Sylpheed/tools/ppc-manual/memory/stwcx.md

# `stwcx` — Store Word Conditional Indexed

> **Category:** [Memory](../categories/memory.md) · **Form:** [X](../forms/X.md) · **Opcode:** `0x7c00012d`

<!-- GENERATED: BEGIN -->

## Assembler Mnemonics

| Mnemonic | XML entry | Flags | Description |
| --- | --- | --- | --- |
| `stwcx` | `stwcx` | — | Store Word Conditional Indexed |

## Syntax

```asm
stwcx. [RS], [RA0], [RB]
```

## Encoding

### `stwcx` — form `X`

- **Opcode word:** `0x7c00012d`
- **Primary opcode (bits 0–5):** `31`
- **Extended opcode:** `150`
- **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` | stwcx: read | Source GPR (alias for RD in some stores). |
| `RA0` | stwcx: read | Source GPR; when the encoded register number is 0 the operand is the literal 64-bit zero, **not** `r0`. |
| `RB` | stwcx: read | Source GPR. |
| `CR` | stwcx: write | Condition-register update. When `Rc=1`, CR field 0 (or CR6 for vector compares, CR1 for FPU) is updated from the result. |

## Register Effects

### `stwcx`

- **Reads (always):** `RS`, `RA0`, `RB`
- **Reads (conditional):** _none_
- **Writes (always):** `CR`
- **Writes (conditional):** _none_

## Status-Register Effects

- `stwcx`: **CR0** ← signed-compare(result, 0) with `SO ← XER[SO]` (always).

## Operation (pseudocode)

```
; No hand-written pseudocode for this instruction yet.
; The authoritative semantics are the Canary emitter snapshot under
; Implementation References; about half of Canary's emitters open
; with the PPC-style definition as a comment (`RD <- (RA) + (RB)`).
; Every side effect is also enumerated in the Register Effects and
; Status-Register Effects tables above.
```

## 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

**`stwcx`**
- Canary XML: [`tools/ppc-instructions.xml` — search for `mnem="stwcx"`](https://github.com/xenia-canary/xenia-canary/blob/f21ebd49e979e44f081f474df78c3fbfee9cb3f2/tools/ppc-instructions.xml)
- Canary emitter: [`src/xenia/cpu/ppc/ppc_emit_memory.cc:868`](https://github.com/xenia-canary/xenia-canary/blob/f21ebd49e979e44f081f474df78c3fbfee9cb3f2/src/xenia/cpu/ppc/ppc_emit_memory.cc#L868)
- Sylpheed opcode: [`crates/sylpheed-ppc/src/opcode.rs:276`](../../../crates/sylpheed-ppc/src/opcode.rs#L276)
- Sylpheed decoder: [`crates/sylpheed-ppc/src/decoder.rs:897`](../../../crates/sylpheed-ppc/src/decoder.rs#L897)
<details><summary>Canary emitter (frozen snapshot @ <code>f21ebd49e9</code>)</summary>

```cpp
int InstrEmit_stwcx(PPCHIRBuilder& f, const InstrData& i) {
  // if RA = 0 then
  //   b <- 0
  // else
  //   b <- (RA)
  // EA <- b + (RB)
  // RESERVE stuff...
  // MEM(EA, 4) <- (RS)[32:63]
  // n <- 1 if store performed
  // CR0[LT GT EQ SO] = 0b00 || n || XER[SO]

  // NOTE: we assume we are within a global lock.
  // As we have been exclusively executing this entire time, we assume that no
  // one else could have possibly touched the memory and must always succeed.
  // We use atomic compare exchange here to support reserved load/store without
  // being under the global lock (flag disable_global_lock - see mtmsr/mtmsrd).
  // This will always succeed if under the global lock, however.

  Value* ea = CalculateEA_0(f, i.X.RA, i.X.RB);

  Value* rt = f.ByteSwap(f.Truncate(f.LoadGPR(i.X.RT), INT32_TYPE));

  if (cvars::no_reserved_ops) {
    f.Store(ea, rt);

    f.StoreContext(offsetof(PPCContext, cr0.cr0_eq), f.LoadConstantInt8(1));
  } else {
    Value* v = f.StoreWithReserve(ea, rt, INT64_TYPE);
    f.StoreContext(offsetof(PPCContext, cr0.cr0_eq), v);
  }

  f.StoreContext(offsetof(PPCContext, cr0.cr0_lt), f.LoadZeroInt8());
  f.StoreContext(offsetof(PPCContext, cr0.cr0_gt), f.LoadZeroInt8());

  // Issue memory barrier for when we go out of lock and want others to see our
  // updates.
  if (!cvars::no_reserved_ops) {
    f.MemoryBarrier();
  }

  return 0;
}
```
</details>

<!-- GENERATED: END -->

## Special Cases & Edge Conditions

- **Always sets `Rc=1` (the trailing dot).** The mnemonic is `stwcx.` — there is no non-Rc variant. CR0 is updated unconditionally to communicate success/failure. `EQ=1` means the conditional store succeeded; `EQ=0` means it failed (the prior reservation was lost; no memory write).
- **Reservation check.** Canary's store helper fails — no write, `EQ=0` — if the thread holds no reservation. Otherwise it writes the low 32 bits of `RS` (big-endian) with `lock cmpxchg`, which succeeds only if memory still holds the value `lwarx` loaded; `EQ=1` only then. The reservation is released either way, so a retry must be preceded by a fresh [`lwarx`](lwarx.md). `LT` and `GT` are cleared; `SO` is left as it was instead of being copied from `XER[SO]`.
- **Hardware granule.** PowerISA defines reservation by aligned word; Xenon implementations widen this to one 128-byte cache line. A store by another agent anywhere in the line clears the reservation. Canary works differently: ordinary stores never clear a reservation. `lwarx` sets a bit for the 64 KiB block holding `EA` in a bitmap shared by all threads, and the conditional store succeeds only if this thread still holds that bit and the word still holds the value `lwarx` read. A write elsewhere in the line — or one that puts back the same value — goes unnoticed, while two threads reserving in the same 64 KiB block make the later store fail.
- **Alignment requirement.** `EA` must be 4-byte aligned. Unaligned `stwcx.` raises an alignment exception on real hardware; Canary does not check.
- **`RA0` semantics.** When `RA = 0`, base is literal zero — `stwcx. RS, 0, RB` writes at exact `RB`.
- **CR0[SO] reflects XER[SO].** Like all CR-updating ops, CR0[SO] is copied from `XER[SO]` rather than computed.
- **Spurious failures permitted.** Hardware may report failure even when no actual conflict occurred (e.g. on context switch). Application code treats failure as a normal retry condition.
- **Pair atomically with [`lwarx`](lwarx.md).** Don't interleave loads/stores between the pair; an [`lwsync`](../alu/sync.md) inside the loop body is common.
- **Stores low 32 bits of `RS`.** The high 32 bits of the source GPR are ignored.

## Related Instructions

- [`lwarx`](lwarx.md) — load-and-reserve word (the matching load).
- [`stdcx`](stdcx.md) / [`ldarx`](ldarx.md) — 64-bit reservation pair.
- [`stw`](stw.md), [`stwx`](stw.md) — non-conditional word stores.
- [`sync`](../alu/sync.md), [`lwsync`](../alu/sync.md), [`isync`](../alu/isync.md) — barriers used around reservation pairs.

## IBM Reference

- [AIX 7.3 — `stwcx.` (Store Word Conditional Indexed)](https://www.ibm.com/docs/en/aix/7.3.0?topic=set-stwcx-store-word-conditional-indexed-instruction)
- `PowerISA v2.07B Book II` § "Atomic Update Primitives" for canonical reservation semantics and granule rules.