# `vnmsubfp` — Vector Negative Multiply-Subtract Floating Point
> **Category:** [VMX (Altivec)](../categories/vmx.md) · **Form:** [VA](../forms/VA.md) · **Opcode:** `0x1000002f`
## 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)
xenia-rs interpreter body (frozen snapshot)
```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;
}
```
xenia-rs interpreter body (frozen snapshot)
```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;
}
```