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

# `fctidx` — Floating Convert to Integer Doubleword

> **Category:** [Floating-Point](../categories/fpu.md) · **Form:** [X](../forms/X.md) · **Opcode:** `0xfc00065c`

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## Assembler Mnemonics

| Mnemonic | XML entry | Flags | Description |
| --- | --- | --- | --- |
| `fctid` | `fctidx` | — | Floating Convert to Integer Doubleword |
| `fctid.` | `fctidx` | Rc=1 | Floating Convert to Integer Doubleword |

## Syntax

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

## Encoding

### `fctidx` — form `X`

- **Opcode word:** `0xfc00065c`
- **Primary opcode (bits 0–5):** `63`
- **Extended opcode:** `814`
- **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` | fctidx: read | Source B floating-point register. |
| `FD` | fctidx: write | Destination floating-point register. |
| `CR` | fctidx: 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` | fctidx: write | Floating-Point Status and Control Register. |

## Register Effects

### `fctidx`

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

## Status-Register Effects

- `fctidx`: **CR0** ← signed-compare(result, 0) with `SO ← XER[SO]`, when `Rc=1`.; **FPSCR** updated per IEEE-754 flags (FX, FEX, FPRF, FR, FI, exceptions).

## 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

**`fctidx`**
- xenia-canary XML: [`tools/ppc-instructions.xml` — search for `mnem="fctidx"`](../../xenia-canary/tools/ppc-instructions.xml)
- xenia-canary emit: [`src/xenia/cpu/ppc/ppc_emit_fpu.cc:280`](../../xenia-canary/src/xenia/cpu/ppc/ppc_emit_fpu.cc#L280)
- xenia-rs opcode: [`crates/xenia-cpu/src/opcode.rs:27`](../../xenia-rs/crates/xenia-cpu/src/opcode.rs#L27)
- xenia-rs decoder: [`crates/xenia-cpu/src/decoder.rs:912`](../../xenia-rs/crates/xenia-cpu/src/decoder.rs#L912)
- xenia-rs interpreter: [`crates/xenia-cpu/src/interpreter.rs:2886-2906`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L2886-L2906)
<details><summary>xenia-rs interpreter body (frozen snapshot)</summary>

```rust
        PpcOpcode::fctidx => {
            // Convert to integer doubleword (round per FPSCR[RN]).
            // PPCBUG-229: set XX on inexact (fractional input).
            let val = ctx.fpr[instr.rb()];
            let result = if val.is_nan() {
                fpscr::set_exception(ctx, fpscr::VXCVI | if fpscr::is_snan(val) { fpscr::VXSNAN } else { 0 });
                0x8000_0000_0000_0000u64
            } else if val >= (i64::MAX as f64) {
                fpscr::set_exception(ctx, fpscr::VXCVI);
                0x7FFF_FFFF_FFFF_FFFFu64
            } else if val < (i64::MIN as f64) {
                fpscr::set_exception(ctx, fpscr::VXCVI);
                0x8000_0000_0000_0000u64
            } else {
                if val != val.trunc() { fpscr::set_exception(ctx, fpscr::XX); }
                fpscr::round_to_i64(ctx, val) as u64
            };
            ctx.fpr[instr.rd()] = f64::from_bits(result);
            if instr.rc_bit() { update_cr1_from_fpscr(ctx); }
            ctx.pc += 4;
        }
```
</details>

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## Special Cases & Edge Conditions

- **binary64 → 64-bit signed integer, current rounding mode.** Result is the integer rounded per `FPSCR[RN]`, packed into the 64-bit FPR as raw bits (the FPR is reinterpreted as an `i64` by subsequent `stfd`/integer code).
- **Saturation on out-of-range.** Per PowerISA, values outside `[i64::MIN, i64::MAX]` (or NaN) yield the most-negative integer (`0x8000_0000_0000_0000`) and set `FPSCR[VXCVI, VX, FX]`. xenia-rs special-cases NaN to `0x8000_0000_0000_0000` but **does not saturate** out-of-range finite values — Rust's `as i64` from a too-large `f64` produces an undefined-then-saturated result that may differ from the PPC convention. **xenia quirk:** very-large finite inputs may round to a different sentinel than hardware.
- **xenia round implementation.** xenia uses Rust's `f64::round`, which rounds half-cases **away from zero** (NOT round-to-nearest-even). PowerISA round-to-nearest in default mode rounds half-cases to even. **xenia quirk:** values like `0.5`, `1.5`, `2.5` may produce different integers (xenia: `1, 2, 3`; PPC default: `0, 2, 2`).
- **Rounding mode.** PPC uses `FPSCR[RN]` for the rounding direction. xenia ignores the FPSCR mode and always uses `f64::round` (i.e. round-half-away-from-zero) regardless of `RN`. **xenia quirk:** non-default rounding modes are not respected.
- **Inexact.** Sets `FPSCR[XX, FX]` on any non-integer input. xenia does not update FPSCR.
- **NaN.** Returns sentinel `0x8000_0000_0000_0000` and sets `FPSCR[VXCVI]`. xenia matches the sentinel, but does not raise the FPSCR bit.
- **`Rc=1` (`fctid.`)** copies `FPSCR[FX, FEX, VX, OX]` into CR1.
- **Encoding.** X-form, primary 63, XO 814. Reads `FRB` only.
- **Pair with `stfd`** to extract the `i64` value to memory or a GPR (Xbox 360 has no direct FPR↔GPR move; round-trip via stack).

## Related Instructions

- [`fctidzx`](fctidzx.md) — same conversion but always rounds toward zero (truncation).
- [`fctiwx`](fctiwx.md), [`fctiwzx`](fctiwzx.md) — 32-bit integer variants (saturate to `i32` range).
- [`fcfidx`](fcfidx.md) — inverse direction (`i64` → binary64).
- [`mffsx`](mffsx.md), [`mtfsfx`](mtfsfx.md) — control `FPSCR[RN]`.
- `stfd`, `stfiwx` — store the integer-bits FPR to memory; `stfiwx` stores only the low 32 bits (use after `fctiwx` / `fctiwzx`).

## IBM Reference

- [AIX 7.3 — `fctid` (Floating Convert to Integer Doubleword)](https://www.ibm.com/docs/en/aix/7.3.0?topic=set-fctid-floating-convert-integer-doubleword-instruction)
- [PowerISA v2.07B, Book I, Chapter 4 — Floating-Point Processor](https://openpowerfoundation.org/specifications/isa/) (`VXCVI` is the invalid-conversion exception bit; saturation sentinel is `0x8000_0000_0000_0000`).