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

# `addmex` — Add to Minus One Extended

> **Category:** [Integer ALU](../categories/alu.md) · **Form:** [XO](../forms/XO.md) · **Opcode:** `0x7c0001d4`

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## Assembler Mnemonics

| Mnemonic | XML entry | Flags | Description |
| --- | --- | --- | --- |
| `addme` | `addmex` | — | Add to Minus One Extended |
| `addmeo` | `addmex` | OE=1 | Add to Minus One Extended |
| `addme.` | `addmex` | Rc=1 | Add to Minus One Extended |
| `addmeo.` | `addmex` | OE=1, Rc=1 | Add to Minus One Extended |

## Syntax

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

## Encoding

### `addmex` — form `XO`

- **Opcode word:** `0x7c0001d4`
- **Primary opcode (bits 0–5):** `31`
- **Extended opcode:** `234`
- **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` | addmex: read | Source GPR (`r0`–`r31`). |
| `CA` | addmex: read; addmex: write | XER[CA] carry bit. Read by add-with-carry/subtract-with-borrow instructions, written by carrying instructions. |
| `RD` | addmex: write | Destination GPR. |
| `OE` | addmex: write (conditional) | Overflow-enable bit. When 1, the instruction updates `XER[OV]` and stickies `XER[SO]` on signed overflow. |
| `CR` | addmex: 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

### `addmex`

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

## Status-Register Effects

- `addmex`: **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 + 0xFFFF_FFFF_FFFF_FFFF
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

**`addmex`**
- xenia-canary XML: [`tools/ppc-instructions.xml` — search for `mnem="addmex"`](../../xenia-canary/tools/ppc-instructions.xml)
- xenia-canary emit: [`src/xenia/cpu/ppc/ppc_emit_alu.cc:152`](../../xenia-canary/src/xenia/cpu/ppc/ppc_emit_alu.cc#L152)
- 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:873`](../../xenia-rs/crates/xenia-cpu/src/decoder.rs#L873)
- xenia-rs interpreter: [`crates/xenia-cpu/src/interpreter.rs:239-254`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L239-L254)
<details><summary>xenia-rs interpreter body (frozen snapshot)</summary>

```rust
        PpcOpcode::addmex => {
            // PPCBUG-016+020: 32-bit truncation. RT = RA + CA - 1.
            let ra32 = ctx.gpr[instr.ra()] as u32;
            let ca = ctx.xer_ca as u32;
            let result32 = ra32.wrapping_add(ca).wrapping_sub(1);
            ctx.xer_ca = if ra32 != 0 || ca != 0 { 1 } else { 0 };
            ctx.gpr[instr.rd()] = result32 as u64;
            if instr.oe() {
                let true_sum = (ra32 as i32 as i128) + (ca as i128) - 1;
                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.** `addmex` is encoded in XO-form but ignores the `RB` slot — assemblers must still emit a value (typically zero). Disassemblers that parse a non-zero `RB` should not flag it as illegal; it is simply unused.
- **Operation is `RA + CA + (−1)`**, i.e. `RA - 1 + CA`. Used to terminate a multi-word *subtract* chain when the high source word is implicitly all-ones (e.g. computing `-x` as `~x + 1` across 128 bits).
- **Carry-out predicate is `RA != 0 OR CA != 0`.** Equivalently, `CA' = NOT(RA == 0 AND CA == 0)`. Adding `−1` to anything except a zero-with-no-carry produces a carry-out (no borrow needed). This terse form in xenia-rs is correct but easy to misread.
- **Overflow not implemented in xenia-rs.** The `OE=1` path is silently a no-op; spec says set `XER[OV]` if the signed result wraps.
- **64-bit CR update on Xenon, 32-bit in xenia-rs.** [`interpreter.rs:139`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L139) — same quirk as the rest of the add family.
- **`XER[CA]` must be initialised** by an earlier carrying instruction. `addme` is a *terminator*, not a seed.

## Related Instructions

- [`addzex`](addzex.md) — terminate a chain with `RA + 0 + CA` (no `−1`).
- [`addex`](addex.md) — middle-of-chain `RA + RB + CA`.
- [`addcx`](addcx.md) — seeds a chain.
- [`subfmex`](subfmex.md) — subtract dual: `~RA + (−1) + CA`.
- [`negx`](negx.md) — single-instruction two's-complement negate.

## IBM Reference

- [AIX 7.3 — `addme` (Add to Minus One Extended)](https://www.ibm.com/docs/en/aix/7.3.0?topic=set-addme-add-minus-one-extended-instruction)