decoder.rs#L589)
- xenia-rs interpreter: [`crates/xenia-cpu/src/interpreter.rs:2074-2089`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L2074-L2089)
<details><summary>xenia-rs interpreter body (frozen snapshot)</summary>

```rust
        PpcOpcode::vnmsubfp => {
            // vD = -(vA * vC - vB) = vB - vA * vC. Same denorm-flush rule as vmaddfp.
            let a = ctx.vr[instr.ra()].as_f32x4();
            let b = ctx.vr[instr.rb()].as_f32x4();
            let c = ctx.vr[instr.rc()].as_f32x4();
            let mut r = [0f32; 4];
            for i in 0..4 {
                let ai = vmx::flush_denorm(a[i]);
                let bi = vmx::flush_denorm(b[i]);
                let ci = vmx::flush_denorm(c[i]);
                // PPCBUG-426: single FMA rounding instead of two-step (b - a*c).
                r[i] = vmx::flush_denorm(-ai.mul_add(ci, -bi));
            }
            ctx.vr[instr.rd()] = xenia_types::Vec128::from_f32x4_array(r);
            ctx.pc += 4;
        }
```
</details>

**`vnmsubfp128`**
- xenia-canary XML: [`tools/ppc-instructions.xml` — search for `mnem="vnmsubfp128"`](../../xenia-canary/tools/ppc-instructions.xml)
- xenia-canary emit: [`src/xenia/cpu/ppc/ppc_emit_altivec.cc:1157`](../../xenia-canary/src/xenia/cpu/ppc/ppc_emit_altivec.cc#L1157)
- xenia-rs opcode: [`crates/xenia-cpu/src/opcode.rs:110`](../../xenia-rs/crates/xenia-cpu/src/opcode.rs#L110)
- xenia-rs decoder: [`crates/xenia-cpu/src/decoder.rs:615`](../../xenia-rs/crates/xenia-cpu/src/decoder.rs#L615)
- xenia-rs interpreter: [`crates/xenia-cpu/src/interpreter.rs:2090-2107`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L2090-L2107)
<details><summary>xenia-rs interpreter body (frozen snapshot)</summary>

```rust
        PpcOpcode::vnmsubfp128 => {
            // VMX128 form: vD <- -((vA * vB) - vD) = vD - (vA * vB). Canary
            // routes through `InstrEmit_vnmsubfp_` with the same arg-swap,
            // which flushes all inputs unconditionally.
            let a = ctx.vr[instr.va128()].as_f32x4();
            let b = ctx.vr[instr.vb128()].as_f32x4();
            let d = ctx.vr[instr.vd128()].as_f32x4();
            let mut r = [0f32; 4];
            for i in 0..4 {
                let ai = vmx::flush_denorm(a[i]);
                let bi = vmx::flush_denorm(b[i]);
                let di = vmx::flush_denorm(d[i]);
                // PPCBUG-427: single FMA rounding.
                r[i] = vmx::flush_denorm(-ai.mul_add(bi, -di));
            }
            ctx.vr[instr.vd128()] = xenia_types::Vec128::from_f32x4_array(r);
            ctx.pc += 4;
        }
```
</details>

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## Special Cases & Edge Conditions

- **Lane-wise negative multiply-subtract.** Each of the four lanes computes `VD[i] = −((VA[i] × VC[i]) − VB[i])`, i.e. `VB[i] − VA[i] × VC[i]`. The multiply and the subsequent add are **not** a single fused rounding step in xenia — they're a multiply, a subtract, then a negate — but the PowerPC ISA specifies the sequence to behave *as if* it were fused (single IEEE-754 rounding). Hardware Xenon indeed rounds only once.
- **IEEE-754 binary32 lanes.** Follows `VSCR[NJ]`: denormal inputs/outputs flush to zero when `NJ = 1`.
- **No VSCR[SAT] update.** VMX float ops never set saturation.
- **No FPSCR effect.** Unlike scalar `fnmsub[s]`, `vnmsubfp` does not touch FPSCR.
- **NaN propagation.** A NaN in any of `VA`, `VB`, or `VC` yields a NaN in the corresponding lane. Sign-of-NaN is unspecified but stable in xenia (matches the x86 host's `vfnmadd`-family output).
- **Big-endian lane indexing.** Lane 0 is the MSB-most 4 bytes.
- **VMX128 sibling: [`vnmsubfp128`](vnmsubfp128.md).** Identical operation with access to `v0..v127`.
- **No `Rc` bit** on this opcode; it never touches CR.

## Related Instructions

- [`vmaddfp`](vmaddfp.md) — the positive-rounded fused MAC `(VA × VC) + VB`.
- [`vaddfp`](vaddfp.md), [`vsubfp`](vsubfp.md) — the underlying adds/subs.
- [`vmulfp`](vmulfp.md) — xenia-convenience lane-wise float multiply (no native Altivec form; usually encoded as `vmaddfp VD, VA, VC, v0_zero`).
- [`vrefp`](vrefp.md), [`vrsqrtefp`](vrsqrtefp.md) — Newton iterations that pair with `vnmsubfp`.
- [`vmaxfp`](vmaxfp.md), [`vminfp`](vminfp.md) — the other float-arithmetic primitives.

## IBM Reference

- [AIX 7.3 — `vnmsubfp` (Vector Negative Multiply-Subtract Floating Point)](https://www.ibm.com/docs/en/aix/7.3.0?topic=set-vnmsubfp-vector-negative-multiply-subtract-floating-point-instruction)
- [IBM AltiVec Technology Programmer's Interface Manual, Chapter 5 — Floating-Point Arithmetic](https://www.nxp.com/docs/en/reference-manual/ALTIVECPIM.pdf)