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

# `stwcx` — Store Word Conditional Indexed

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

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## 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)

```
; 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

**`stwcx`**
- xenia-canary XML: [`tools/ppc-instructions.xml` — search for `mnem="stwcx"`](../../xenia-canary/tools/ppc-instructions.xml)
- xenia-canary emit: [`src/xenia/cpu/ppc/ppc_emit_memory.cc:868`](../../xenia-canary/src/xenia/cpu/ppc/ppc_emit_memory.cc#L868)
- xenia-rs opcode: [`crates/xenia-cpu/src/opcode.rs:81`](../../xenia-rs/crates/xenia-cpu/src/opcode.rs#L81)
- xenia-rs decoder: [`crates/xenia-cpu/src/decoder.rs:782`](../../xenia-rs/crates/xenia-cpu/src/decoder.rs#L782)
- xenia-rs interpreter: [`crates/xenia-cpu/src/interpreter.rs:1225-1288`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L1225-L1288)
<details><summary>xenia-rs interpreter body (frozen snapshot)</summary>

```rust
        PpcOpcode::stwcx => {
            let ea = if instr.ra() == 0 { 0u64 } else { ctx.gpr[instr.ra()] };
            let ea = ea.wrapping_add(ctx.gpr[instr.rb()]) as u32;
            let line = ea & !RESERVATION_MASK;
            let table_route = ctx
                .reservation_table
                .as_ref()
                .filter(|t| t.is_enabled())
                .cloned();
            // PPCBUG-151: stwcx. requires a word (lwarx) reservation;
            // a doubleword (ldarx) reservation must not commit here.
            let width_ok = ctx.reservation_width == 4;
            let success = if let Some(t) = &table_route {
                // Table-routed: success iff the slot still holds our
                // reservation AND the per-ctx flag agrees (the per-ctx
                // flag would be cleared by an intervening write or
                // context switch).
                ctx.has_reservation
                    && width_ok
                    && ctx.reserved_line == line
                    && t.try_commit(ea, ctx.reserved_generation, ctx.hw_id)
            } else {
                // Legacy per-ctx path (M2 default / lockstep).
                // PPCBUG-108: fires on non-primary HW slots under misconfig —
                // if the table is disabled while workers are active, slots
                // 1..N will trip this assert, surfacing the misconfiguration
                // early in debug builds. Note: hw_id==0 (primary slot) taking
                // this path while other slots run in parallel would NOT be
                // caught; that case requires the table to be enabled instead.
                debug_assert!(
                    ctx.hw_id == 0,
                    "PPCBUG-108: legacy per-ctx stwcx. on non-primary HW slot \
                     (hw_id={}) — ReservationTable must be enabled under --parallel",
                    ctx.hw_id
                );
                ctx.has_reservation && width_ok && ctx.reserved_line == line
            };
            if success {
                mem.write_u32(ea, ctx.gpr[instr.rs()] as u32);
                ctx.cr[0] = crate::context::CrField {
                    lt: false,
                    gt: false,
                    eq: true,
                    so: ctx.xer_so != 0,
                };
            } else {
                ctx.cr[0] = crate::context::CrField {
                    lt: false,
                    gt: false,
                    eq: false,
                    so: ctx.xer_so != 0,
                };
                // Failed stwcx: if we held the reservation in the table
                // (someone else displaced our gen), release it from the
                // counter so `has_active_reservers` returns to zero
                // when no real reserver exists.
                if let Some(t) = &table_route {
                    t.release(ea, ctx.reserved_generation, ctx.hw_id);
                }
            }
            ctx.has_reservation = false;
            ctx.reservation_width = 0; // PPCBUG-151: always clear on exit
            ctx.pc += 4;
        }
```
</details>

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## 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.** Xenia's snapshot tests `has_reservation && reserved_addr == ea`. On match it performs `mem.write_u32` (low 32 bits of `RS`, big-endian), sets `EQ=1`. On mismatch, no memory write and `EQ=0`. In both cases the reservation is cleared, so a retry must begin with a fresh [`lwarx`](lwarx.md).
- **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. Xenia's per-address check is more permissive than hardware.
- **Alignment requirement.** `EA` must be 4-byte aligned. Unaligned `stwcx.` raises an alignment exception on real hardware; xenia 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`](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`](sync.md), [`lwsync`](sync.md), [`isync`](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.