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

# `srawx` — Shift Right Algebraic Word

> **Category:** [Integer ALU](../categories/alu.md) · **Form:** [X](../forms/X.md) · **Opcode:** `0x7c000630`

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## Assembler Mnemonics

| Mnemonic | XML entry | Flags | Description |
| --- | --- | --- | --- |
| `sraw` | `srawx` | — | Shift Right Algebraic Word |
| `sraw.` | `srawx` | Rc=1 | Shift Right Algebraic Word |

## Syntax

```asm
sraw[Rc] [RA], [RS], [RB]
```

## Encoding

### `srawx` — form `X`

- **Opcode word:** `0x7c000630`
- **Primary opcode (bits 0–5):** `31`
- **Extended opcode:** `792`
- **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` | srawx: read | Source GPR (alias for RD in some stores). |
| `RB` | srawx: read | Source GPR. |
| `RA` | srawx: write | Source GPR (`r0`–`r31`). |
| `CR` | srawx: 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` | srawx: write | XER[CA] carry bit. Read by add-with-carry/subtract-with-borrow instructions, written by carrying instructions. |

## Register Effects

### `srawx`

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

## Status-Register Effects

- `srawx`: **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)

```
n <- (RB)[58:63]
RA <- ((RS)[32:63] >>a n) sign-extended
CA <- 1 if (signed RS < 0) && any_bit_shifted_out else 0
```

## 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

**`srawx`**
- xenia-canary XML: [`tools/ppc-instructions.xml` — search for `mnem="srawx"`](../../xenia-canary/tools/ppc-instructions.xml)
- xenia-canary emit: [`src/xenia/cpu/ppc/ppc_emit_alu.cc:1262`](../../xenia-canary/src/xenia/cpu/ppc/ppc_emit_alu.cc#L1262)
- 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:840`](../../xenia-rs/crates/xenia-cpu/src/decoder.rs#L840)
- xenia-rs interpreter: [`crates/xenia-cpu/src/interpreter.rs:642-660`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L642-L660)
<details><summary>xenia-rs interpreter body (frozen snapshot)</summary>

```rust
        PpcOpcode::srawx => {
            // PPCBUG-041+043 coupled: 32-bit ABI writeback truncation + CR0 i32.
            // CA logic is independently correct (uses u32 shifted-out test).
            let rs = ctx.gpr[instr.rs()] as i32;
            let sh = ctx.gpr[instr.rb()] as u32 & 0x3F;
            if sh == 0 {
                ctx.gpr[instr.ra()] = rs as u32 as u64;
                ctx.xer_ca = 0;
            } else if sh < 32 {
                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;
            } else {
                ctx.gpr[instr.ra()] = if rs < 0 { 0xFFFF_FFFFu64 } else { 0 };
                ctx.xer_ca = if rs < 0 { 1 } else { 0 };
            }
            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

- **32-bit arithmetic right shift, sign-extended to 64.** `RA ← ((i32)RS >> n) sign-extended`, with `XER[CA]` set when `RS[32] = 1` (negative) AND any low bit was shifted out.
- **Shift count is 6 bits**, `RB[58:63]`. Counts `≥ 32` saturate: `RA = -1` (all-ones, sign-extended) if `RS < 0`, else `0`. Xenia handles this in three branches ([`interpreter.rs:432-444`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L432-L444)).
- **`SH = 0`** sign-extends `RS` to 64 bits and clears `XER[CA]` — like `extsw`, but additionally writing CA.
- **Result is always sign-extended to 64 bits.** `RA[0:31]` matches the sign of `RA[32]`. This is the key difference from [`srwx`](srwx.md) (zero-extension).
- **`Rc=1` CR0 update truncates to 32 bits in xenia-rs.** [`interpreter.rs:443`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L443) — but since the result is sign-extended, the low 32 bits' sign matches the full 64-bit sign, so spec and xenia agree here.
- **Used with [`addzex`](addzex.md)** for signed divide by `2^n` rounding toward zero.
- **No `OE` bit.**

## Related Instructions

- [`srawix`](srawix.md) — immediate-shift form.
- [`sradx`](sradx.md), [`sradix`](sradix.md) — 64-bit arithmetic right shifts.
- [`srwx`](srwx.md) — 32-bit *logical* right shift (no `XER[CA]`).
- [`addzex`](addzex.md) — companion for divide-rounding idiom.
- [`slwx`](slwx.md) — left shift.

## IBM Reference

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