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

# `divwux` — Divide Word Unsigned

> **Category:** [Integer ALU](../categories/alu.md) · **Form:** [XO](../forms/XO.md) · **Opcode:** `0x7c000396`

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## Assembler Mnemonics

| Mnemonic | XML entry | Flags | Description |
| --- | --- | --- | --- |
| `divwu` | `divwux` | — | Divide Word Unsigned |
| `divwuo` | `divwux` | OE=1 | Divide Word Unsigned |
| `divwu.` | `divwux` | Rc=1 | Divide Word Unsigned |
| `divwuo.` | `divwux` | OE=1, Rc=1 | Divide Word Unsigned |

## Syntax

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

## Encoding

### `divwux` — form `XO`

- **Opcode word:** `0x7c000396`
- **Primary opcode (bits 0–5):** `31`
- **Extended opcode:** `459`
- **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` | divwux: read | Source GPR (`r0`–`r31`). |
| `RB` | divwux: read | Source GPR. |
| `RD` | divwux: write | Destination GPR. |
| `OE` | divwux: write (conditional) | Overflow-enable bit. When 1, the instruction updates `XER[OV]` and stickies `XER[SO]` on signed overflow. |
| `CR` | divwux: 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

### `divwux`

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

## Status-Register Effects

- `divwux`: **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)[32:63] /u (RB)[32:63]) zero-extended to 64 ; undefined if RB=0
```

## 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

**`divwux`**
- xenia-canary XML: [`tools/ppc-instructions.xml` — search for `mnem="divwux"`](../../xenia-canary/tools/ppc-instructions.xml)
- xenia-canary emit: [`src/xenia/cpu/ppc/ppc_emit_alu.cc:269`](../../xenia-canary/src/xenia/cpu/ppc/ppc_emit_alu.cc#L269)
- xenia-rs opcode: [`crates/xenia-cpu/src/opcode.rs:21`](../../xenia-rs/crates/xenia-cpu/src/opcode.rs#L21)
- xenia-rs decoder: [`crates/xenia-cpu/src/decoder.rs:877`](../../xenia-rs/crates/xenia-cpu/src/decoder.rs#L877)
- xenia-rs interpreter: [`crates/xenia-cpu/src/interpreter.rs:413-430`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L413-L430)
<details><summary>xenia-rs interpreter body (frozen snapshot)</summary>

```rust
        PpcOpcode::divwux => {
            // PPCBUG-020: 32-bit ABI CR0 view.
            let ra = ctx.gpr[instr.ra()] as u32;
            let rb = ctx.gpr[instr.rb()] as u32;
            let ov = overflow::divw_ov_unsigned(rb);
            if ov {
                ctx.gpr[instr.rd()] = 0;
            } else {
                ctx.gpr[instr.rd()] = (ra / rb) as u64;
            }
            if instr.oe() {
                overflow::apply(ctx, ov);
            }
            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

- **32-bit operands, zero-extended result.** Both `RA` and `RB` are read as their low 32 bits, unsigned (`as u32`); the quotient is computed as `u32`, then *zero-extended* to 64 bits. The high 32 bits of `RA`/`RB` are ignored on input and the high 32 bits of `RT` are zero on output.
- **Single undefined case.** Division by zero (`RB == 0`); xenia-rs returns 0 ([`interpreter.rs:251`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L251)). No `INT_MIN/-1` case because the operands are unsigned.
- **No trap on Xenon.** Same as [`divdx`](divdx.md) — silent undefined result.
- **`OE=1` should set `XER[OV]` on `RB == 0`**; xenia-rs ignores this.
- **`Rc=1` CR0 update.** [`interpreter.rs:256`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L256) uses `as i32 as i64` — for an unsigned 32-bit quotient stored in the low 32 bits with high zeros, this matches spec exactly; the i32 view will be negative iff the unsigned quotient ≥ 2^31. Worth flagging when comparing CR0 against zero after a large `divwu`.
- **Truncating quotient.** Floor division for non-negative integers; matches C `unsigned` semantics.
- **Same slow non-pipelined latency** as `divw`.

## Related Instructions

- [`divwx`](divwx.md) — signed 32-bit divide.
- [`divdux`](divdux.md), [`divdx`](divdx.md) — 64-bit variants.
- [`mullwx`](mullwx.md) — pair to recover the remainder.
- [`cmplwi`](cmpli.md) (simplified) — guard the divisor.

## IBM Reference

- [AIX 7.3 — `divwu` (Divide Word Unsigned)](https://www.ibm.com/docs/en/aix/7.3.0?topic=set-divwu-divide-word-unsigned-instruction)