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

# `stmw` — Store Multiple Word

> **Category:** [Memory](../categories/memory.md) · **Form:** [D](../forms/D.md) · **Opcode:** `0xbc000000`

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## Assembler Mnemonics

| Mnemonic | XML entry | Flags | Description |
| --- | --- | --- | --- |
| `stmw` | `stmw` | — | Store Multiple Word |

## Syntax

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

## Encoding

### `stmw` — form `D`

- **Opcode word:** `0xbc000000`
- **Primary opcode (bits 0–5):** `47`
- **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

### `stmw`

- **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

**`stmw`**
- xenia-canary XML: [`tools/ppc-instructions.xml` — search for `mnem="stmw"`](../../xenia-canary/tools/ppc-instructions.xml)
- xenia-canary emit: [`src/xenia/cpu/ppc/ppc_emit_memory.cc:527`](../../xenia-canary/src/xenia/cpu/ppc/ppc_emit_memory.cc#L527)
- xenia-rs opcode: [`crates/xenia-cpu/src/opcode.rs:75`](../../xenia-rs/crates/xenia-cpu/src/opcode.rs#L75)
- xenia-rs decoder: [`crates/xenia-cpu/src/decoder.rs:370`](../../xenia-rs/crates/xenia-cpu/src/decoder.rs#L370)
- xenia-rs interpreter: [`crates/xenia-cpu/src/interpreter.rs:1735-1759`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L1735-L1759)
<details><summary>xenia-rs interpreter body (frozen snapshot)</summary>

```rust
        PpcOpcode::stmw => {
            let mut ea = if instr.ra() == 0 { 0u64 } else { ctx.gpr[instr.ra()] };
            ea = ea.wrapping_add(instr.d() as i64 as u64);
            // PPCBUG-160: stmw can span two cache lines when (32-rs)*4 > one line.
            // Iterate over every touched line so any reservation on a later line
            // is also invalidated (same guarantee as single-word stores).
            if let Some(t) = ctx.reservation_table.as_ref().filter(|t| t.is_enabled()) {
                if t.has_active_reservers() {
                    let start_ea = ea as u32;
                    let last_ea = start_ea.wrapping_add((32 - instr.rs() as u32) * 4).wrapping_sub(1);
                    let line_size = RESERVATION_MASK + 1;
                    let mut line = start_ea & !RESERVATION_MASK;
                    loop {
                        t.invalidate_for_write(line);
                        if line >= (last_ea & !RESERVATION_MASK) { break; }
                        line = line.wrapping_add(line_size);
                    }
                }
            }
            for r in instr.rs()..32 {
                mem.write_u32(ea as u32, ctx.gpr[r] as u32);
                ea = ea.wrapping_add(4);
            }
            ctx.pc += 4;
        }
```
</details>

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## Special Cases & Edge Conditions

- **Bulk register save.** Stores `(32 - RS)` consecutive 32-bit words taken from `r[RS]`, `r[RS+1]`, …, `r31` to memory starting at `EA`. The symmetric counterpart of [`lmw`](lmw.md). Used by AIX/PowerPC ABI prologues to save non-volatile GPRs in one instruction.
- **Each store is the low 32 bits of the GPR.** Xenia's snapshot writes `ctx.gpr[r] as u32` — only the low half of the 64-bit GPR. The high 32 bits are discarded; `stmw` cannot save 64-bit values (use a sequence of [`std`](std.md) instead).
- **Big-endian write.** Word from `r[RS]` lands at `EA`, word from `r[RS+1]` at `EA+4`, etc. Each word is itself written most-significant-byte first.
- **`RA0` semantics.** When `RA = 0`, base is the literal zero. Useful for absolute-address restoration.
- **Alignment.** PowerISA requires word-aligned `EA`; an unaligned `stmw` may raise an alignment exception on hardware. Xenia tolerates it.
- **Performance trap.** Modern PowerPC implementations microcode `stmw` — typically slower than the same number of `stw` instructions. Compilers prefer the unrolled form.
- **Cache-line behaviour.** When the run of words crosses several 128-byte cache lines, each cold line triggers a read-allocate. Pre-clearing with [`dcbz128`](dcbz.md) helps for fresh frames.

## Related Instructions

- [`lmw`](lmw.md) — symmetric "load multiple words" (the matching epilogue partner).
- [`stw`](stw.md), [`stwx`](stw.md) — single-word stores; the modern preferred form.
- [`stswi`](stswi.md), [`stswx`](stswx.md) — store string (byte-granular bulk transfer).
- [`std`](std.md) — for 64-bit values (no "store multiple doubleword" exists).

## IBM Reference

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