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

# `addx` — Add

> **Category:** [Integer ALU](../categories/alu.md) · **Form:** [XO](../forms/XO.md) · **Opcode:** `0x7c000214`

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## Assembler Mnemonics

| Mnemonic | XML entry | Flags | Description |
| --- | --- | --- | --- |
| `add` | `addx` | — | Add |
| `addo` | `addx` | OE=1 | Add |
| `add.` | `addx` | Rc=1 | Add |
| `addo.` | `addx` | OE=1, Rc=1 | Add |

## Syntax

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

## Encoding

### `addx` — form `XO`

- **Opcode word:** `0x7c000214`
- **Primary opcode (bits 0–5):** `31`
- **Extended opcode:** `266`
- **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` | addx: read | Source GPR (`r0`–`r31`). |
| `RB` | addx: read | Source GPR. |
| `RD` | addx: write | Destination GPR. |
| `OE` | addx: write (conditional) | Overflow-enable bit. When 1, the instruction updates `XER[OV]` and stickies `XER[SO]` on signed overflow. |
| `CR` | addx: 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

### `addx`

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

## Status-Register Effects

- `addx`: **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)
```

## C Translation Example

```c
/* add / add. / addo / addo.  (XO-form)                         */
uint64_t a = r[insn.RA], b = r[insn.RB];
uint64_t result = a + b;
r[insn.RT] = result;
if (insn.OE) { bool ov = (~(a ^ b) & (a ^ result)) >> 63;
               if (ov) { xer.OV = 1; xer.SO = 1; } else xer.OV = 0; }
if (insn.Rc) update_cr0_signed((int64_t)result);
```

## Implementation References

**`addx`**
- xenia-canary XML: [`tools/ppc-instructions.xml` — search for `mnem="addx"`](../../xenia-canary/tools/ppc-instructions.xml)
- xenia-canary emit: [`src/xenia/cpu/ppc/ppc_emit_alu.cc:50`](../../xenia-canary/src/xenia/cpu/ppc/ppc_emit_alu.cc#L50)
- 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:875`](../../xenia-rs/crates/xenia-cpu/src/decoder.rs#L875)
- xenia-rs interpreter: [`crates/xenia-cpu/src/interpreter.rs:175-189`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L175-L189)
<details><summary>xenia-rs interpreter body (frozen snapshot)</summary>

```rust
        PpcOpcode::addx => {
            // PPCBUG-012+020: 32-bit ABI writeback truncation + CR0 i32 view.
            let ra32 = ctx.gpr[instr.ra()] as u32;
            let rb32 = ctx.gpr[instr.rb()] as u32;
            let result32 = ra32.wrapping_add(rb32);
            ctx.gpr[instr.rd()] = result32 as u64;
            if instr.oe() {
                let true_sum = (ra32 as i32 as i128) + (rb32 as i32 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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## Extended Pseudocode

```
RT <- (RA) + (RB)                                ; modulo 2^64, carry discarded
if OE then
    XER[OV] <- (~(RA ^ RB) & (RA ^ RT))[0]       ; signed overflow: same-sign inputs, opposite-sign result
    XER[SO] <- XER[SO] | XER[OV]
if Rc then
    CR0[LT,GT,EQ] <- signed_compare(RT, 0)        ; 64-bit comparison on the Xenon
    CR0[SO]      <- XER[SO]
```

## Special Cases & Edge Conditions

- **No trap on overflow.** `addo` / `addo.` record overflow in `XER[OV]` and sticky-set `XER[SO]`. A trap can only be produced by a separate `td`/`tw` instruction examining the result.
- **Signed-overflow predicate.** Overflow occurs iff both addends share a sign bit and the result has the opposite sign bit: `OV = ((~(a ^ b)) & (a ^ rt)) >> 63`. Unsigned carry is *not* tracked — use [`addcx`](addcx.md) when you need `XER[CA]`.
- **`XER[SO]` is sticky.** Once set, it remains set until cleared by `mcrxr`. The `.` record forms copy it into `CR0[SO]`.
- **64-bit CR update on Xenon.** The Xbox 360 Xenon CPU is 64-bit, so `add.` compares the full 64-bit result against zero. **Xenia-rs presently truncates to 32 bits** before the CR update (`result as i32 as i64` in [`interpreter.rs:95`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L95)). If your translator must match xenia bit-for-bit, emit a 32-bit compare; if it must be spec-correct, emit a 64-bit compare. Most Xbox 360 object code works either way because results that overflow 32 bits are rare outside of explicit 64-bit math.
- **OE overflow detection not emulated in xenia-rs.** The `addo` / `addo.` branch in `interpreter.rs` is a `TODO` stub. A faithful translator should still emit the overflow check — titles rarely observe `XER[OV]`, but it's occasionally used by profiling / sanity-checking code paths.
- **Operand aliasing.** `add r3, r3, r3`, `add r3, r3, r4`, `add r3, r4, r3` are all legal. The addition reads both source operands before writing `RT`.
- **No immediate form.** For `RT = RA + imm` use [`addi`](addi.md) / [`addis`](addis.md). Those are distinct opcodes, not a flag on `add`.

## Related Instructions

- [`addcx`](addcx.md) — produces the carry-out in `XER[CA]`.
- [`addex`](addex.md) — sums `(RA) + (RB) + XER[CA]` (carry-in chain).
- [`addmex`](addmex.md), [`addzex`](addzex.md) — add to `−1` or `0` with carry-in (used to propagate borrows across multiword subtracts).
- [`addi`](addi.md), [`addis`](addis.md) — D-form immediate adds; no `Rc`/`OE`.
- [`addic`](addic.md), [`addicx`](addicx.md) — D-form adds that set `XER[CA]`.
- [`subfx`](subfx.md) — the dual: `RT ← (RB) − (RA)`.
- [`negx`](negx.md) — two's-complement negate.

## IBM Reference

- [AIX 7.3 — `add` (Add)](https://www.ibm.com/docs/en/aix/7.3.0?topic=set-add-instruction)
- [PowerISA v2.07B, Book I, §3.3.8 — Fixed-Point Arithmetic Instructions](https://openpowerfoundation.org/specifications/isa/) (overflow predicate, CR0 / `XER[SO]` semantics).