# `crnor` — Condition Register NOR

> **Category:** [Control / CR / SPR](../categories/control.md) · **Form:** [XL](../forms/XL.md) · **Opcode:** `0x4c000042`

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## Assembler Mnemonics

| Mnemonic | XML entry | Flags | Description |
| --- | --- | --- | --- |
| `crnor` | `crnor` | — | Condition Register NOR |

## Syntax

```asm
crnor [CRBD], [CRBA], [CRBB]
```

## Encoding

### `crnor` — form `XL`

- **Opcode word:** `0x4c000042`
- **Primary opcode (bits 0–5):** `19`
- **Extended opcode:** `33`
- **Synchronising:** no

| Bits | Field | Meaning |
| --- | --- | --- |
| 0–5 | `OPCD` | primary opcode (19) |
| 6–10 | `BT/BO` | target / branch options |
| 11–15 | `BA/BI` | source A / CR bit to test |
| 16–20 | `BB` | source B |
| 21–30 | `XO` | extended opcode (10 bits) |
| 31 | `LK` | link flag |

## Operands

| Field | Role | Description |
| --- | --- | --- |
| `CRBA` | crnor: read | CR source bit A (0–31). |
| `CRBB` | crnor: read | CR source bit B (0–31). |
| `CRBD` | crnor: write | CR destination bit (0–31). |

## Register Effects

### `crnor`

- **Reads (always):** `CRBA`, `CRBB`
- **Reads (conditional):** _none_
- **Writes (always):** `CRBD`
- **Writes (conditional):** _none_

## Status-Register Effects

_No condition-register or status-register effects._

## 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

**`crnor`**
- xenia-canary XML: [`tools/ppc-instructions.xml` — search for `mnem="crnor"`](../../xenia-canary/tools/ppc-instructions.xml)
- xenia-canary emit: [`src/xenia/cpu/ppc/ppc_emit_control.cc:388`](../../xenia-canary/src/xenia/cpu/ppc/ppc_emit_control.cc#L388)
- xenia-rs opcode: [`crates/xenia-cpu/src/opcode.rs:17`](../../xenia-rs/crates/xenia-cpu/src/opcode.rs#L17)
- xenia-rs decoder: [`crates/xenia-cpu/src/decoder.rs:712`](../../xenia-rs/crates/xenia-cpu/src/decoder.rs#L712)

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## Special Cases & Edge Conditions

- **Operation.** `CR[CRBD] ← ¬(CR[CRBA] OR CR[CRBB])`. Result is 1 only when both source bits are 0; otherwise 0.
- **`crnot BT, BA` idiom.** With `BA == BB`, `crnor BT, BA, BA` ≡ `¬CR[BA]` — this is the canonical PowerPC **single-bit invert**, recognised by assemblers as the simplified mnemonic `crnot BT, BA`.
- **Bit-level operands.** 5-bit absolute CR-bit indices (0..31). The three bits may live in any combination of the eight CR fields.
- **Use case.** Branch on "neither A nor B"; or, with `crnot`, simply complement a CR bit before consuming it in a `bcx`.
- **No `Rc` / `OE`.** Pure CR-bit dataflow; CR0/XER untouched.
- **Not synchronising.** Reorderable.
- **xenia status.** Decoded via the generic CR-logical handler. xenia-canary's `InstrEmit_crnor` emits a host OR followed by NOT.

## Related Instructions

- [`cror`](cror.md), [`crorc`](crorc.md) — non-negated OR siblings.
- [`crnand`](crnand.md) — negated AND (the De Morgan dual).
- [`crand`](crand.md), [`crandc`](crandc.md) — AND family.
- [`crxor`](crxor.md), [`creqv`](creqv.md) — XOR / XNOR.
- [`mcrf`](mcrf.md) — bulk CR-field copy.
- [`bcx`](../branch/bcx.md) — typical consumer of synthesised CR bits.

### Simplified Mnemonics

| Simplified      | Expansion              | Effect |
| ---             | ---                    | --- |
| `crnot BT, BA`  | `crnor BT, BA, BA`     | `CR[BT] ← ¬CR[BA]` (invert single bit) |

## IBM Reference

- [AIX 7.3 — `crnor` (Condition Register NOR)](https://www.ibm.com/docs/en/aix/7.3.0?topic=set-crnor-condition-register-nor-instruction)
- [AIX 7.3 — Condition register simplified mnemonics](https://www.ibm.com/docs/en/aix/7.3.0?topic=mnemonics-condition-register-logical-simplified)