xenia-rs/migration/project-root/ppc-manual/fpu/fnabsx.md

# `fnabsx` — Floating Negative Absolute Value

> **Category:** [Floating-Point](../categories/fpu.md) · **Form:** [X](../forms/X.md) · **Opcode:** `0xfc000110`

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## Assembler Mnemonics

| Mnemonic | XML entry | Flags | Description |
| --- | --- | --- | --- |
| `fnabs` | `fnabsx` | — | Floating Negative Absolute Value |
| `fnabs.` | `fnabsx` | Rc=1 | Floating Negative Absolute Value |

## Syntax

```asm
fnabs[Rc] [FD], [FB]
```

## Encoding

### `fnabsx` — form `X`

- **Opcode word:** `0xfc000110`
- **Primary opcode (bits 0–5):** `63`
- **Extended opcode:** `136`
- **Synchronising:** no

| Bits | Field | Meaning |
| --- | --- | --- |
| 0–5 | `OPCD` | primary opcode |
| 6–10 | `RT/FRT/VRT` | destination |
| 11–15 | `RA/FRA/VRA` | source A |
| 16–20 | `RB/FRB/VRB` | source B |
| 21–30 | `XO` | extended opcode (10 bits) |
| 31 | `Rc` | record-form flag |

## Operands

| Field | Role | Description |
| --- | --- | --- |
| `FB` | fnabsx: read | Source B floating-point register. |
| `FD` | fnabsx: write | Destination floating-point register. |
| `CR` | fnabsx: write (conditional) | Condition-register update. When `Rc=1`, CR field 0 (or CR6 for vector compares, CR1 for FPU) is updated from the result. |

## Register Effects

### `fnabsx`

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

## Status-Register Effects

- `fnabsx`: **CR0** ← signed-compare(result, 0) with `SO ← XER[SO]`, when `Rc=1`.

## Operation (pseudocode)

```
FRT <- set_sign(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

**`fnabsx`**
- xenia-canary XML: [`tools/ppc-instructions.xml` — search for `mnem="fnabsx"`](../../xenia-canary/tools/ppc-instructions.xml)
- xenia-canary emit: [`src/xenia/cpu/ppc/ppc_emit_fpu.cc:504`](../../xenia-canary/src/xenia/cpu/ppc/ppc_emit_fpu.cc#L504)
- xenia-rs opcode: [`crates/xenia-cpu/src/opcode.rs:29`](../../xenia-rs/crates/xenia-cpu/src/opcode.rs#L29)
- xenia-rs decoder: [`crates/xenia-cpu/src/decoder.rs:908`](../../xenia-rs/crates/xenia-cpu/src/decoder.rs#L908)
- xenia-rs interpreter: [`crates/xenia-cpu/src/interpreter.rs:2767-2771`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L2767-L2771)
<details><summary>xenia-rs interpreter body (frozen snapshot)</summary>

```rust
        PpcOpcode::fnabsx => {
            ctx.fpr[instr.rd()] = -(ctx.fpr[instr.rb()].abs());
            if instr.rc_bit() { update_cr1_from_fpscr(ctx); }
            ctx.pc += 4;
        }
```
</details>

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## Special Cases & Edge Conditions

- **Bit-pattern operation, no rounding.** `fnabs` **sets** the sign bit (bit 0) of the source binary64 value to 1, producing `-|FRB|`. No precision change, no exception bits. xenia-rs implements this as `-(b.abs())` — the abs clears the sign bit, then negation sets it.
- **NaN handling.** Returns the source NaN with the sign bit set to 1; payload preserved; signalling/quiet bit unchanged. `FPSCR[VXSNAN]` is **not** raised.
- **Special values.** `fnabs(±0) = -0`; `fnabs(±∞) = -∞`; `fnabs(±NaN) = -NaN` (sign set, payload preserved).
- **FPSCR.** Hardware does not update `FPRF` and does not raise any exception bit. Sign-bit ops are not arithmetic.
- **`Rc=1` (`fnabs.`)** copies `FPSCR[FX, FEX, VX, OX]` into CR1.
- **No `FRA`.** X-form, primary 63, XO 136. Reads `FRB` only.
- **Use case.** Less common than `fabs`/`fneg`. Useful for unconditional negative-magnitude values, e.g. forcing a value to be on the negative side of zero before a subsequent compare or for bit-pattern setup.

## Related Instructions

- [`fabsx`](fabsx.md) — clear sign bit (positive).
- [`fnegx`](fnegx.md) — toggle sign bit.
- [`fmrx`](fmrx.md) — plain register copy.
- [`fselx`](fselx.md) — branch-free select; with `fabs`/`fnabs` synthesises `copysign`-like helpers.

## IBM Reference

- [AIX 7.3 — `fnabs` (Floating Negative Absolute Value)](https://www.ibm.com/docs/en/aix/7.3.0?topic=set-fnabs-floating-negative-absolute-value-instruction)
- [PowerISA v2.07B, Book I, Chapter 4 — Floating-Point Processor](https://openpowerfoundation.org/specifications/isa/).