Sylpheed/tools/ppc-manual/alu/rldimix.md

# `rldimix` — Rotate Left Doubleword Immediate then Mask Insert

> **Category:** [Integer ALU](../categories/alu.md) · **Form:** [MD](../forms/MD.md) · **Opcode:** `0x7800000c`

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## Assembler Mnemonics

| Mnemonic | XML entry | Flags | Description |
| --- | --- | --- | --- |
| `rldimi` | `rldimix` | — | Rotate Left Doubleword Immediate then Mask Insert |
| `rldimi.` | `rldimix` | Rc=1 | Rotate Left Doubleword Immediate then Mask Insert |

## Syntax

```asm
rldimi[Rc] [RA], [RS], [SH], [MB]
```

## Encoding

### `rldimix` — form `MD`

- **Opcode word:** `0x7800000c`
- **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 | `sh` | shift amount low 5 bits |
| 21–26 | `mb/me` | 6-bit mask field (swapped halves) |
| 27–29 | `XO` | extended opcode |
| 30 | `sh5` | shift amount high bit |
| 31 | `Rc` | record-form flag |

## Operands

| Field | Role | Description |
| --- | --- | --- |
| `RS` | rldimix: read | Source GPR (alias for RD in some stores). |
| `SH` | rldimix: read | Shift amount. |
| `MB` | rldimix: read | Mask begin bit. |
| `RA` | rldimix: write | Source GPR (`r0`–`r31`). |
| `CR` | rldimix: 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

### `rldimix`

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

## Status-Register Effects

- `rldimix`: **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

**`rldimix`**
- xenia-canary XML: [`tools/ppc-instructions.xml` — search for `mnem="rldimix"`](../../xenia-canary/tools/ppc-instructions.xml)
- xenia-canary emit: [`src/xenia/cpu/ppc/ppc_emit_alu.cc:985`](../../xenia-canary/src/xenia/cpu/ppc/ppc_emit_alu.cc#L985)
- 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:731`](../../xenia-rs/crates/xenia-cpu/src/decoder.rs#L731)
- xenia-rs interpreter: [`crates/xenia-cpu/src/interpreter.rs:792-801`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L792-L801)
<details><summary>xenia-rs interpreter body (frozen snapshot)</summary>

```rust
        PpcOpcode::rldimix => {
            let rs = ctx.gpr[instr.rs()];
            let sh = instr.sh64();
            let mb = instr.mb_md();
            let rotated = rs.rotate_left(sh);
            let mask = rld_mask_left(mb) & rld_mask_right(63 - sh);
            ctx.gpr[instr.ra()] = (rotated & mask) | (ctx.gpr[instr.ra()] & !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, SH) & MASK) | (RA & ~MASK)`.** *Reads* the prior `RA` so it can preserve the bits outside the mask — this is the only `rld*` instruction with `RA` as both source and destination.
- **Mask is `MASK_LEFT(MB) AND MASK_RIGHT(63 - SH)`** ([`interpreter.rs:578`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L578)) — same span as [`rldicx`](rldicx.md), but the un-masked region is preserved in the destination instead of being zeroed.
- **Use to insert a bit-field.** Common idiom: `rldimi RA, RS, b, mask_start` writes `RS`'s low (`64 - mask_start`) bits into `RA` starting at bit `b`.
- **`SH` and `MB` decoding** is identical to the rest of the family (6-bit `sh` via `instr.sh64()`, 6-bit `mb` via the swap layout).
- **`Rc=1` CR0 is correctly 64-bit.** Uses `as i64` directly.
- **No `XER` effect.**
- **Compile-time pattern.** When you see `rldimi r3, r4, n, m`, the compiler is splicing a value into `r3`; recover the meaning by computing the mask `MASK(m, 63 - n)`.

## Related Instructions

- [`rldicx`](rldicx.md), [`rldiclx`](rldiclx.md), [`rldicrx`](rldicrx.md) — same form family, but they zero outside the mask instead of preserving.
- [`rlwimix`](rlwimix.md) — 32-bit insert cousin.
- [`rldclx`](rldclx.md), [`rldcrx`](rldcrx.md) — register-shift forms (no insert variant).

## IBM Reference

- [AIX 7.3 — `rldimi` (Rotate Left Doubleword Immediate then Mask Insert)](https://www.ibm.com/docs/en/aix/7.3.0?topic=set-rldimi-rotate-left-double-word-immediate-then-mask-insert-instruction)