xenia-rs/migration/project-root/ppc-manual/memory/lmw.md

# `lmw` — Load Multiple Word

> **Category:** [Memory](../categories/memory.md) · **Form:** [D](../forms/D.md) · **Opcode:** `0xb8000000`

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## Assembler Mnemonics

| Mnemonic | XML entry | Flags | Description |
| --- | --- | --- | --- |
| `lmw` | `lmw` | — | Load Multiple Word |

## Syntax

```asm
(no disassembly template)
```

## Encoding

### `lmw` — form `D`

- **Opcode word:** `0xb8000000`
- **Primary opcode (bits 0–5):** `46`
- **Extended opcode:** —
- **Synchronising:** no

| Bits | Field | Meaning |
| --- | --- | --- |
| 0–5 | `OPCD` | primary opcode |
| 6–10 | `RT` | destination GPR (or RS when storing) |
| 11–15 | `RA` | source GPR (0 ⇒ literal 0 for RA0 forms) |
| 16–31 | `D/SI/UI` | 16-bit signed or unsigned immediate |

## Operands

| Field | Role | Description |
| --- | --- | --- |

## Register Effects

### `lmw`

- **Reads (always):** _none_
- **Reads (conditional):** _none_
- **Writes (always):** _none_
- **Writes (conditional):** _none_

## Status-Register Effects

_No condition-register or status-register effects._

## 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

**`lmw`**
- xenia-canary XML: [`tools/ppc-instructions.xml` — search for `mnem="lmw"`](../../xenia-canary/tools/ppc-instructions.xml)
- xenia-canary emit: [`src/xenia/cpu/ppc/ppc_emit_memory.cc:705`](../../xenia-canary/src/xenia/cpu/ppc/ppc_emit_memory.cc#L705)
- xenia-rs opcode: [`crates/xenia-cpu/src/opcode.rs:42`](../../xenia-rs/crates/xenia-cpu/src/opcode.rs#L42)
- xenia-rs decoder: [`crates/xenia-cpu/src/decoder.rs:369`](../../xenia-rs/crates/xenia-cpu/src/decoder.rs#L369)
- xenia-rs interpreter: [`crates/xenia-cpu/src/interpreter.rs:1720-1734`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L1720-L1734)
<details><summary>xenia-rs interpreter body (frozen snapshot)</summary>

```rust
        PpcOpcode::lmw => {
            // PPCBUG-125: PowerISA marks `lmw` invalid when rA is in [rT..31];
            // canary skips the write to rA in that case to preserve the EA base.
            let mut ea = if instr.ra() == 0 { 0u64 } else { ctx.gpr[instr.ra()] };
            ea = ea.wrapping_add(instr.d() as i64 as u64);
            for r in instr.rd()..32 {
                if r == instr.ra() {
                    ea = ea.wrapping_add(4);
                    continue;
                }
                ctx.gpr[r] = mem.read_u32(ea as u32) as u64;
                ea = ea.wrapping_add(4);
            }
            ctx.pc += 4;
        }
```
</details>

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## Special Cases & Edge Conditions

- **Bulk register restore.** Loads `(32 - RT)` consecutive 32-bit words starting at `EA` into `RT`, `RT+1`, …, `r31`. Used by AIX/PowerPC ABI prologues/epilogues to restore non-volatile GPRs in one instruction. Modern compilers prefer multiple `lwz` for scheduling; `lmw` survives in older code and hand-rolled context-switch routines.
- **Loop bound from encoding.** Xenia's snapshot iterates `for r in instr.rd()..32`, exactly matching IBM's "load until r31 inclusive" semantic. With `RT = 28`, four registers (r28..r31) are loaded.
- **Each word is zero-extended.** Like `lwz`, every loaded 32-bit word zero-extends into the destination's 64-bit GPR. The high 32 bits of each `r[k]` become zero.
- **Big-endian read.** Word at `EA` goes to `r[RT]`, word at `EA+4` goes to `r[RT+1]`, etc. Each word is itself loaded most-significant-byte-first.
- **`RA0` semantics.** When `RA = 0`, base is literal zero. Useful for absolute-address restoration.
- **Invalid forms.** AIX docs declare it invalid for `RA` to be in the destination range `[RT, 31]` — a load could overwrite the base register mid-sequence. Xenia performs loads in order without this check.
- **Alignment.** PowerISA requires word-aligned `EA`; an unaligned `lmw` may raise an alignment exception on real hardware. Xenia tolerates it.
- **Performance trap.** On modern PowerPC implementations `lmw` is microcoded — slower than the equivalent sequence of `lwz`. Compilers avoid it.

## Related Instructions

- [`stmw`](stmw.md) — symmetric "store multiple words" (the matching epilogue/prologue partner).
- [`lwz`](lwz.md), [`lwzx`](lwz.md) — single-word loads; the modern preferred form.
- [`lswi`](lswi.md), [`lswx`](lswx.md) — load string (byte-granular bulk transfer).

## IBM Reference

- [AIX 7.3 — `lmw` (Load Multiple Word)](https://www.ibm.com/docs/en/aix/7.3.0?topic=set-lmw-load-multiple-word-instruction)
- `PowerISA v2.07B Book II` § "Load and Store Multiple" for invalid-form rules.