Sylpheed/tools/ppc-manual/alu/cmpi.md

# `cmpi` — Compare Immediate

> **Category:** [Integer ALU](../categories/alu.md) · **Form:** [D](../forms/D.md) · **Opcode:** `0x2c000000`

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## Assembler Mnemonics

| Mnemonic | XML entry | Flags | Description |
| --- | --- | --- | --- |
| `cmpi` | `cmpi` | — | Compare Immediate |

## Syntax

```asm
cmpi [CRFD], [L], [RA], [SIMM]
```

## Encoding

### `cmpi` — form `D`

- **Opcode word:** `0x2c000000`
- **Primary opcode (bits 0–5):** `11`
- **Extended opcode:** —
- **Synchronising:** no

| Bits | Field | Meaning |
| --- | --- | --- |
| 0–5 | `OPCD` | primary opcode |
| 6–10 | `RT` | destination GPR (or RS when storing) |
| 11–15 | `RA` | source GPR (0 ⇒ literal 0 for RA0 forms) |
| 16–31 | `D/SI/UI` | 16-bit signed or unsigned immediate |

## Operands

| Field | Role | Description |
| --- | --- | --- |
| `L` | cmpi: read | Operand-length bit for compare instructions (`0 ⇒ 32-bit`, `1 ⇒ 64-bit`). |
| `RA` | cmpi: read | Source GPR (`r0`–`r31`). |
| `SIMM` | cmpi: read | 16-bit signed immediate. Sign-extended to 64 bits before use. |
| `RD` | cmpi: write | Destination GPR. |
| `CRFD` | cmpi: write | CR destination field (`crf`, 0–7). |

## Register Effects

### `cmpi`

- **Reads (always):** `L`, `RA`, `SIMM`
- **Reads (conditional):** _none_
- **Writes (always):** `RD`, `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(SIMM)
else             a,b <- (RA), EXTS(SIMM)
CR[BF] <- signed_compare(a, b) || XER[SO]
```

## C Translation Example

```c
/* No hand-written C yet. Translate the Canary emitter snapshot   */
/* under Implementation References; its HIR maps directly:        */
/*   f.LoadGPR(n) / f.StoreGPR(n, v)  -> r[n] / r[n] = v          */
/*   f.LoadFPR / StoreFPR, f.LoadVR / StoreVR -> f[n], v[n]        */
/*   f.Load(ea, T), f.Store(ea, v) -> raw read / write; emitters   */
/*     wrap them in f.ByteSwap for the big-endian guest value      */
/*   f.UpdateCR(n, v)  -> CR field n from v's LOW 32 BITS vs 0     */
/*   f.LoadCA / f.StoreCA -> xer.CA;  f.StoreSAT -> vscr.SAT       */
/*   i.XO.RA, i.D.DS, ... -> the bit-fields listed under Operands  */
/* The Register Effects and Status-Register Effects tables above  */
/* enumerate every side effect a faithful translation must emit.  */
```

## Implementation References

**`cmpi`**
- Canary XML: [`tools/ppc-instructions.xml` — search for `mnem="cmpi"`](https://github.com/xenia-canary/xenia-canary/blob/f21ebd49e979e44f081f474df78c3fbfee9cb3f2/tools/ppc-instructions.xml)
- Canary emitter: [`src/xenia/cpu/ppc/ppc_emit_alu.cc:552`](https://github.com/xenia-canary/xenia-canary/blob/f21ebd49e979e44f081f474df78c3fbfee9cb3f2/src/xenia/cpu/ppc/ppc_emit_alu.cc#L552)
- Sylpheed opcode: [`crates/sylpheed-ppc/src/opcode.rs:28`](../../../crates/sylpheed-ppc/src/opcode.rs#L28)
- Sylpheed decoder: [`crates/sylpheed-ppc/src/decoder.rs:450`](../../../crates/sylpheed-ppc/src/decoder.rs#L450)
<details><summary>Canary emitter (frozen snapshot @ <code>f21ebd49e9</code>)</summary>

```cpp
int InstrEmit_cmpi(PPCHIRBuilder& f, const InstrData& i) {
  // if L = 0 then
  //   a <- EXTS((RA)[32:63])
  // else
  //   a <- (RA)
  // if a < EXTS(SI) then
  //   c <- 0b100
  // else if a > EXTS(SI) then
  //   c <- 0b010
  // else
  //   c <- 0b001
  // CR[4×BF+32:4×BF+35] <- c || XER[SO]
  uint32_t BF = i.D.RT >> 2;
  uint32_t L = i.D.RT & 1;
  Value* lhs;
  Value* rhs;
  if (L) {
    lhs = f.LoadGPR(i.D.RA);
    rhs = f.LoadConstantInt64(XEEXTS16(i.D.DS));
  } else {
    lhs = f.Truncate(f.LoadGPR(i.D.RA), INT32_TYPE);
    rhs = f.LoadConstantInt32(int32_t(XEEXTS16(i.D.DS)));
  }
  f.UpdateCR(BF, lhs, rhs);
  return 0;
}
```
</details>

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## Special Cases & Edge Conditions

- **Immediate is sign-extended.** `SIMM` is treated as a signed 16-bit value in the range `[-32768, 32767]` and sign-extended to the operand width. Use [`cmpli`](cmpli.md) for unsigned comparisons against a 16-bit value.
- **`L` bit selects width.** `L = 0` (the usual `cmpwi`) compares the low 32 bits of `RA` (sign-extended) against `EXTS(SIMM)`; `L = 1` (`cmpdi`) does a full 64-bit signed compare. Most Xbox 360 code uses `cmpwi` because pointers and counters are 32-bit ABI.
- **Simplified mnemonics dominate disassembly.** `cmpwi crN, RA, SIMM` ≡ `cmpi crN, 0, RA, SIMM` and `cmpdi crN, RA, SIMM` ≡ `cmpi crN, 1, RA, SIMM`. The default CR field is `cr0` if omitted.
- **`BF` is a CR field (0–7), not a bit.** Same convention as [`cmp`](cmp.md). Distinct standalone compares should target `cr1..cr7` to avoid clobbering the implicit `cr0` chain set up by `Rc=1` arithmetic.
- **SO is copied from `XER[SO]`.** This makes overflow observable downstream of an `addo.` / `mulo.` etc. via `bso`/`bns`.
- **Canary is one direct compare.** As in [`cmp`](cmp.md): the operand and `EXTS(SI)` are compared at 64 or 32 bits by `L`, via `f.UpdateCR(BF, lhs, rhs)`, which does not write the field's `SO` bit (spec copies `XER[SO]`).
- **No register written** other than the 4-bit CR field — there is no `Rc` or `OE` bit.

## Related Instructions

- [`cmp`](cmp.md) — register-register signed compare.
- [`cmpli`](cmpli.md) — unsigned compare against immediate (zero-extended).
- [`cmpl`](cmpl.md) — unsigned register compare.
- `cmpwi`, `cmpdi` (simplified mnemonics) — select `L=0` / `L=1`.
- [`mcrxr`](../control/mcrxr.md) — clear sticky overflow before a fresh compare sequence.

## IBM Reference

- [AIX 7.3 — `cmpi` (Compare Immediate)](https://www.ibm.com/docs/en/aix/7.3.0?topic=set-cmpi-compare-immediate-instruction)
- [AIX 7.3 — `cmpwi` / `cmpdi` (simplified mnemonics)](https://www.ibm.com/docs/en/aix/7.3.0?topic=mnemonics-cmpwi-compare-word-immediate)