# `rldclx` — Rotate Left Doubleword then Clear Left

> **Category:** [Integer ALU](../categories/alu.md) · **Form:** [MDS](../forms/MDS.md) · **Opcode:** `0x78000010`

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## Assembler Mnemonics

| Mnemonic | XML entry | Flags | Description |
| --- | --- | --- | --- |
| `rldcl` | `rldclx` | — | Rotate Left Doubleword then Clear Left |
| `rldcl.` | `rldclx` | Rc=1 | Rotate Left Doubleword then Clear Left |

## Syntax

```asm
rldcl[Rc] [RA], [RS], [RB], [MB]
```

## Encoding

### `rldclx` — form `MDS`

- **Opcode word:** `0x78000010`
- **Primary opcode (bits 0–5):** `30`
- **Extended opcode:** —
- **Synchronising:** no

| Bits | Field | Meaning |
| --- | --- | --- |
| 0–5 | `OPCD` | primary opcode (30) |
| 6–10 | `RS` | source GPR |
| 11–15 | `RA` | destination GPR |
| 16–20 | `RB` | source B GPR |
| 21–26 | `mb/me` | 6-bit mask field (swapped halves) |
| 27–30 | `XO` | extended opcode |
| 31 | `Rc` | record-form flag |

## Operands

| Field | Role | Description |
| --- | --- | --- |
| `RS` | rldclx: read | Source GPR (alias for RD in some stores). |
| `RB` | rldclx: read | Source GPR. |
| `MB` | rldclx: read | Mask begin bit. |
| `RA` | rldclx: write | Source GPR (`r0`–`r31`). |
| `CR` | rldclx: write (conditional) | Condition-register update. When `Rc=1`, CR field 0 (or CR6 for vector compares, CR1 for FPU) is updated from the result. |

## Register Effects

### `rldclx`

- **Reads (always):** `RS`, `RB`, `MB`
- **Reads (conditional):** _none_
- **Writes (always):** `RA`
- **Writes (conditional):** `CR`

## Status-Register Effects

- `rldclx`: **CR0** ← signed-compare(result, 0) with `SO ← XER[SO]`, when `Rc=1`.

## 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

**`rldclx`**
- xenia-canary XML: [`tools/ppc-instructions.xml` — search for `mnem="rldclx"`](../../xenia-canary/tools/ppc-instructions.xml)
- xenia-canary emit: [`src/xenia/cpu/ppc/ppc_emit_alu.cc:856`](../../xenia-canary/src/xenia/cpu/ppc/ppc_emit_alu.cc#L856)
- xenia-rs opcode: [`crates/xenia-cpu/src/opcode.rs:61`](../../xenia-rs/crates/xenia-cpu/src/opcode.rs#L61)
- xenia-rs decoder: [`crates/xenia-cpu/src/decoder.rs:733`](../../xenia-rs/crates/xenia-cpu/src/decoder.rs#L733)
- xenia-rs interpreter: [`crates/xenia-cpu/src/interpreter.rs:802-811`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L802-L811)
<details><summary>xenia-rs interpreter body (frozen snapshot)</summary>

```rust
        PpcOpcode::rldclx => {
            let rs = ctx.gpr[instr.rs()];
            let sh = ctx.gpr[instr.rb()] & 0x3F;
            let mb = instr.mb_md();
            let rotated = rs.rotate_left(sh as u32);
            let mask = rld_mask_left(mb);
            ctx.gpr[instr.ra()] = rotated & mask;
            if instr.rc_bit() { ctx.update_cr_signed(0, ctx.gpr[instr.ra()] as i64); }
            ctx.pc += 4;
        }
```
</details>

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## Special Cases & Edge Conditions

- **`RA ← ROTL64(RS, RB[58:63]) & MASK(MB, 63)`.** Rotate `RS` left by `RB & 0x3F`, then *clear* bits to the left of `MB` — i.e. keep bits `MB..63`, force bits `0..MB-1` to zero.
- **Shift comes from a register.** Unlike [`rldiclx`](rldiclx.md), the rotate amount is dynamic. Only the low 6 bits of `RB` are used (`& 0x3F`); the upper 58 bits are silently ignored.
- **`MB` is a split 6-bit field.** Bit 5 of the encoded `mb/me` is *swapped* into bit position 5 (raw bit 30) — xenia decodes via `(instr.mb() << 1) | ((raw >> 1) & 1)` ([`interpreter.rs:587`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L587)). This MDS form is unusual; if you write a decoder, follow this exact bit assembly.
- **Mask generation.** `rld_mask_left(MB)` is `(1 << (64 - MB)) - 1` — i.e. clear bits `0..MB-1`, keep bits `MB..63`. When `MB = 0` the mask is all ones; when `MB = 63` only bit 63 survives.
- **`Rc=1` CR0 is correctly 64-bit.** [`interpreter.rs:592`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L592) uses `as i64` directly — no truncation. The rotate-and-mask family is one of the few xenia-rs instruction groups that already does the spec-correct 64-bit CR0 compare.
- **No `XER` effect.**
- **Use over [`rldiclx`](rldiclx.md)** when the shift amount is computed at runtime (e.g. via `cntlzd` for normalisation).

## Related Instructions

- [`rldcrx`](rldcrx.md) — sister: clear *right* instead of left.
- [`rldiclx`](rldiclx.md), [`rldicrx`](rldicrx.md), [`rldicx`](rldicx.md) — immediate-shift variants.
- [`rldimix`](rldimix.md) — rotate and mask insert.
- [`rlwnmx`](rlwnmx.md), [`rlwinmx`](rlwinmx.md) — 32-bit cousins.
- [`sldx`](sldx.md), [`srdx`](srdx.md) — preferred for plain 64-bit shifts.

## IBM Reference

- [AIX 7.3 — `rldcl` (Rotate Left Doubleword then Clear Left)](https://www.ibm.com/docs/en/aix/7.3.0?topic=set-rldcl-rotate-left-double-word-then-clear-left-instruction)