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

# `addzex` — Add to Zero Extended

> **Category:** [Integer ALU](../categories/alu.md) · **Form:** [XO](../forms/XO.md) · **Opcode:** `0x7c000194`

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## Assembler Mnemonics

| Mnemonic | XML entry | Flags | Description |
| --- | --- | --- | --- |
| `addze` | `addzex` | — | Add to Zero Extended |
| `addzeo` | `addzex` | OE=1 | Add to Zero Extended |
| `addze.` | `addzex` | Rc=1 | Add to Zero Extended |
| `addzeo.` | `addzex` | OE=1, Rc=1 | Add to Zero Extended |

## Syntax

```asm
addze[OE][Rc] [RD], [RA]
```

## Encoding

### `addzex` — form `XO`

- **Opcode word:** `0x7c000194`
- **Primary opcode (bits 0–5):** `31`
- **Extended opcode:** `202`
- **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` | addzex: read | Source GPR (`r0`–`r31`). |
| `CA` | addzex: read; addzex: write | XER[CA] carry bit. Read by add-with-carry/subtract-with-borrow instructions, written by carrying instructions. |
| `RD` | addzex: write | Destination GPR. |
| `OE` | addzex: write (conditional) | Overflow-enable bit. When 1, the instruction updates `XER[OV]` and stickies `XER[SO]` on signed overflow. |
| `CR` | addzex: 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

### `addzex`

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

## Status-Register Effects

- `addzex`: **CR0** ← signed-compare(result, 0) with `SO ← XER[SO]`, when `Rc=1`.; **XER[OV]** ← signed-overflow(result); **XER[SO]** stickies, when `OE=1`.; **XER[CA]** ← carry-out of the add / borrow-in of the subtract (always).

## Operation (pseudocode)

```
RT <- (RA) + CA
CA <- carry_out
```

## 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

**`addzex`**
- xenia-canary XML: [`tools/ppc-instructions.xml` — search for `mnem="addzex"`](../../xenia-canary/tools/ppc-instructions.xml)
- xenia-canary emit: [`src/xenia/cpu/ppc/ppc_emit_alu.cc:172`](../../xenia-canary/src/xenia/cpu/ppc/ppc_emit_alu.cc#L172)
- xenia-rs opcode: [`crates/xenia-cpu/src/opcode.rs:8`](../../xenia-rs/crates/xenia-cpu/src/opcode.rs#L8)
- xenia-rs decoder: [`crates/xenia-cpu/src/decoder.rs:870`](../../xenia-rs/crates/xenia-cpu/src/decoder.rs#L870)
- xenia-rs interpreter: [`crates/xenia-cpu/src/interpreter.rs:223-238`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L223-L238)
<details><summary>xenia-rs interpreter body (frozen snapshot)</summary>

```rust
        PpcOpcode::addzex => {
            // PPCBUG-015+020: 32-bit truncation.
            let ra32 = ctx.gpr[instr.ra()] as u32;
            let ca = ctx.xer_ca as u32;
            let result32 = ra32.wrapping_add(ca);
            ctx.xer_ca = if result32 < ra32 { 1 } else { 0 };
            ctx.gpr[instr.rd()] = result32 as u64;
            if instr.oe() {
                let true_sum = (ra32 as i32 as i128) + (ca as i128);
                overflow::apply(ctx, true_sum != (result32 as i32) as i128);
            }
            if instr.rc_bit() {
                ctx.update_cr_signed(0, result32 as i32 as i64);
            }
            ctx.pc += 4;
        }
```
</details>

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## Special Cases & Edge Conditions

- **No `RB` field used.** Like [`addmex`](addmex.md), this XO-form instruction ignores the `RB` slot. Assemblers emit zero there.
- **Operation is `RA + 0 + CA` ≡ `RA + CA`.** Used to terminate the *high word* of a multi-word add chain seeded by [`addcx`](addcx.md). After the low-word `addc` produces the carry, all middle words use [`addex`](addex.md), and the final word that has no register operand uses `addze`.
- **Carry-out is the simple unsigned overflow test** `result < ra` — same predicate as [`addcx`](addcx.md). `CA' = 1` only if `RA == ~0 && CA == 1`.
- **`OE=1` not implemented in xenia-rs.** The interpreter has no overflow branch at all; spec asks for the standard signed-overflow detect.
- **64-bit CR update on Xenon, 32-bit in xenia-rs** (truncation in [`interpreter.rs:128`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L128) — see [`addx`](addx.md) for context).
- **Common idiom: extracting a carry as a 0/1.** `addze rT, 0` (or `addze rT, rN` where `rN == 0`) materialises `XER[CA]` into `rT` as a plain integer.

## Related Instructions

- [`addmex`](addmex.md) — terminate with `RA + (−1) + CA` instead of `+0`.
- [`addex`](addex.md) — middle of a multi-word add chain.
- [`addcx`](addcx.md) — seeds the chain.
- [`subfzex`](subfzex.md) — subtract dual: `~RA + 0 + CA`.

## IBM Reference

- [AIX 7.3 — `addze` (Add to Zero Extended)](https://www.ibm.com/docs/en/aix/7.3.0?topic=set-addze-add-zero-extended-instruction)