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

# `bclrx` — Branch Conditional to Link Register

> **Category:** [Branch & System](../categories/branch.md) · **Form:** [XL](../forms/XL.md) · **Opcode:** `0x4c000020` · _sync_

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## Assembler Mnemonics

| Mnemonic | XML entry | Flags | Description |
| --- | --- | --- | --- |
| `bclr` | `bclrx` | — | Branch Conditional to Link Register |
| `bclrl` | `bclrx` | LK=1 | Branch Conditional to Link Register |

## Syntax

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

## Encoding

### `bclrx` — form `XL`

- **Opcode word:** `0x4c000020`
- **Primary opcode (bits 0–5):** `19`
- **Extended opcode:** `16`
- **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` | bclrx: read | Link bit. When 1, LR ← address-of-next-instruction before the branch is taken. |
| `BO` | bclrx: read | 5-bit branch options — selects CTR decrement, CTR test polarity, and CR bit test polarity. See `forms/XL.md`. |
| `BI` | bclrx: read | CR bit index (0–31) selected by BO's condition test. |
| `CR` | bclrx: read (conditional) | Condition-register update. When `Rc=1`, CR field 0 (or CR6 for vector compares, CR1 for FPU) is updated from the result. |
| `CTR` | bclrx: read (conditional); bclrx: write (conditional) | Count register. Decremented and optionally tested by conditional branches when `BO[2]=0`. |
| `LR` | bclrx: write (conditional) | Link register. Written by `bl`/`bla`/`bcl`/`bclrl`/`bcctrl`; read by `bclr`/`bclrl`. |

## Register Effects

### `bclrx`

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

## Status-Register Effects

_No condition-register or status-register effects._

## Operation (pseudocode)

```
if ¬BO[2] then CTR <- CTR − 1
ctr_ok  <- BO[2] | ((CTR ≠ 0) XOR BO[3])
cond_ok <- BO[0] | (CR[BI] ≡ BO[1])
if ctr_ok & cond_ok then NIA <- LR[0:61] || 0b00
if LK then LR <- CIA + 4
```

## C Translation Example

```c
/* bclr/bclrl — branch conditional to LR                           */
if (!(insn.BO & 4)) ctr -= 1;
bool ctr_ok  = (insn.BO & 4) || ((ctr != 0) ^ !!(insn.BO & 2));
bool cond_ok = (insn.BO & 16) || (cr_bit(insn.BI) == !!(insn.BO & 8));
uint32_t next = pc + 4;
if (ctr_ok && cond_ok) pc = lr & ~3u; else pc = next;
if (insn.LK) lr = next;
```

## Implementation References

**`bclrx`**
- xenia-canary XML: [`tools/ppc-instructions.xml` — search for `mnem="bclrx"`](../../xenia-canary/tools/ppc-instructions.xml)
- xenia-canary emit: [`src/xenia/cpu/ppc/ppc_emit_control.cc:282`](../../xenia-canary/src/xenia/cpu/ppc/ppc_emit_control.cc#L282)
- 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:711`](../../xenia-rs/crates/xenia-cpu/src/decoder.rs#L711)
- xenia-rs interpreter: [`crates/xenia-cpu/src/interpreter.rs:939-961`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L939-L961)
<details><summary>xenia-rs interpreter body (frozen snapshot)</summary>

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

            if bo & 0b00100 == 0 {
                ctx.ctr = ctx.ctr.wrapping_sub(1);
            }

            let ctr_ok = (bo & 0b00100) != 0
                || (((ctx.ctr as u32) != 0) ^ ((bo & 0b00010) != 0));
            let cond_ok = (bo & 0b10000) != 0
                || (ctx.get_cr_bit(bi) == ((bo & 0b01000) != 0));

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

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## BO Encoding (5 bits)

`BO` controls two independent tests and two "hints". Bit 0 is the MSB.

| BO (binary) | CTR decrement? | CTR test | CR test | Meaning |
| --- | --- | --- | --- | --- |
| `0000z` | yes | `CTR ≠ 0` | `¬CR[BI]` | decrement, branch if CTR ≠ 0 **and** CR[BI] false |
| `0001z` | yes | `CTR = 0` | `¬CR[BI]` | decrement, branch if CTR = 0 **and** CR[BI] false |
| `001at` | yes | `CTR ≠ 0` / `CTR = 0` | — | decrement, branch on CTR only |
| `0100z` | no  | — | `¬CR[BI]` | branch if CR[BI] false |
| `0101z` | no  | — | `CR[BI]` | branch if CR[BI] true |
| `011at` | no  | — | — | branch always (`z` and `t` are prediction hints) |
| `1z00z` | yes | `CTR ≠ 0` | — | decrement, branch if CTR ≠ 0 |
| `1z01z` | yes | `CTR = 0` | — | decrement, branch if CTR = 0 |
| `1z1zz` | no  | — | — | branch always |

Bit **`BO[0]` = 1** disables the CR test; **`BO[2]` = 1** disables the CTR decrement/test. `BO[1]` and `BO[3]` select the polarity of each test. `BO[4]` is a branch-prediction hint (0 = not taken, 1 = taken; ignored on the Xenon in most cases).

The most common bclr instance in Xbox 360 disassembly is `BO = 0b10100` → `blr` (branch always to LR), the function epilogue. `BO = 0b01100, BI = 2` → `beqlr` (return if `cr0.EQ`), also common.

## Special Cases & Edge Conditions

- **LR alignment mask.** The target address is `LR & ~3` — the low 2 bits are cleared. This silently ignores a misaligned LR; incoming code should always produce 4-byte-aligned LR values.
- **Ordering of CTR decrement and branch.** The CTR is decremented **first**, then compared to zero **after** the decrement. So after `bdnz` at `CTR = 1`, the CTR becomes `0` and the branch is not taken.
- **Self-referential LR write.** `bclrl` writes `LR ← CIA + 4` **before** reading `LR` to set `NIA`. Per the PowerISA, `bclrl` reads the *old* `LR` for the branch target and writes the *new* `LR` with the return address, atomically from software's perspective. Xenia implements it this way (`next_pc` captured first, then `lr` written).
- **Branch prediction hints (`BO[4]`).** The Xenon does static prediction on the basis of these hints, but behaviour is architecturally unobservable. Translators may ignore them.
- **Synchronisation.** `bclr` is **context-synchronising** (hence the `sync` flag in xenia's XML). Translators must ensure side-effecting instructions preceding the branch have committed — trivial in a sequential C translation but relevant for JIT backends.
- **xenia's `LR_HALT_SENTINEL`.** Xenia sets `LR` to `0xBCBCBCBC` at thread start; when the top-level guest function returns via `blr`, the interpreter loop halts cleanly. Translators replicating guest behaviour don't need this — but if you generate a test harness, the sentinel is a convenient "function returned" signal.

## Related Instructions

- [`bcctrx`](bcctrx.md) — branch conditional to **CTR** (used by indirect calls / vtables).
- [`bcx`](bcx.md) — branch conditional to an immediate displacement (D-form).
- [`bx`](bx.md) — unconditional branch (I-form).
- [`mtlr`](../control/mtspr.md), [`mflr`](../control/mfspr.md) — set/get LR via `mtspr 8, …` / `mfspr …, 8`.
- [`sc`](sc.md) — system call (alternative control-flow exit).

## Simplified Mnemonics

Assemblers fold common `BO`/`BI` patterns to single mnemonics:

| Simplified | Expansion |
| --- | --- |
| `blr`       | `bclr BO=0b10100, BI=0`  — branch always to LR |
| `blrl`      | `bclrl BO=0b10100, BI=0` — branch always to LR with link (tail-call trampoline) |
| `beqlr crN` | `bclr BO=0b01100, BI=4·N+2` — return if `crN.EQ` |
| `bnelr crN` | `bclr BO=0b00100, BI=4·N+2` — return if `crN.NE` |
| `bltlr crN` | `bclr BO=0b01100, BI=4·N+0` — return if `crN.LT` |
| `bgelr crN` | `bclr BO=0b00100, BI=4·N+0` — return if `crN.GE` |
| `bgtlr crN` | `bclr BO=0b01100, BI=4·N+1` — return if `crN.GT` |
| `blelr crN` | `bclr BO=0b00100, BI=4·N+1` — return if `crN.LE` |

Xbox 360 disassemblers almost always emit the simplified form; the translation agent should learn to recognise them.

## IBM Reference

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