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

# `mulhdux` — Multiply High Doubleword Unsigned

> **Category:** [Integer ALU](../categories/alu.md) · **Form:** [XO](../forms/XO.md) · **Opcode:** `0x7c000012`

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## Assembler Mnemonics

| Mnemonic | XML entry | Flags | Description |
| --- | --- | --- | --- |
| `mulhdu` | `mulhdux` | — | Multiply High Doubleword Unsigned |
| `mulhdu.` | `mulhdux` | Rc=1 | Multiply High Doubleword Unsigned |

## Syntax

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

## Encoding

### `mulhdux` — form `XO`

- **Opcode word:** `0x7c000012`
- **Primary opcode (bits 0–5):** `31`
- **Extended opcode:** `9`
- **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` | mulhdux: read | Source GPR (`r0`–`r31`). |
| `RB` | mulhdux: read | Source GPR. |
| `RD` | mulhdux: write | Destination GPR. |
| `CR` | mulhdux: 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

### `mulhdux`

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

## Status-Register Effects

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

## Operation (pseudocode)

```
RT <- ((RA) * (RB))[0:63]               ; high 64 of unsigned 64×64
```

## 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

**`mulhdux`**
- xenia-canary XML: [`tools/ppc-instructions.xml` — search for `mnem="mulhdux"`](../../xenia-canary/tools/ppc-instructions.xml)
- xenia-canary emit: [`src/xenia/cpu/ppc/ppc_emit_alu.cc:311`](../../xenia-canary/src/xenia/cpu/ppc/ppc_emit_alu.cc#L311)
- 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:860`](../../xenia-rs/crates/xenia-cpu/src/decoder.rs#L860)
- xenia-rs interpreter: [`crates/xenia-cpu/src/interpreter.rs:454-462`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L454-L462)
<details><summary>xenia-rs interpreter body (frozen snapshot)</summary>

```rust
        PpcOpcode::mulhdux => {
            let ra = ctx.gpr[instr.ra()] as u128;
            let rb = ctx.gpr[instr.rb()] as u128;
            ctx.gpr[instr.rd()] = (ra.wrapping_mul(rb) >> 64) as u64;
            if instr.rc_bit() {
                ctx.update_cr_signed(0, ctx.gpr[instr.rd()] as i64);
            }
            ctx.pc += 4;
        }
```
</details>

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## Special Cases & Edge Conditions

- **Returns the high 64 bits of an unsigned 64×64 product.** Operands are zero-extended (treated as unsigned) before multiplication. Pair with [`mulldx`](mulldx.md) for the low 64 bits to form a full 128-bit unsigned product.
- **No `OE` bit.** No overflow signal — the high half is a defined function of the inputs even when the product fills 128 bits.
- **Xenia uses native `u128`.** [`interpreter.rs:284`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L284) widens both operands then shifts. Note `as u128` *zero*-extends, in contrast to [`mulhdx`](mulhdx.md)'s `as i64 as i128` which sign-extends — this is the entire semantic difference.
- **`Rc=1` CR0 update is correctly 64-bit.** Uses `as i64` directly. Because the high half is unsigned, treating it as signed for CR0 means very large unsigned values appear `LT` — keep this in mind when interpreting the CR0 bits after `mulhdu.`.
- **Used in reciprocal-multiply division strategies.** Compilers may strength-reduce divide-by-constant into `mulhdu` plus a shift; appears in optimised disassembly.
- **Slow.** Same multi-cycle cost as the signed variant.

## Related Instructions

- [`mulldx`](mulldx.md) — low 64 bits of the same unsigned product.
- [`mulhdx`](mulhdx.md) — signed high half.
- [`mulhwux`](mulhwux.md) — 32-bit unsigned high half.
- [`divdux`](divdux.md) — 64-bit unsigned divide; sometimes replaced by reciprocal `mulhdu`.

## IBM Reference

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