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

# `srawix` — Shift Right Algebraic Word Immediate

> **Category:** [Integer ALU](../categories/alu.md) · **Form:** [X](../forms/X.md) · **Opcode:** `0x7c000670`

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## Assembler Mnemonics

| Mnemonic | XML entry | Flags | Description |
| --- | --- | --- | --- |
| `srawi` | `srawix` | — | Shift Right Algebraic Word Immediate |
| `srawi.` | `srawix` | Rc=1 | Shift Right Algebraic Word Immediate |

## Syntax

```asm
srawi[Rc] [RA], [RS], [SH]
```

## Encoding

### `srawix` — form `X`

- **Opcode word:** `0x7c000670`
- **Primary opcode (bits 0–5):** `31`
- **Extended opcode:** `824`
- **Synchronising:** no

| Bits | Field | Meaning |
| --- | --- | --- |
| 0–5 | `OPCD` | primary opcode |
| 6–10 | `RT/FRT/VRT` | destination |
| 11–15 | `RA/FRA/VRA` | source A |
| 16–20 | `RB/FRB/VRB` | source B |
| 21–30 | `XO` | extended opcode (10 bits) |
| 31 | `Rc` | record-form flag |

## Operands

| Field | Role | Description |
| --- | --- | --- |
| `RS` | srawix: read | Source GPR (alias for RD in some stores). |
| `SH` | srawix: read | Shift amount. |
| `RA` | srawix: write | Source GPR (`r0`–`r31`). |
| `CR` | srawix: write (conditional) | Condition-register update. When `Rc=1`, CR field 0 (or CR6 for vector compares, CR1 for FPU) is updated from the result. |
| `CA` | srawix: write | XER[CA] carry bit. Read by add-with-carry/subtract-with-borrow instructions, written by carrying instructions. |

## Register Effects

### `srawix`

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

## Status-Register Effects

- `srawix`: **CR0** ← signed-compare(result, 0) with `SO ← XER[SO]`, when `Rc=1`.; **XER[CA]** ← carry-out of the add / borrow-in of the subtract (always).

## Operation (pseudocode)

```
RA <- ((RS)[32:63] >>a SH) sign-extended
CA <- (RS[32] signed) && any_low_bit_shifted_out
```

## 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

**`srawix`**
- xenia-canary XML: [`tools/ppc-instructions.xml` — search for `mnem="srawix"`](../../xenia-canary/tools/ppc-instructions.xml)
- xenia-canary emit: [`src/xenia/cpu/ppc/ppc_emit_alu.cc:1291`](../../xenia-canary/src/xenia/cpu/ppc/ppc_emit_alu.cc#L1291)
- xenia-rs opcode: [`crates/xenia-cpu/src/opcode.rs:65`](../../xenia-rs/crates/xenia-cpu/src/opcode.rs#L65)
- xenia-rs decoder: [`crates/xenia-cpu/src/decoder.rs:843`](../../xenia-rs/crates/xenia-cpu/src/decoder.rs#L843)
- xenia-rs interpreter: [`crates/xenia-cpu/src/interpreter.rs:661-675`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L661-L675)
<details><summary>xenia-rs interpreter body (frozen snapshot)</summary>

```rust
        PpcOpcode::srawix => {
            // PPCBUG-042+043 coupled: same shape as srawx for the sh-immediate form.
            let rs = ctx.gpr[instr.rs()] as i32;
            let sh = instr.sh();
            if sh == 0 {
                ctx.gpr[instr.ra()] = rs as u32 as u64;
                ctx.xer_ca = 0;
            } else {
                let result = rs >> sh;
                ctx.xer_ca = if rs < 0 && (rs as u32) << (32 - sh) != 0 { 1 } else { 0 };
                ctx.gpr[instr.ra()] = result as u32 as u64;
            }
            if instr.rc_bit() { ctx.update_cr_signed(0, ctx.gpr[instr.ra()] as u32 as i32 as i64); }
            ctx.pc += 4;
        }
```
</details>

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## Special Cases & Edge Conditions

- **`RA ← ((i32)RS >> SH) sign-extended`** with `XER[CA]` set when `RS` is negative AND any low bit was shifted out.
- **`SH` is 5 bits** (immediate, range `0..31`). Unlike [`srawx`](srawx.md), there is no saturation case because the count cannot exceed 31. Xenia reads it via `instr.sh()`.
- **`SH = 0`** sign-extends `RS[32:63]` to 64 bits and clears `CA`. This is *not* a no-op when `RS`'s high 32 bits differ from the sign extension of bit 32.
- **Common idiom: `srawi rA, rS, 31`** materialises the 32-bit sign of `rS` as `0` or `−1` — the canonical "sign mask" pattern. Often used for branchless `abs` or conditional negation.
- **Idiom: `srawi rA, rS, n; addze rA, rA`** — divide signed by `2^n` rounding toward zero.
- **`Rc=1` CR0 update truncates to 32 bits in xenia-rs.** [`interpreter.rs:457`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L457) — matches spec because the sign-extended result has consistent low/high 32-bit signs.
- **No `OE` bit.**

## Related Instructions

- [`srawx`](srawx.md) — register-shift form.
- [`sradix`](sradix.md), [`sradx`](sradx.md) — 64-bit arithmetic right.
- [`addzex`](addzex.md) — divide-rounding companion.
- [`extswx`](extswx.md) — `srawi rA, rS, 0` is functionally a sign-extend-32-to-64 plus `CA = 0` clear; `extsw` is preferred when CA isn't wanted.

## IBM Reference

- [AIX 7.3 — `srawi` (Shift Right Algebraic Word Immediate)](https://www.ibm.com/docs/en/aix/7.3.0?topic=set-srawi-shift-right-algebraic-word-immediate-instruction)