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

# `bcctrx` — Branch Conditional to Count Register

> **Category:** [Branch & System](../categories/branch.md) · **Form:** [XL](../forms/XL.md) · **Opcode:** `0x4c000420` · _sync_

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## Assembler Mnemonics

| Mnemonic | XML entry | Flags | Description |
| --- | --- | --- | --- |
| `bcctr` | `bcctrx` | — | Branch Conditional to Count Register |
| `bcctrl` | `bcctrx` | LK=1 | Branch Conditional to Count Register |

## Syntax

```asm
bcctr[LK] [BO], [BI]
```

## Encoding

### `bcctrx` — form `XL`

- **Opcode word:** `0x4c000420`
- **Primary opcode (bits 0–5):** `19`
- **Extended opcode:** `528`
- **Synchronising:** yes

| Bits | Field | Meaning |
| --- | --- | --- |
| 0–5 | `OPCD` | primary opcode (19) |
| 6–10 | `BT/BO` | target / branch options |
| 11–15 | `BA/BI` | source A / CR bit to test |
| 16–20 | `BB` | source B |
| 21–30 | `XO` | extended opcode (10 bits) |
| 31 | `LK` | link flag |

## Operands

| Field | Role | Description |
| --- | --- | --- |
| `LK` | bcctrx: read | Link bit. When 1, LR ← address-of-next-instruction before the branch is taken. |
| `BO` | bcctrx: read | 5-bit branch options — selects CTR decrement, CTR test polarity, and CR bit test polarity. See `forms/XL.md`. |
| `BI` | bcctrx: read | CR bit index (0–31) selected by BO's condition test. |
| `CR` | bcctrx: read | Condition-register update. When `Rc=1`, CR field 0 (or CR6 for vector compares, CR1 for FPU) is updated from the result. |
| `CTR` | bcctrx: read | Count register. Decremented and optionally tested by conditional branches when `BO[2]=0`. |
| `LR` | bcctrx: write (conditional) | Link register. Written by `bl`/`bla`/`bcl`/`bclrl`/`bcctrl`; read by `bclr`/`bclrl`. |

## Register Effects

### `bcctrx`

- **Reads (always):** `LK`, `BO`, `BI`, `CR`, `CTR`
- **Reads (conditional):** _none_
- **Writes (always):** _none_
- **Writes (conditional):** `LR`

## Status-Register Effects

_No condition-register or status-register effects._

## Operation (pseudocode)

```
cond_ok <- BO[0] | (CR[BI] ≡ BO[1])
if cond_ok then NIA <- CTR[0:61] || 0b00
if LK then LR <- CIA + 4
```

## 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

**`bcctrx`**
- xenia-canary XML: [`tools/ppc-instructions.xml` — search for `mnem="bcctrx"`](../../xenia-canary/tools/ppc-instructions.xml)
- xenia-canary emit: [`src/xenia/cpu/ppc/ppc_emit_control.cc:250`](../../xenia-canary/src/xenia/cpu/ppc/ppc_emit_control.cc#L250)
- xenia-rs opcode: [`crates/xenia-cpu/src/opcode.rs:11`](../../xenia-rs/crates/xenia-cpu/src/opcode.rs#L11)
- xenia-rs decoder: [`crates/xenia-cpu/src/decoder.rs:721`](../../xenia-rs/crates/xenia-cpu/src/decoder.rs#L721)
- xenia-rs interpreter: [`crates/xenia-cpu/src/interpreter.rs:962-981`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L962-L981)
<details><summary>xenia-rs interpreter body (frozen snapshot)</summary>

```rust
        PpcOpcode::bcctrx => {
            let bo = instr.bo();
            let bi = instr.bi();

            let cond_ok = (bo & 0b10000) != 0
                || (ctx.get_cr_bit(bi) == ((bo & 0b01000) != 0));

            if cond_ok {
                let next_pc = ctx.pc + 4;
                ctx.pc = (ctx.ctr as u32) & !3;
                if instr.lk() {
                    ctx.lr = next_pc as u64;
                }
            } else {
                if instr.lk() {
                    ctx.lr = (ctx.pc + 4) as u64;
                }
                ctx.pc += 4;
            }
        }
```
</details>

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## Special Cases & Edge Conditions

- **No CTR decrement.** Unlike [`bcx`](bcx.md) and [`bclrx`](bclrx.md), `bcctr` cannot decrement CTR (the CTR is the *target*). The PowerISA reserves `BO[2] = 0` encodings — they are *invalid* on `bcctrx`. xenia silently ignores `BO[2]`/`BO[3]` and treats every `bcctr` as a pure CR-conditional branch, which matches both the canary emit and real Xenon hardware behaviour.
- **CTR alignment mask.** The target is `CTR & ~3`. Like `bclr`, the low two bits are stripped — a misaligned CTR is silently rounded down rather than trapping.
- **BO encoding (CR-only subset).** Because CTR-test bits are unused, only four `BO` patterns are meaningful:

  | BO (binary) | Meaning |
  | --- | --- |
  | `0100z` | branch if `CR[BI]` false |
  | `0101z` | branch if `CR[BI]` true |
  | `1z1zz` | branch always (`bctr`) |
  | `0000z`/`001at`/etc. | reserved — implementation-defined |

- **Indirect call/dispatch idiom.** `mtctr rN; bctrl` is the canonical PPC indirect call: load function pointer into CTR, call. The xenia interpreter writes `LR ← CIA + 4` only when the branch is taken — this matches the PowerISA, but contrast with `bcx` where `LK` always writes LR (even if the branch is not taken). The C-translation reference handles this asymmetry explicitly.
- **`bctr` for switch tables.** Compilers emit `bctr` (not `bctrl`) for jump-table dispatch, with CTR loaded from a base + (index*4) lookup. Xenia honours this by simply jumping to `CTR & ~3`.
- **Synchronisation.** Marked `sync` in xenia's XML — context-synchronising. JIT backends must ensure prior side effects have committed before the indirect transfer.
- **No prediction hint sensitivity.** Xenon predicts indirect branches via a separate target cache; the `BO[4]` hint is mostly cosmetic for `bcctr`.

## Related Instructions

- [`bclrx`](bclrx.md) — branch conditional to **LR** (function returns).
- [`bcx`](bcx.md) — branch conditional to displacement (B-form).
- [`bx`](bx.md) — unconditional displacement branch (I-form).
- [`mtctr`](../control/mtspr.md), [`mfctr`](../control/mfspr.md) — load/read CTR via `mtspr 9` / `mfspr 9`.
- [`sc`](sc.md) — alternative control-flow exit (system call).

### Simplified Mnemonics

| Simplified | Expansion |
| --- | --- |
| `bctr`        | `bcctr BO=0b10100, BI=0` — unconditional indirect branch |
| `bctrl`       | `bcctrl BO=0b10100, BI=0` — unconditional indirect call |
| `beqctr crN`  | `bcctr BO=0b01100, BI=4·N+2` — call CTR if `crN.EQ` |
| `bnectr crN`  | `bcctr BO=0b00100, BI=4·N+2` — call CTR if `crN.NE` |
| `bltctr crN`  | `bcctr BO=0b01100, BI=4·N+0` — call CTR if `crN.LT` |
| `bgectr crN`  | `bcctr BO=0b00100, BI=4·N+0` — call CTR if `crN.GE` |
| `bgtctr crN`  | `bcctr BO=0b01100, BI=4·N+1` — call CTR if `crN.GT` |
| `blectr crN`  | `bcctr BO=0b00100, BI=4·N+1` — call CTR if `crN.LE` |

The unconditional `bctr`/`bctrl` are by far the most common in Xbox 360 disassembly (compiler-emitted indirect calls and switch dispatch).

## IBM Reference

- [AIX 7.3 — `bcctr` (Branch Conditional to Count Register)](https://www.ibm.com/docs/en/aix/7.3.0?topic=set-bcctr-bcctrl-branch-conditional-count-register-instruction)
- [AIX 7.3 — Branch simplified mnemonics](https://www.ibm.com/docs/en/aix/7.3.0?topic=mnemonics-branch-simplified)