# `srdx` — Shift Right Doubleword

> **Category:** [Integer ALU](../categories/alu.md) · **Form:** [X](../forms/X.md) · **Opcode:** `0x7c000436`

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## Assembler Mnemonics

| Mnemonic | XML entry | Flags | Description |
| --- | --- | --- | --- |
| `srd` | `srdx` | — | Shift Right Doubleword |
| `srd.` | `srdx` | Rc=1 | Shift Right Doubleword |

## Syntax

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

## Encoding

### `srdx` — form `X`

- **Opcode word:** `0x7c000436`
- **Primary opcode (bits 0–5):** `31`
- **Extended opcode:** `539`
- **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` | srdx: read | Source GPR (alias for RD in some stores). |
| `RB` | srdx: read | Source GPR. |
| `RA` | srdx: write | Source GPR (`r0`–`r31`). |
| `CR` | srdx: 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

### `srdx`

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

## Status-Register Effects

- `srdx`: **CR0** ← signed-compare(result, 0) with `SO ← XER[SO]`, when `Rc=1`.

## Operation (pseudocode)

```
n <- (RB)[57:63]
RA <- ((RS) >> n)                       if n < 64 else 0
```

## C Translation Example

```c
/* No hand-written C yet. Translate the Canary emitter snapshot   */
/* under Implementation References; its HIR maps directly:        */
/*   f.LoadGPR(n) / f.StoreGPR(n, v)  -> r[n] / r[n] = v          */
/*   f.LoadFPR / StoreFPR, f.LoadVR / StoreVR -> f[n], v[n]        */
/*   f.Load(ea, T), f.Store(ea, v) -> raw read / write; emitters   */
/*     wrap them in f.ByteSwap for the big-endian guest value      */
/*   f.UpdateCR(n, v)  -> CR field n from v's LOW 32 BITS vs 0     */
/*   f.LoadCA / f.StoreCA -> xer.CA;  f.StoreSAT -> vscr.SAT       */
/*   i.XO.RA, i.D.DS, ... -> the bit-fields listed under Operands  */
/* The Register Effects and Status-Register Effects tables above  */
/* enumerate every side effect a faithful translation must emit.  */
```

## Implementation References

**`srdx`**
- Canary XML: [`tools/ppc-instructions.xml` — search for `mnem="srdx"`](https://github.com/xenia-canary/xenia-canary/blob/f21ebd49e979e44f081f474df78c3fbfee9cb3f2/tools/ppc-instructions.xml)
- Canary emitter: [`src/xenia/cpu/ppc/ppc_emit_alu.cc:1161`](https://github.com/xenia-canary/xenia-canary/blob/f21ebd49e979e44f081f474df78c3fbfee9cb3f2/src/xenia/cpu/ppc/ppc_emit_alu.cc#L1161)
- Sylpheed opcode: [`crates/sylpheed-ppc/src/opcode.rs:222`](../../../crates/sylpheed-ppc/src/opcode.rs#L222)
- Sylpheed decoder: [`crates/sylpheed-ppc/src/decoder.rs:936`](../../../crates/sylpheed-ppc/src/decoder.rs#L936)
<details><summary>Canary emitter (frozen snapshot @ <code>f21ebd49e9</code>)</summary>

```cpp
int InstrEmit_srdx(PPCHIRBuilder& f, const InstrData& i) {
  // n <- (RB)[58:63]
  // r <- ROTL64((RS), 64-n)
  // if (RB)[57] = 0 then
  //   m <- MASK(n, 63)
  // else
  //   m <- i64.0
  // RA <- r & m
  Value* sh =
      f.And(f.Truncate(f.LoadGPR(i.X.RB), INT8_TYPE), f.LoadConstantInt8(0x7F));
  Value* v = f.Select(f.IsTrue(f.And(sh, f.LoadConstantInt8(0x40))),
                      f.LoadZeroInt64(), f.Shr(f.LoadGPR(i.X.RT), sh));
  f.StoreGPR(i.X.RA, v);
  if (i.X.Rc) {
    f.UpdateCR(0, v);
  }
  return 0;
}
```
</details>

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## Special Cases & Edge Conditions

- **64-bit logical right shift.** `RA ← RS >> (RB & 0x7F)` if the count is `< 64`, else `RA = 0`. Bits shifted in from the high end are zero (no sign extension).
- **Shift count is 7 bits** (`RB[57:63]`). Counts `64..127` produce zero, not `RS >> (count mod 64)`. Xenia respects this with `& 0x7F` and an explicit `if sh < 64` check ([`interpreter.rs:472`](https://git.mc02.dev/fabi/xenia-rs/src/commit/8401d4d5112849bf7b0f78f9d620f3620b1ce58d/crates/xenia-cpu/src/interpreter.rs)).
- **No `XER[CA]` produced.** This is the logical right shift; for arithmetic shift with `XER[CA]` use [`sradx`](sradx.md).
- **`Rc=1` CR0 is correctly 64-bit.** [`interpreter.rs:475`](https://git.mc02.dev/fabi/xenia-rs/src/commit/8401d4d5112849bf7b0f78f9d620f3620b1ce58d/crates/xenia-cpu/src/interpreter.rs). Result is non-negative as a signed value (high bit is always cleared by the shift), so CR0 will only ever be `EQ` or `GT`.
- **No `OE` bit.**
- **The `srdi` simplified mnemonic** uses [`rldiclx`](rldiclx.md) instead — `rldicl rA, rS, 64-n, n` — because it can be combined with masking. `srd` is for runtime-variable counts.

## Related Instructions

- [`srwx`](srwx.md) — 32-bit logical right shift.
- [`sradx`](sradx.md), [`sradix`](sradix.md) — 64-bit arithmetic right.
- [`sldx`](sldx.md) — 64-bit left shift.
- [`rldiclx`](rldiclx.md) — `srdi` immediate-shift expansion.

## IBM Reference

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