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

# `mulldx` — Multiply Low Doubleword

> **Category:** [Integer ALU](../categories/alu.md) · **Form:** [XO](../forms/XO.md) · **Opcode:** `0x7c0001d2`

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## Assembler Mnemonics

| Mnemonic | XML entry | Flags | Description |
| --- | --- | --- | --- |
| `mulld` | `mulldx` | — | Multiply Low Doubleword |
| `mulldo` | `mulldx` | OE=1 | Multiply Low Doubleword |
| `mulld.` | `mulldx` | Rc=1 | Multiply Low Doubleword |
| `mulldo.` | `mulldx` | OE=1, Rc=1 | Multiply Low Doubleword |

## Syntax

```asm
mulld[OE][Rc] [RD], [RA], [RB]
```

## Encoding

### `mulldx` — form `XO`

- **Opcode word:** `0x7c0001d2`
- **Primary opcode (bits 0–5):** `31`
- **Extended opcode:** `233`
- **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` | mulldx: read | Source GPR (`r0`–`r31`). |
| `RB` | mulldx: read | Source GPR. |
| `RD` | mulldx: write | Destination GPR. |
| `CR` | mulldx: write (conditional) | Condition-register update. When `Rc=1`, CR field 0 (or CR6 for vector compares, CR1 for FPU) is updated from the result. |
| `OE` | mulldx: write (conditional) | Overflow-enable bit. When 1, the instruction updates `XER[OV]` and stickies `XER[SO]` on signed overflow. |

## Register Effects

### `mulldx`

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

## Status-Register Effects

- `mulldx`: **CR0** ← signed-compare(result, 0) with `SO ← XER[SO]`, when `Rc=1`.; **XER[OV]** ← signed-overflow(result); **XER[SO]** stickies, when `OE=1`.

## Operation (pseudocode)

```
RT <- ((RA) * (RB))[64:127]               ; low 64 of signed 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

**`mulldx`**
- xenia-canary XML: [`tools/ppc-instructions.xml` — search for `mnem="mulldx"`](../../xenia-canary/tools/ppc-instructions.xml)
- xenia-canary emit: [`src/xenia/cpu/ppc/ppc_emit_alu.cc:368`](../../xenia-canary/src/xenia/cpu/ppc/ppc_emit_alu.cc#L368)
- 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:872`](../../xenia-rs/crates/xenia-cpu/src/decoder.rs#L872)
- xenia-rs interpreter: [`crates/xenia-cpu/src/interpreter.rs:433-444`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L433-L444)
<details><summary>xenia-rs interpreter body (frozen snapshot)</summary>

```rust
        PpcOpcode::mulldx => {
            let ra = ctx.gpr[instr.ra()] as i64;
            let rb = ctx.gpr[instr.rb()] as i64;
            ctx.gpr[instr.rd()] = ra.wrapping_mul(rb) as u64;
            if instr.oe() {
                overflow::apply(ctx, overflow::mulld_ov(ra, rb));
            }
            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 low 64 bits of a signed 64×64 product.** Equivalent to `(int64_t)(RA * RB)` modulo `2^64`. Both operands are full 64-bit signed; no truncation on input.
- **High bits silently lost.** The high 64 bits of the true product are discarded; pair with [`mulhdx`](mulhdx.md) (signed) or [`mulhdux`](mulhdux.md) (unsigned) to recover them.
- **`OE=1` should set `XER[OV]`** when the 128-bit signed product cannot be represented in 64 bits — i.e. when `mulhd RA, RB` is not the sign-extension of `mulld RA, RB`. **Xenia-rs does not implement** `OE` ([`interpreter.rs:264`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L264) has no `oe()` branch).
- **`Rc=1` CR0 update is correctly 64-bit.** [`interpreter.rs:269`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L269) uses `as i64` — full 64-bit signed compare. One of the few non-truncating CR0 sites in xenia-rs; means `mulld.` gives spec-correct CR0 even when the result has non-zero high 32 bits.
- **Same instruction for signed and unsigned low halves.** Modular arithmetic is identical; only the high half (`mulhd` vs `mulhdu`) distinguishes the interpretations.
- **Multi-cycle latency** — slowest of the ALU pipelines after divide.

## Related Instructions

- [`mulhdx`](mulhdx.md), [`mulhdux`](mulhdux.md) — signed/unsigned high halves of the same multiply.
- [`mullwx`](mullwx.md) — 32-bit signed multiply (low 64).
- [`mulli`](mulli.md) — D-form: `RT ← (RA[32:63]) × SIMM`.
- [`divdx`](divdx.md), [`divdux`](divdux.md) — 64-bit divide, often paired with `mulld` for remainders.

## IBM Reference

- [AIX 7.3 — `mulld` (Multiply Low Doubleword)](https://www.ibm.com/docs/en/aix/7.3.0?topic=set-mulld-multiply-low-double-word-instruction)