xenia-rs/migration/project-root/ppc-manual/alu/negx.md

# `negx` — Negate

> **Category:** [Integer ALU](../categories/alu.md) · **Form:** [XO](../forms/XO.md) · **Opcode:** `0x7c0000d0`

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## Assembler Mnemonics

| Mnemonic | XML entry | Flags | Description |
| --- | --- | --- | --- |
| `neg` | `negx` | — | Negate |
| `nego` | `negx` | OE=1 | Negate |
| `neg.` | `negx` | Rc=1 | Negate |
| `nego.` | `negx` | OE=1, Rc=1 | Negate |

## Syntax

```asm
neg[OE][Rc] [RD], [RA]
```

## Encoding

### `negx` — form `XO`

- **Opcode word:** `0x7c0000d0`
- **Primary opcode (bits 0–5):** `31`
- **Extended opcode:** `104`
- **Synchronising:** no

| Bits | Field | Meaning |
| --- | --- | --- |
| 0–5 | `OPCD` | primary opcode (31) |
| 6–10 | `RT` | destination GPR |
| 11–15 | `RA` | source A |
| 16–20 | `RB` | source B |
| 21 | `OE` | overflow-enable flag |
| 22–30 | `XO` | extended opcode (9 bits) |
| 31 | `Rc` | record-form flag |

## Operands

| Field | Role | Description |
| --- | --- | --- |
| `RA` | negx: read | Source GPR (`r0`–`r31`). |
| `RD` | negx: write | Destination GPR. |
| `CR` | negx: write (conditional) | Condition-register update. When `Rc=1`, CR field 0 (or CR6 for vector compares, CR1 for FPU) is updated from the result. |
| `OE` | negx: write (conditional) | Overflow-enable bit. When 1, the instruction updates `XER[OV]` and stickies `XER[SO]` on signed overflow. |

## Register Effects

### `negx`

- **Reads (always):** `RA`
- **Reads (conditional):** _none_
- **Writes (always):** `RD`
- **Writes (conditional):** `CR`, `OE`

## Status-Register Effects

- `negx`: **CR0** ← signed-compare(result, 0) with `SO ← XER[SO]`, when `Rc=1`.; **XER[OV]** ← signed-overflow(result); **XER[SO]** stickies, when `OE=1`.

## Operation (pseudocode)

```
RT <- ~(RA) + 1
```

## 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

**`negx`**
- xenia-canary XML: [`tools/ppc-instructions.xml` — search for `mnem="negx"`](../../xenia-canary/tools/ppc-instructions.xml)
- xenia-canary emit: [`src/xenia/cpu/ppc/ppc_emit_alu.cc:406`](../../xenia-canary/src/xenia/cpu/ppc/ppc_emit_alu.cc#L406)
- xenia-rs opcode: [`crates/xenia-cpu/src/opcode.rs:59`](../../xenia-rs/crates/xenia-cpu/src/opcode.rs#L59)
- xenia-rs decoder: [`crates/xenia-cpu/src/decoder.rs:866`](../../xenia-rs/crates/xenia-cpu/src/decoder.rs#L866)
- xenia-rs interpreter: [`crates/xenia-cpu/src/interpreter.rs:342-356`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L342-L356)
<details><summary>xenia-rs interpreter body (frozen snapshot)</summary>

```rust
        PpcOpcode::negx => {
            // PPCBUG-006: 32-bit ABI. `(!ra).wrapping_add(1)` on u64 always
            // sets upper 32 bits — every neg poisoned the GPR. neg_ov also
            // checks at 64-bit INT_MIN; should be 32-bit INT_MIN.
            let ra32 = ctx.gpr[instr.ra()] as u32;
            let result32 = (!ra32).wrapping_add(1);
            ctx.gpr[instr.rd()] = result32 as u64;
            if instr.oe() {
                overflow::apply(ctx, ra32 == 0x8000_0000);
            }
            if instr.rc_bit() {
                ctx.update_cr_signed(0, result32 as i32 as i64);
            }
            ctx.pc += 4;
        }
```
</details>

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## Special Cases & Edge Conditions

- **Two's-complement negate.** `RT ← ~RA + 1`, equivalent to `0 − RA`. A specialisation of [`subfx`](subfx.md) where `RB` is implicit zero.
- **`INT64_MIN` is its own negation.** `neg(0x8000000000000000) = 0x8000000000000000` — the only fixed point. `nego` should set `XER[OV]` in this case (it is the canonical signed-overflow trigger), but **xenia-rs does not implement `OE`** ([`interpreter.rs:201`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L201) has no `oe()` branch).
- **`RB` field unused.** Set to 0 by assemblers; ignored.
- **`Rc=1` CR0 update truncates to 32 bits in xenia-rs.** [`interpreter.rs:204`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L204). Important: `neg.` of a 64-bit value with high bits set will give a CR0 that doesn't match spec (which compares the full 64-bit `~RA + 1` to zero).
- **No carry produced.** Use [`subfic`](subficx.md) `RT, RA, 0` (`RT ← 0 − RA` with carry) when you need the borrow.
- **Latency: single cycle.** Negate is the cheapest XO-form ALU operation (cheaper than `subf` despite being a special case, because there's no `RB` operand fetch).

## Related Instructions

- [`subfx`](subfx.md) — generalisation: `neg RT, RA` ≡ `subf RT, RA, 0` (but the latter requires materialising 0 in a register).
- [`subfic`](subficx.md) — `RT ← SIMM − RA` with `XER[CA]`; `subfic RT, RA, 0` produces a borrow.
- [`addx`](addx.md), [`addmex`](addmex.md), [`addzex`](addzex.md) — for chained negation (multi-word two's complement).
- `not` (simplified) — bit-wise complement via `nor RA, RS, RS`; distinct from negate.

## IBM Reference

- [AIX 7.3 — `neg` (Negate)](https://www.ibm.com/docs/en/aix/7.3.0?topic=set-neg-negate-instruction)