# `cmp` — Compare

> **Category:** [Integer ALU](../categories/alu.md) · **Form:** [X](../forms/X.md) · **Opcode:** `0x7c000000`

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## Assembler Mnemonics

| Mnemonic | XML entry | Flags | Description |
| --- | --- | --- | --- |
| `cmp` | `cmp` | — | Compare |

## Syntax

```asm
cmp [CRFD], [L], [RA], [RB]
```

## Encoding

### `cmp` — form `X`

- **Opcode word:** `0x7c000000`
- **Primary opcode (bits 0–5):** `31`
- **Extended opcode:** `0`
- **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 |
| --- | --- | --- |
| `L` | cmp: read | Operand-length bit for compare instructions (`0 ⇒ 32-bit`, `1 ⇒ 64-bit`). |
| `RA` | cmp: read | Source GPR (`r0`–`r31`). |
| `RB` | cmp: read | Source GPR. |
| `CRFD` | cmp: write | CR destination field (`crf`, 0–7). |

## Register Effects

### `cmp`

- **Reads (always):** `L`, `RA`, `RB`
- **Reads (conditional):** _none_
- **Writes (always):** `CRFD`
- **Writes (conditional):** _none_

## Status-Register Effects

_No condition-register or status-register effects._

## Operation (pseudocode)

```
if L = 0 then a,b <- EXTS((RA)[32:63]), EXTS((RB)[32:63])
else             a,b <- (RA), (RB)
CR[BF] <- signed_compare(a, b) || XER[SO]
```

## 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

**`cmp`**
- xenia-canary XML: [`tools/ppc-instructions.xml` — search for `mnem="cmp"`](../../xenia-canary/tools/ppc-instructions.xml)
- xenia-canary emit: [`src/xenia/cpu/ppc/ppc_emit_alu.cc:523`](../../xenia-canary/src/xenia/cpu/ppc/ppc_emit_alu.cc#L523)
- xenia-rs opcode: [`crates/xenia-cpu/src/opcode.rs:13`](../../xenia-rs/crates/xenia-cpu/src/opcode.rs#L13)
- xenia-rs decoder: [`crates/xenia-cpu/src/decoder.rs:749`](../../xenia-rs/crates/xenia-cpu/src/decoder.rs#L749)
- xenia-rs interpreter: [`crates/xenia-cpu/src/interpreter.rs:863-885`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L863-L885)
<details><summary>xenia-rs interpreter body (frozen snapshot)</summary>

```rust
        PpcOpcode::cmp => {
            let bf = instr.crfd();
            if instr.l() {
                let ra = ctx.gpr[instr.ra()] as i64;
                let rb = ctx.gpr[instr.rb()] as i64;
                ctx.cr[bf] = crate::context::CrField {
                    lt: ra < rb,
                    gt: ra > rb,
                    eq: ra == rb,
                    so: ctx.xer_so != 0,
                };
            } else {
                let ra = ctx.gpr[instr.ra()] as i32;
                let rb = ctx.gpr[instr.rb()] as i32;
                ctx.cr[bf] = crate::context::CrField {
                    lt: ra < rb,
                    gt: ra > rb,
                    eq: ra == rb,
                    so: ctx.xer_so != 0,
                };
            }
            ctx.pc += 4;
        }
```
</details>

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## Extended Pseudocode

```
if L = 0 then                                   ; 32-bit compare
    a <- EXTS((RA)[32:63])                      ; sign-extend low word to 64
    b <- EXTS((RB)[32:63])
else                                            ; 64-bit compare
    a <- (RA)
    b <- (RB)
CR[BF] <- { LT: a <s b, GT: a >s b, EQ: a = b, SO: XER[SO] }   ; signed
```

## Special Cases & Edge Conditions

- **`BF` is a CR field (0–7), not a bit.** The `crfD` operand encodes which of the eight 4-bit CR fields is updated. Assemblers write it as `crN` where `N ∈ 0..7`. The simplified mnemonic `cmpw RA, RB` ≡ `cmp cr0, 0, RA, RB` is universal in Xbox 360 code.
- **`L` bit selects width.** `L = 0` (the usual `cmpw` / `cmpd`-is-rare path) performs a *32-bit* signed compare of `RA[32:63]` and `RB[32:63]`, both sign-extended to 64 bits. `L = 1` (`cmpd`) performs a full 64-bit signed compare.
- **Signed.** Use [`cmpl`](cmpl.md) / [`cmpli`](cmpli.md) for unsigned comparisons. Confusing signed/unsigned is the most common compare-family bug in hand-written asm.
- **SO is always copied from `XER[SO]`.** This makes overflow observable across arithmetic/compare sequences: an `addo.` followed by `beq` can branch on the record-form flag while `bso` can inspect the sticky overflow.
- **`cr0` is the default for record-form ALU**; by convention assemblers and generators reserve `cr0` for the chain of `Rc=1` instructions and use `cr1..cr7` (or `cmp` to an explicit field) for standalone compares. Don't assume `cmp` writes `cr0` unless the `BF` operand says so.
- **No register is written** beyond the 4-bit CR field. `cmp` has no `Rc` or `OE` bit.
- **Xenia-rs quirk.** The interpreter recomputes `EQ` after the signed compare to guard against a subtract-cancellation edge case; this is a defensive belt-and-braces against the 32-bit narrowing path. Functionally equivalent to the spec.

## Related Instructions

- [`cmpi`](cmpi.md) — signed compare against a 16-bit immediate.
- [`cmpl`](cmpl.md), [`cmpli`](cmpli.md) — unsigned versions.
- [`cmpw`](cmp.md), [`cmpd`](cmp.md) — simplified mnemonics selecting `L`.
- [`mcrxr`](mcrxr.md) — move `XER[SO..CA]` into a CR field and clear them; used to reset sticky overflow.
- Every `Rc=1` ALU instruction ([`addx`](addx.md), [`subfx`](subfx.md), [`andx`](andx.md), …) — these implicitly perform a signed-compare-to-zero into `cr0`; use explicit `cmp` only when comparing two non-zero values or using a non-zero CR field.

## IBM Reference

- [AIX 7.3 — `cmp` (Compare)](https://www.ibm.com/docs/en/aix/7.3.0?topic=set-cmp-compare-instruction)
- [AIX 7.3 — `cmpw` / `cmpd` (simplified mnemonics)](https://www.ibm.com/docs/en/aix/7.3.0?topic=mnemonics-cmpw-compare-word)