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

# `subfcx` — Subtract From Carrying

> **Category:** [Integer ALU](../categories/alu.md) · **Form:** [XO](../forms/XO.md) · **Opcode:** `0x7c000010`

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## Assembler Mnemonics

| Mnemonic | XML entry | Flags | Description |
| --- | --- | --- | --- |
| `subfc` | `subfcx` | — | Subtract From Carrying |
| `subfco` | `subfcx` | OE=1 | Subtract From Carrying |
| `subfc.` | `subfcx` | Rc=1 | Subtract From Carrying |
| `subfco.` | `subfcx` | OE=1, Rc=1 | Subtract From Carrying |

## Syntax

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

## Encoding

### `subfcx` — form `XO`

- **Opcode word:** `0x7c000010`
- **Primary opcode (bits 0–5):** `31`
- **Extended opcode:** `8`
- **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` | subfcx: read | Source GPR (`r0`–`r31`). |
| `RB` | subfcx: read | Source GPR. |
| `RD` | subfcx: write | Destination GPR. |
| `CR` | subfcx: write (conditional) | Condition-register update. When `Rc=1`, CR field 0 (or CR6 for vector compares, CR1 for FPU) is updated from the result. |
| `OE` | subfcx: write (conditional) | Overflow-enable bit. When 1, the instruction updates `XER[OV]` and stickies `XER[SO]` on signed overflow. |

## Register Effects

### `subfcx`

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

## Status-Register Effects

- `subfcx`: **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) + 1
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

**`subfcx`**
- xenia-canary XML: [`tools/ppc-instructions.xml` — search for `mnem="subfcx"`](../../xenia-canary/tools/ppc-instructions.xml)
- xenia-canary emit: [`src/xenia/cpu/ppc/ppc_emit_alu.cc:441`](../../xenia-canary/src/xenia/cpu/ppc/ppc_emit_alu.cc#L441)
- 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:859`](../../xenia-rs/crates/xenia-cpu/src/decoder.rs#L859)
- xenia-rs interpreter: [`crates/xenia-cpu/src/interpreter.rs:270-287`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L270-L287)
<details><summary>xenia-rs interpreter body (frozen snapshot)</summary>

```rust
        PpcOpcode::subfcx => {
            // PPCBUG-007: 32-bit ABI. The `rb >= ra` u64 unsigned compare is
            // exactly the shape that broke addis. Defensive 32-bit truncation
            // is required for correct CA even after upstream cleanup.
            let ra32 = ctx.gpr[instr.ra()] as u32;
            let rb32 = ctx.gpr[instr.rb()] as u32;
            let result32 = rb32.wrapping_sub(ra32);
            ctx.xer_ca = if rb32 >= ra32 { 1 } else { 0 };
            ctx.gpr[instr.rd()] = result32 as u64;
            if instr.oe() {
                let true_diff = (rb32 as i32 as i128) - (ra32 as i32 as i128);
                overflow::apply(ctx, true_diff != (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

- **`RT ← RB − RA` with `XER[CA]` set on no-borrow.** Same operand-order convention as [`subfx`](subfx.md): the *first* source is subtracted *from* the second.
- **`XER[CA] = 1` means *no borrow occurred*** — i.e. `RB >= RA` as unsigned. PowerISA encodes this as the carry-out of `~RA + RB + 1`, not as a borrow flag. Xenia's `if rb >= ra` test ([`interpreter.rs:157`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L157)) is the correct boolean encoding.
- **No trap on signed overflow.** `subfco` / `subfco.` set `XER[OV]` and sticky `XER[SO]`; xenia-rs leaves the `OE` arm unimplemented.
- **64-bit CR update on Xenon, 32-bit in xenia-rs.** [`interpreter.rs:160`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L160) truncates with `as i32 as i64`. Spec demands a full 64-bit signed compare for `subfc.`.
- **Seeds a multi-word subtract chain.** Use as the low-word op; continue with [`subfex`](subfex.md) for middle words and [`subfmex`](subfmex.md)/[`subfzex`](subfzex.md) for the high word.
- **Operand aliasing fine.** `subfc r3, r3, r3` always yields `0` with `CA = 1`.

## Related Instructions

- [`subfx`](subfx.md) — same op without `XER[CA]`.
- [`subfex`](subfex.md) — `~RA + RB + CA` (chain continuation).
- [`subfmex`](subfmex.md), [`subfzex`](subfzex.md) — chain terminators.
- [`subfic`](subficx.md) — D-form: `RT ← SIMM − RA` with `XER[CA]`.
- [`addcx`](addcx.md) — dual: addition seed-of-chain.

## IBM Reference

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