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

# `subfx` — Subtract From

> **Category:** [Integer ALU](../categories/alu.md) · **Form:** [XO](../forms/XO.md) · **Opcode:** `0x7c000050`

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## Assembler Mnemonics

| Mnemonic | XML entry | Flags | Description |
| --- | --- | --- | --- |
| `subf` | `subfx` | — | Subtract From |
| `subfo` | `subfx` | OE=1 | Subtract From |
| `subf.` | `subfx` | Rc=1 | Subtract From |
| `subfo.` | `subfx` | OE=1, Rc=1 | Subtract From |

## Syntax

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

## Encoding

### `subfx` — form `XO`

- **Opcode word:** `0x7c000050`
- **Primary opcode (bits 0–5):** `31`
- **Extended opcode:** `40`
- **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` | subfx: read | Source GPR (`r0`–`r31`). |
| `RB` | subfx: read | Source GPR. |
| `RD` | subfx: write | Destination GPR. |
| `CR` | subfx: 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` | subfx: write (conditional) | Overflow-enable bit. When 1, the instruction updates `XER[OV]` and stickies `XER[SO]` on signed overflow. |

## Register Effects

### `subfx`

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

## Status-Register Effects

- `subfx`: **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       ; = (RB) − (RA)
```

## 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

**`subfx`**
- xenia-canary XML: [`tools/ppc-instructions.xml` — search for `mnem="subfx"`](../../xenia-canary/tools/ppc-instructions.xml)
- xenia-canary emit: [`src/xenia/cpu/ppc/ppc_emit_alu.cc:427`](../../xenia-canary/src/xenia/cpu/ppc/ppc_emit_alu.cc#L427)
- 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:863`](../../xenia-rs/crates/xenia-cpu/src/decoder.rs#L863)
- xenia-rs interpreter: [`crates/xenia-cpu/src/interpreter.rs:255-269`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L255-L269)
<details><summary>xenia-rs interpreter body (frozen snapshot)</summary>

```rust
        PpcOpcode::subfx => {
            // PPCBUG-017+020: 32-bit truncation.
            let ra32 = ctx.gpr[instr.ra()] as u32;
            let rb32 = ctx.gpr[instr.rb()] as u32;
            let result32 = rb32.wrapping_sub(ra32);
            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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## Extended Pseudocode

```
RT <- ~(RA) + (RB) + 1                       ; = (RB) − (RA)
if OE then
    XER[OV] <- signed_overflow_of_subtract((RB), (RA), RT)
    XER[SO] <- XER[SO] | XER[OV]
if Rc then
    CR0[LT,GT,EQ] <- signed_compare(RT, 0)
    CR0[SO]      <- XER[SO]
```

## Special Cases & Edge Conditions

- **Operand order gotcha.** `subf RT, RA, RB` computes `RT ← RB − RA`, **not** `RA − RB`. This reverses the intuitive ordering seen in x86/ARM. The assembler exposes a simplified mnemonic `sub RT, RX, RY` ≡ `subf RT, RY, RX` that restores the natural order — watch for both forms in disassembly.
- **Implemented as add-with-complement.** Hardware (and xenia) compute `~RA + RB + 1`. All overflow/CR semantics are the same as [`addx`](addx.md) with one operand complemented.
- **No `XER[CA]` update** — use [`subfcx`](subfcx.md) if you need a borrow-out bit.
- **No trap on overflow.** `subfo` / `subfo.` only record the event in `XER[OV]` and sticky-set `XER[SO]`.
- **Signed-overflow predicate.** `OV = ((RA ^ RB) & (RB ^ RT)) >> 63` — set when operands have different signs and the result's sign differs from `RB`'s.
- **64-bit CR update on Xenon** (xenia-rs truncates to 32 bits; see [`addx`](addx.md) note).

## Related Instructions

- [`subfcx`](subfcx.md) — subtract-from producing `XER[CA]` (borrow-out).
- [`subfex`](subfex.md) — `~RA + RB + XER[CA]` (subtract-with-borrow chain).
- [`subfmex`](subfmex.md), [`subfzex`](subfzex.md) — subtract-from `−1` / `0` with carry-in (propagates borrows).
- [`subfic`](subfic.md) — D-form: `RT ← SIMM − RA` with `XER[CA]`.
- [`negx`](negx.md) — specialises to `0 − RA`.
- [`addx`](addx.md) — inverse; shares overflow machinery.

## IBM Reference

- [AIX 7.3 — `subf` (Subtract From)](https://www.ibm.com/docs/en/aix/7.3.0?topic=set-subf-subtract-from-instruction)
- [AIX 7.3 — `sub` (simplified mnemonic)](https://www.ibm.com/docs/en/aix/7.3.0?topic=mnemonics-sub-subtract)