# `fsubx` — Floating Subtract

> **Category:** [Floating-Point](../categories/fpu.md) · **Form:** [A](../forms/A.md) · **Opcode:** `0xfc000028`

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## Assembler Mnemonics

| Mnemonic | XML entry | Flags | Description |
| --- | --- | --- | --- |
| `fsub` | `fsubx` | — | Floating Subtract |
| `fsub.` | `fsubx` | Rc=1 | Floating Subtract |

## Syntax

```asm
fsub[Rc] [FD], [FA], [FB]
```

## Encoding

### `fsubx` — form `A`

- **Opcode word:** `0xfc000028`
- **Primary opcode (bits 0–5):** `63`
- **Extended opcode:** `20`
- **Synchronising:** no

| Bits | Field | Meaning |
| --- | --- | --- |
| 0–5 | `OPCD` | primary opcode (59 or 63) |
| 6–10 | `FRT` | destination FPR |
| 11–15 | `FRA` | source A FPR |
| 16–20 | `FRB` | source B FPR |
| 21–25 | `FRC` | source C FPR (multiplier for madd-style ops) |
| 26–30 | `XO` | extended opcode (5 bits) |
| 31 | `Rc` | record-form flag (updates CR1) |

## Operands

| Field | Role | Description |
| --- | --- | --- |
| `FA` | fsubx: read | Source A floating-point register (`fr0`–`fr31`). |
| `FB` | fsubx: read | Source B floating-point register. |
| `FD` | fsubx: write | Destination floating-point register. |
| `CR` | fsubx: write (conditional) | Condition-register update. When `Rc=1`, CR field 0 (or CR6 for vector compares, CR1 for FPU) is updated from the result. |
| `FPSCR` | fsubx: write | Floating-Point Status and Control Register. |

## Register Effects

### `fsubx`

- **Reads (always):** `FA`, `FB`
- **Reads (conditional):** _none_
- **Writes (always):** `FD`, `FPSCR`
- **Writes (conditional):** `CR`

## Status-Register Effects

- `fsubx`: **CR1** ← FPSCR[FX, FEX, VX, OX] when `Rc=1`.; **FPSCR** updated per IEEE-754 flags (FX, FEX, FPRF, FR, FI, exceptions).

## Operation (pseudocode)

```
FRT <- FRA − FRB
```

## 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

**`fsubx`**
- xenia-canary XML: [`tools/ppc-instructions.xml` — search for `mnem="fsubx"`](../../xenia-canary/tools/ppc-instructions.xml)
- xenia-canary emit: [`src/xenia/cpu/ppc/ppc_emit_fpu.cc:127`](../../xenia-canary/src/xenia/cpu/ppc/ppc_emit_fpu.cc#L127)
- xenia-rs opcode: [`crates/xenia-cpu/src/opcode.rs:30`](../../xenia-rs/crates/xenia-cpu/src/opcode.rs#L30)
- xenia-rs decoder: [`crates/xenia-cpu/src/decoder.rs:921`](../../xenia-rs/crates/xenia-cpu/src/decoder.rs#L921)
- xenia-rs interpreter: [`crates/xenia-cpu/src/interpreter.rs:2575-2584`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L2575-L2584)
<details><summary>xenia-rs interpreter body (frozen snapshot)</summary>

```rust
        PpcOpcode::fsubx => {
            let a = ctx.fpr[instr.ra()];
            let b = ctx.fpr[instr.rb()];
            fpscr::check_invalid_add(ctx, a, b, true);
            let result = a - b;
            ctx.fpr[instr.rd()] = result;
            fpscr::update_after_op(ctx, result, a.is_finite() && b.is_finite());
            if instr.rc_bit() { update_cr1_from_fpscr(ctx); }
            ctx.pc += 4;
        }
```
</details>

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## Special Cases & Edge Conditions

- **Double precision.** `fsub` operates on IEEE-754 binary64. The single-precision sibling is [`fsubsx`](fsubsx.md), which rounds the result to binary32 before re-encoding it into the 64-bit FPR.
- **`±∞ − ±∞` is the canonical invalid case.** Same-signed infinity subtraction (or opposite-signed addition) yields `QNaN(VXISI)` and sets `FPSCR[VXISI, VX, FX]`.
- **FPSCR side effects.** Hardware updates `FPRF`, `FR`, `FI`, `FX` plus exception bits `OX`, `UX`, `XX`, `VXISI`, `VXSNAN` as appropriate. xenia-rs's interpreter does **not** model FPSCR updates — a xenia quirk that almost never matters in practice.
- **`Rc=1` (`fsub.`)** writes `CR1` from `FPSCR[FX, FEX, VX, OX]`.
- **NaN propagation.** Any NaN operand yields a quiet NaN; a signalling NaN input is quietened (signalling bit cleared) per PowerISA. Host `f64 -` is relied on for the value.
- **Sign of zero.** `+0 − +0 = +0` in round-to-nearest, `−0` in round-toward-negative-infinity. xenia inherits host semantics.
- **Denormal flush.** Xenon boots with `FPSCR[NI]=1` (non-IEEE mode) so subnormal results flush to zero on hardware. Xenia produces IEEE-compliant denormals from the host FPU; titles relying on flush-to-zero typically see no observable difference for game logic but may see subtle differences in audio DSP.
- **Encoding.** A-form, primary 63, XO 20. `FRC` is don't-care for sub.

## Related Instructions

- [`fsubsx`](fsubsx.md) — single-precision subtract (rounds to binary32).
- [`faddx`](faddx.md), [`faddsx`](faddsx.md) — add counterparts; subtract is implemented as add-with-negated-B on most cores.
- [`fnegx`](fnegx.md) — sign flip (the bit-pattern operation behind `−FRB`).
- [`fmsubx`](fmsubx.md), [`fnmsubx`](fnmsubx.md) — fused multiply-subtract (single rounding step).
- [`mffsx`](mffsx.md), [`mtfsfx`](mtfsfx.md), [`mtfsb0x`](mtfsb0x.md), [`mtfsb1x`](mtfsb1x.md) — FPSCR control.

## IBM Reference

- [AIX 7.3 — `fsub` (Floating Subtract)](https://www.ibm.com/docs/en/aix/7.3.0?topic=set-fs-fsub-floating-subtract-instruction)
- [PowerISA v2.07B, Book I, Chapter 4 — Floating-Point Processor](https://openpowerfoundation.org/specifications/isa/).