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

# `mulhwux` — Multiply High Word Unsigned

> **Category:** [Integer ALU](../categories/alu.md) · **Form:** [XO](../forms/XO.md) · **Opcode:** `0x7c000016`

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## Assembler Mnemonics

| Mnemonic | XML entry | Flags | Description |
| --- | --- | --- | --- |
| `mulhwu` | `mulhwux` | — | Multiply High Word Unsigned |
| `mulhwu.` | `mulhwux` | Rc=1 | Multiply High Word Unsigned |

## Syntax

```asm
mulhwu[Rc] [RD], [RA], [RB]
```

## Encoding

### `mulhwux` — form `XO`

- **Opcode word:** `0x7c000016`
- **Primary opcode (bits 0–5):** `31`
- **Extended opcode:** `11`
- **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` | mulhwux: read | Source GPR (`r0`–`r31`). |
| `RB` | mulhwux: read | Source GPR. |
| `RD` | mulhwux: write | Destination GPR. |
| `CR` | mulhwux: 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

### `mulhwux`

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

## Status-Register Effects

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

## Operation (pseudocode)

```
RT <- high_32_of_unsigned_multiply((RA)[32:63], (RB)[32:63]) zero-extended to 64
```

## 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

**`mulhwux`**
- xenia-canary XML: [`tools/ppc-instructions.xml` — search for `mnem="mulhwux"`](../../xenia-canary/tools/ppc-instructions.xml)
- xenia-canary emit: [`src/xenia/cpu/ppc/ppc_emit_alu.cc:347`](../../xenia-canary/src/xenia/cpu/ppc/ppc_emit_alu.cc#L347)
- xenia-rs opcode: [`crates/xenia-cpu/src/opcode.rs:57`](../../xenia-rs/crates/xenia-cpu/src/opcode.rs#L57)
- xenia-rs decoder: [`crates/xenia-cpu/src/decoder.rs:862`](../../xenia-rs/crates/xenia-cpu/src/decoder.rs#L862)
- xenia-rs interpreter: [`crates/xenia-cpu/src/interpreter.rs:383-393`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L383-L393)
<details><summary>xenia-rs interpreter body (frozen snapshot)</summary>

```rust
        PpcOpcode::mulhwux => {
            // PPCBUG-020: 32-bit ABI CR0 view.
            let ra = ctx.gpr[instr.ra()] as u32 as u64;
            let rb = ctx.gpr[instr.rb()] as u32 as u64;
            let result = ra.wrapping_mul(rb);
            ctx.gpr[instr.rd()] = (result >> 32) & 0xFFFF_FFFF;
            if instr.rc_bit() {
                ctx.update_cr_signed(0, ctx.gpr[instr.rd()] as u32 as i32 as i64);
            }
            ctx.pc += 4;
        }
```
</details>

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## Special Cases & Edge Conditions

- **Inputs are the low 32 bits, zero-extended.** `RA[32:63]` and `RB[32:63]` are treated as unsigned, widened to 64-bit `u64`, multiplied; the high 32 bits of the 64-bit product land in `RT[32:63]`. Xenia masks the high 32 bits of `RT` to zero ([`interpreter.rs:231`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L231)).
- **Pair with [`mullwx`](mullwx.md) for the full 64-bit unsigned product.** `mullw` returns the low 32 sign-extended; for unsigned use, pair `mulhwu` with `rlwinm` to mask the low half. Xbox 360 compilers commonly emit this combination.
- **No `OE` bit.** Same family rule.
- **`Rc=1` CR0 update.** Uses `as i32 as i64` ([`interpreter.rs:234`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L234)). Because the result is bounded by `0xFFFFFFFF` and stored only in the low 32 bits, this CR0 will report `LT` for any unsigned high half ≥ `0x80000000` — a known signed/unsigned interpretation pitfall when `Rc=1` is used with `mulhwu`.
- **Common idiom for multi-precision arithmetic.** `mulhwu` + `mullw` + `addc` chains build extended-precision multiplies entirely in 32-bit ops; useful for cryptographic code that targets the Xenon's 32-bit ABI.
- **Multi-cycle latency** like the rest of the multiply family.

## Related Instructions

- [`mullwx`](mullwx.md) — low 32 bits of the same product (sign-extended).
- [`mulhwx`](mulhwx.md) — signed high half.
- [`mulhdux`](mulhdux.md) — 64-bit unsigned high half.
- [`addcx`](addcx.md), [`addex`](addex.md) — used to chain 32-bit products into wider precision.

## IBM Reference

- [AIX 7.3 — `mulhwu` (Multiply High Word Unsigned)](https://www.ibm.com/docs/en/aix/7.3.0?topic=set-mulhwu-multiply-high-word-unsigned-instruction)