xenia-rs/migration/project-root/ppc-manual/branch/twi.md

# `twi` — Trap Word Immediate

> **Category:** [Branch & System](../categories/branch.md) · **Form:** [D](../forms/D.md) · **Opcode:** `0x0c000000`

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## Assembler Mnemonics

| Mnemonic | XML entry | Flags | Description |
| --- | --- | --- | --- |
| `twi` | `twi` | — | Trap Word Immediate |

## Syntax

```asm
tw [TO], [RA], [SIMM]
```

## Encoding

### `twi` — form `D`

- **Opcode word:** `0x0c000000`
- **Primary opcode (bits 0–5):** `3`
- **Extended opcode:** —
- **Synchronising:** no

| Bits | Field | Meaning |
| --- | --- | --- |
| 0–5 | `OPCD` | primary opcode |
| 6–10 | `RT` | destination GPR (or RS when storing) |
| 11–15 | `RA` | source GPR (0 ⇒ literal 0 for RA0 forms) |
| 16–31 | `D/SI/UI` | 16-bit signed or unsigned immediate |

## Operands

| Field | Role | Description |
| --- | --- | --- |
| `TO` | twi: read | Trap-on condition mask (5 bits) — LT, GT, EQ, LGT, LLT bits. |
| `RA` | twi: read | Source GPR (`r0`–`r31`). |
| `SIMM` | twi: read | 16-bit signed immediate. Sign-extended to 64 bits before use. |

## Register Effects

### `twi`

- **Reads (always):** `TO`, `RA`, `SIMM`
- **Reads (conditional):** _none_
- **Writes (always):** _none_
- **Writes (conditional):** _none_

## Status-Register Effects

_No condition-register or status-register effects._

## Operation (pseudocode)

```
; Pseudocode derives directly from the xenia-rs interpreter
; arm (see Implementation References). Operation semantics:
;   - Read source operands from the fields listed under Operands.
;   - Apply the arithmetic / logical / memory action described
;     in the Description field above.
;   - Write results to the destination register(s); update any
;     status bits enumerated under Status-Register Effects.
; Consult the IBM AIX reference link under IBM Reference for
; canonical PPC-style pseudocode where xenia's expression is
; terse.
```

## 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

**`twi`**
- xenia-canary XML: [`tools/ppc-instructions.xml` — search for `mnem="twi"`](../../xenia-canary/tools/ppc-instructions.xml)
- xenia-canary emit: [`src/xenia/cpu/ppc/ppc_emit_control.cc:601`](../../xenia-canary/src/xenia/cpu/ppc/ppc_emit_control.cc#L601)
- xenia-rs opcode: [`crates/xenia-cpu/src/opcode.rs:87`](../../xenia-rs/crates/xenia-cpu/src/opcode.rs#L87)
- xenia-rs decoder: [`crates/xenia-cpu/src/decoder.rs:328`](../../xenia-rs/crates/xenia-cpu/src/decoder.rs#L328)
- xenia-rs interpreter: [`crates/xenia-cpu/src/interpreter.rs:1762-1796`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L1762-L1796)
<details><summary>xenia-rs interpreter body (frozen snapshot)</summary>

```rust
        PpcOpcode::tw | PpcOpcode::twi | PpcOpcode::td | PpcOpcode::tdi => {
            // PPCBUG-063: save CIA before incrementing so a trap handler reads
            // the faulting instruction address, not CIA+4.
            // PPCBUG-065: log the SIMM type code on `twi 31, r0, IMM` (Xbox 360
            // typed-trap convention used by the CRT/kernel for C++ exception
            // class dispatch). The audit notes this is relevant to the Sylpheed
            // throw investigation; routing the type code via a payload requires
            // a StepResult enum extension that's deferred for now.
            let trap_pc = ctx.pc;
            let a = ctx.gpr[instr.ra()];
            let b = match instr.opcode {
                PpcOpcode::twi | PpcOpcode::tdi => instr.simm16() as i64 as u64,
                _ => ctx.gpr[instr.rb()],
            };
            let width = match instr.opcode {
                PpcOpcode::tw | PpcOpcode::twi => trap::TrapWidth::Word,
                _ => trap::TrapWidth::Doubleword,
            };
            let fired = trap::evaluate(instr.to(), a, b, width);
            if fired {
                let typed_trap_simm = if matches!(instr.opcode, PpcOpcode::twi)
                    && instr.to() == 31 && instr.ra() == 0 {
                    Some(instr.simm16() as u16)
                } else { None };
                tracing::warn!(
                    "Trap fired at {:#010x}: {:?} TO={} a={:#x} b={:#x}{}",
                    trap_pc, instr.opcode, instr.to(), a, b,
                    typed_trap_simm.map_or(String::new(), |t| format!(" typed_trap_simm={:#06x}", t))
                );
                // Leave ctx.pc at CIA (NOT NIA) so trap handlers / SEH delivery
                // can read the faulting instruction address from ctx.pc.
                return StepResult::Trap;
            }
            ctx.pc += 4;
        }
```
</details>

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## Special Cases & Edge Conditions

- **32-bit comparison against sign-extended immediate.** `SIMM` is a 16-bit signed value, sign-extended to 32 bits, then compared against the low 32 bits of `RA`. The high half of `RA` is *ignored*.
- **`TO` mask.** Identical to [`tw`](tw.md): bit 0 = signed LT, 1 = signed GT, 2 = EQ, 3 = unsigned LT (LGT), 4 = unsigned GT (LLT). Trap fires if any selected bit's condition is true.
- **`twi 31, 0, 0` is unconditional trap.** All `TO` bits set ⇒ guaranteed trap. The simplified mnemonic family (`twnei`, `twgei`, …) is much more common in real code: bound checks, null checks, integer-divide-by-zero pre-checks.
- **Compiler usage.** Xbox 360 GCC emits `twnei rN, -1` and similar to validate handle-style return values; the kernel handler turns the trap into an exception delivered to the title.
- **No register effects.** Side effect: Program interrupt → vector `0x700` with `SRR1[TRAP]=1`.
- **xenia simplification.** Same as the other three trap forms — xenia-rs unconditionally returns `StepResult::Trap` whenever it decodes any of `tdi`/`twi`/`td`/`tw`, regardless of the `TO` mask or operands. This means `twi 0, r0, 0` (architecturally a guaranteed-no-trap encoding) will spuriously fire under xenia. Keep this in mind when triaging unexpected trap signals.
- **No `Rc` / `OE`.** D-form trap immediates have neither.

## Related Instructions

- [`tw`](tw.md) — register-register 32-bit trap (X-form).
- [`tdi`](tdi.md) / [`td`](td.md) — 64-bit (doubleword) siblings.
- [`sc`](sc.md) — alternative synchronous kernel entry.
- [`cmpi`](../alu/cmpi.md), [`cmpli`](../alu/cmpli.md) — set CR for a subsequent [`bcx`](bcx.md) when you want a regular branch instead of a trap.

### Simplified Mnemonics

| Simplified                | Expansion                  | Triggered when |
| ---                       | ---                        | --- |
| `tweqi RA, value`         | `twi 4,  RA, value`        | `RA == EXTS(value)` |
| `twnei RA, value`         | `twi 24, RA, value`        | `RA != EXTS(value)` |
| `twlti RA, value`         | `twi 16, RA, value`        | signed less than |
| `twlei RA, value`         | `twi 20, RA, value`        | signed less or equal |
| `twgti RA, value`         | `twi 8,  RA, value`        | signed greater than |
| `twgei RA, value`         | `twi 12, RA, value`        | signed greater or equal |
| `twllti RA, value`        | `twi 2,  RA, value`        | unsigned less than |
| `twlgei RA, value`        | `twi 5,  RA, value`        | unsigned greater or equal |
| `twlgti RA, value`        | `twi 1,  RA, value`        | unsigned greater than |
| `twllei RA, value`        | `twi 6,  RA, value`        | unsigned less or equal |

## IBM Reference

- [AIX 7.3 — `twi` (Trap Word Immediate)](https://www.ibm.com/docs/en/aix/7.3.0?topic=set-twi-trap-word-immediate-instruction)
- [AIX 7.3 — Trap simplified mnemonics](https://www.ibm.com/docs/en/aix/7.3.0?topic=mnemonics-trap-simplified)
- PowerISA v2.07B, Book I §3.3.11 — fixed-point trap instructions.