# `xorx` — XOR

> **Category:** [Integer ALU](../categories/alu.md) · **Form:** [X](../forms/X.md) · **Opcode:** `0x7c000278`

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## Assembler Mnemonics

| Mnemonic | XML entry | Flags | Description |
| --- | --- | --- | --- |
| `xor` | `xorx` | — | XOR |
| `xor.` | `xorx` | Rc=1 | XOR |

## Syntax

```asm
xor[Rc] [RA], [RS], [RB]
```

## Encoding

### `xorx` — form `X`

- **Opcode word:** `0x7c000278`
- **Primary opcode (bits 0–5):** `31`
- **Extended opcode:** `316`
- **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 |
| --- | --- | --- |
| `RS` | xorx: read | Source GPR (alias for RD in some stores). |
| `RB` | xorx: read | Source GPR. |
| `RA` | xorx: write | Source GPR (`r0`–`r31`). |
| `CR` | xorx: 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

### `xorx`

- **Reads (always):** `RS`, `RB`
- **Reads (conditional):** _none_
- **Writes (always):** `RA`
- **Writes (conditional):** `CR`

## Status-Register Effects

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

## Operation (pseudocode)

```
RA <- (RS) ^ (RB)
```

## 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

**`xorx`**
- xenia-canary XML: [`tools/ppc-instructions.xml` — search for `mnem="xorx"`](../../xenia-canary/tools/ppc-instructions.xml)
- xenia-canary emit: [`src/xenia/cpu/ppc/ppc_emit_alu.cc:829`](../../xenia-canary/src/xenia/cpu/ppc/ppc_emit_alu.cc#L829)
- xenia-rs opcode: [`crates/xenia-cpu/src/opcode.rs:132`](../../xenia-rs/crates/xenia-cpu/src/opcode.rs#L132)
- xenia-rs decoder: [`crates/xenia-cpu/src/decoder.rs:798`](../../xenia-rs/crates/xenia-cpu/src/decoder.rs#L798)
- xenia-rs interpreter: [`crates/xenia-cpu/src/interpreter.rs:556-561`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L556-L561)
<details><summary>xenia-rs interpreter body (frozen snapshot)</summary>

```rust
        PpcOpcode::xorx => {
            // PPCBUG-032+020: 32-bit ABI CR0 view.
            ctx.gpr[instr.ra()] = ctx.gpr[instr.rs()] ^ ctx.gpr[instr.rb()];
            if instr.rc_bit() { ctx.update_cr_signed(0, ctx.gpr[instr.ra()] as u32 as i32 as i64); }
            ctx.pc += 4;
        }
```
</details>

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## Special Cases & Edge Conditions

- **`RA ← RS XOR RB`.** Bit-wise XOR.
- **Idiom: `xor RA, RS, RS`** zeroes `RA` — the canonical "clear register" instruction. Cheaper than `li RA, 0` because no immediate-extraction stage is involved.
- **Operand convention** is X-form (`RA` destination, `RS`/`RB` sources).
- **64-bit operation** on Xenon.
- **No `OE` or `XER` side effects.** Only `Rc=1` updates `CR0`.
- **64-bit CR update on Xenon, 32-bit in xenia-rs.** [`interpreter.rs:367`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L367) truncates with `as i32 as i64`. For `xor.` whose result has differing high/low halves, spec and xenia diverge; `xor. RA, RS, RS` gives `EQ` either way.
- **Useful as bitmask toggle.** `xor r3, r3, r4` flips in `r3` every bit set in `r4`.
- **No `XER[CA]`.**

## Related Instructions

- [`xori`](xori.md), [`xoris`](xoris.md) — D-form immediate variants.
- [`eqvx`](eqvx.md) — NXOR (`~(RS ^ RB)`).
- [`andx`](andx.md), [`orx`](orx.md), [`norx`](norx.md), [`nandx`](nandx.md) — sister logicals.

## IBM Reference

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