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

# `subficx` — Subtract From Immediate Carrying

> **Category:** [Integer ALU](../categories/alu.md) · **Form:** [D](../forms/D.md) · **Opcode:** `0x20000000`

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## Assembler Mnemonics

| Mnemonic | XML entry | Flags | Description |
| --- | --- | --- | --- |
| `subfic` | `subficx` | — | Subtract From Immediate Carrying |

## Syntax

```asm
subfic [RD], [RA], [SIMM]
```

## Encoding

### `subficx` — form `D`

- **Opcode word:** `0x20000000`
- **Primary opcode (bits 0–5):** `8`
- **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 |
| --- | --- | --- |
| `RA` | subficx: read | Source GPR (`r0`–`r31`). |
| `SIMM` | subficx: read | 16-bit signed immediate. Sign-extended to 64 bits before use. |
| `RD` | subficx: write | Destination GPR. |
| `CA` | subficx: write | XER[CA] carry bit. Read by add-with-carry/subtract-with-borrow instructions, written by carrying instructions. |

## Register Effects

### `subficx`

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

## Status-Register Effects

- `subficx`: **XER[CA]** ← carry-out of the add / borrow-in of the subtract (always).

## 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

**`subficx`**
- xenia-canary XML: [`tools/ppc-instructions.xml` — search for `mnem="subficx"`](../../xenia-canary/tools/ppc-instructions.xml)
- xenia-canary emit: [`src/xenia/cpu/ppc/ppc_emit_alu.cc:459`](../../xenia-canary/src/xenia/cpu/ppc/ppc_emit_alu.cc#L459)
- xenia-rs opcode: [`crates/xenia-cpu/src/opcode.rs:83`](../../xenia-rs/crates/xenia-cpu/src/opcode.rs#L83)
- xenia-rs decoder: [`crates/xenia-cpu/src/decoder.rs:333`](../../xenia-rs/crates/xenia-cpu/src/decoder.rs#L333)
- xenia-rs interpreter: [`crates/xenia-cpu/src/interpreter.rs:155-164`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L155-L164)
<details><summary>xenia-rs interpreter body (frozen snapshot)</summary>

```rust
        PpcOpcode::subficx => {
            // PPCBUG-005: 32-bit ABI. Sign-extended imm has bits 32-63 set for
            // negative SIMM, poisoning the writeback. Canary uses 32-bit form.
            let ra32 = ctx.gpr[instr.ra()] as u32;
            let imm32 = instr.simm16() as i32 as u32;
            let result32 = imm32.wrapping_sub(ra32);
            ctx.xer_ca = if imm32 >= ra32 { 1 } else { 0 };
            ctx.gpr[instr.rd()] = result32 as u64;
            ctx.pc += 4;
        }
```
</details>

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## Special Cases & Edge Conditions

- **`RT ← SIMM − RA` with `XER[CA]` always set.** Note the operand order: the *immediate* is the minuend, not the subtrahend. `subfic rD, rA, 1` computes `1 - rA`, useful for negation-plus-one or one's-complement-style operations.
- **Immediate is sign-extended** to 64 bits before the subtract. So `subfic rD, rA, -1` computes `-1 - rA`, equivalent to `~rA`.
- **`XER[CA] = 1` when `SIMM >= RA`** (no borrow). Computed in xenia as `if imm >= ra` ([`interpreter.rs:73`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L73)) — comparing the sign-extended unsigned representations.
- **No `Rc`, no `OE`.** This D-form has no flag bits beyond the implicit `CA` write.
- **No record-form variant.** There is no `subfic.` in the ISA; if you need CR0 to also reflect the result, follow with a `cmpwi`.
- **Synthesised "subtract immediate"**. Assemblers sometimes accept `subi rD, rA, value` as a shorthand for `addi rD, rA, -value`, but for the carry-producing variant you must use `subfic` explicitly.
- **Common idiom: `subfic rD, rA, 0`** computes `-rA` and sets `CA` according to whether `rA == 0` (no borrow) or `rA != 0` (borrow). Equivalent to [`negx`](negx.md) when `CA` doesn't matter.

## Related Instructions

- [`subfcx`](subfcx.md) — XO-form register version.
- [`subfx`](subfx.md) — register subtract without `XER[CA]`.
- [`addic`](addic.md), [`addicx`](addicx.md) — dual: add immediate carrying.
- [`negx`](negx.md) — equivalent to `subfic rD, rA, 0` when `CA` is unwanted.

## IBM Reference

- [AIX 7.3 — `subfic` (Subtract From Immediate Carrying)](https://www.ibm.com/docs/en/aix/7.3.0?topic=set-subfic-subtract-from-immediate-carrying-instruction)