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

# `rldcrx` — Rotate Left Doubleword then Clear Right

> **Category:** [Integer ALU](../categories/alu.md) · **Form:** [MDS](../forms/MDS.md) · **Opcode:** `0x78000012`

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## Assembler Mnemonics

| Mnemonic | XML entry | Flags | Description |
| --- | --- | --- | --- |
| `rldcr` | `rldcrx` | — | Rotate Left Doubleword then Clear Right |
| `rldcr.` | `rldcrx` | Rc=1 | Rotate Left Doubleword then Clear Right |

## Syntax

```asm
rldcr[Rc] [RA], [RS], [RB], [ME]
```

## Encoding

### `rldcrx` — form `MDS`

- **Opcode word:** `0x78000012`
- **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` | rldcrx: read | Source GPR (alias for RD in some stores). |
| `RB` | rldcrx: read | Source GPR. |
| `ME` | rldcrx: read | Mask end bit. |
| `RA` | rldcrx: write | Source GPR (`r0`–`r31`). |
| `CR` | rldcrx: 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

### `rldcrx`

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

## Status-Register Effects

- `rldcrx`: **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

**`rldcrx`**
- xenia-canary XML: [`tools/ppc-instructions.xml` — search for `mnem="rldcrx"`](../../xenia-canary/tools/ppc-instructions.xml)
- xenia-canary emit: [`src/xenia/cpu/ppc/ppc_emit_alu.cc:881`](../../xenia-canary/src/xenia/cpu/ppc/ppc_emit_alu.cc#L881)
- 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:734`](../../xenia-rs/crates/xenia-cpu/src/decoder.rs#L734)
- xenia-rs interpreter: [`crates/xenia-cpu/src/interpreter.rs:812-821`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L812-L821)
<details><summary>xenia-rs interpreter body (frozen snapshot)</summary>

```rust
        PpcOpcode::rldcrx => {
            let rs = ctx.gpr[instr.rs()];
            let sh = ctx.gpr[instr.rb()] & 0x3F;
            let me = instr.mb_md();
            let rotated = rs.rotate_left(sh as u32);
            let mask = rld_mask_right(me);
            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(0, ME)`.** Rotate `RS` left by `RB & 0x3F`, then *clear* bits to the right of `ME` — keep bits `0..ME`, force bits `ME+1..63` to zero.
- **Shift from register.** Same as [`rldclx`](rldclx.md): only the low 6 bits of `RB` count.
- **`ME` is a split 6-bit field.** Same swap-decoded layout as `MB` in `rldclx`: `(instr.mb() << 1) | ((raw >> 1) & 1)` ([`interpreter.rs:597`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L597)). Note that even though it represents `ME` here, xenia reads it from `instr.mb()` because the field shares the same encoding slot.
- **Mask generation.** `rld_mask_right(ME)` = `~((1 << (63 - ME)) - 1)` keeping bits `0..ME`. When `ME = 63` the mask is all ones; when `ME = 0` only bit 0 survives.
- **`Rc=1` CR0 is correctly 64-bit.** Uses `as i64` directly ([`interpreter.rs:602`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L602)).
- **No `XER` effect.**
- **Useful for left-shift with arbitrary discard.** `rldcr RA, RS, RB, 63 - n` is functionally close to a left-shift-and-mask sequence, with the rotate variant additionally allowing wrap-around.

## Related Instructions

- [`rldclx`](rldclx.md) — sister: clear *left* instead of right.
- [`rldicrx`](rldicrx.md) — immediate-shift form.
- [`rldicx`](rldicx.md), [`rldiclx`](rldiclx.md), [`rldimix`](rldimix.md) — full immediate rotate-and-mask family.
- [`sldx`](sldx.md) — plain 64-bit logical left shift (often a strength-reduced equivalent).

## IBM Reference

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