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

# `stvrx` — Store Vector Right Indexed

> **Category:** [Memory](../categories/memory.md) · **Form:** [X](../forms/X.md) · **Opcode:** `0x7c00054e`

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## Assembler Mnemonics

| Mnemonic | XML entry | Flags | Description |
| --- | --- | --- | --- |
| `stvrx` | `stvrx` | — | Store Vector Right Indexed |
| `stvrx128` | `stvrx128` | — | Store Vector Right Indexed 128 |

## Syntax

```asm
stvrx [VS], [RA0], [RB]
stvrx128 [VS], [RA0], [RB]
```

## Encoding

### `stvrx` — form `X`

- **Opcode word:** `0x7c00054e`
- **Primary opcode (bits 0–5):** `31`
- **Extended opcode:** `679`
- **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 |

### `stvrx128` — form `VX128_1`

- **Opcode word:** `0x10000543`
- **Primary opcode (bits 0–5):** `4`
- **Extended opcode:** `1347`
- **Synchronising:** no

| Bits | Field | Meaning |
| --- | --- | --- |
| 0–5 | `OPCD` | primary opcode (4) |
| 6–10 | `VD128l` | destination low 5 bits |
| 11–15 | `RA` | address register |
| 16–20 | `RB` | offset register |
| 21–27 | `XO` | extended opcode |
| 28–29 | `VD128h` | destination high 2 bits |
| 30–31 | `—` | reserved |

## Operands

| Field | Role | Description |
| --- | --- | --- |
| `VS` | stvrx: read; stvrx128: read | Source vector register (alias for VD on stores). |
| `RA0` | stvrx: read; stvrx128: read | Source GPR; when the encoded register number is 0 the operand is the literal 64-bit zero, **not** `r0`. |
| `RB` | stvrx: read; stvrx128: read | Source GPR. |

## Register Effects

### `stvrx`

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

### `stvrx128`

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

## Status-Register Effects

_No condition-register or status-register effects._

## 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

**`stvrx`**
- xenia-canary XML: [`tools/ppc-instructions.xml` — search for `mnem="stvrx"`](../../xenia-canary/tools/ppc-instructions.xml)
- xenia-canary emit: [`src/xenia/cpu/ppc/ppc_emit_altivec.cc:290`](../../xenia-canary/src/xenia/cpu/ppc/ppc_emit_altivec.cc#L290)
- xenia-rs opcode: [`crates/xenia-cpu/src/opcode.rs:78`](../../xenia-rs/crates/xenia-cpu/src/opcode.rs#L78)
- xenia-rs decoder: [`crates/xenia-cpu/src/decoder.rs:833`](../../xenia-rs/crates/xenia-cpu/src/decoder.rs#L833)
- xenia-rs interpreter: [`crates/xenia-cpu/src/interpreter.rs:3134-3150`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L3134-L3150)
<details><summary>xenia-rs interpreter body (frozen snapshot)</summary>

```rust
        PpcOpcode::stvrx | PpcOpcode::stvrxl => {
            let ea = ea_indexed(ctx, instr);
            // PPCBUG-514: stvrx/stvrxl were missing invalidate_for_write.
            // store_vector_right writes [ea & !0xF, ea-1] (up to 15 bytes, all within a single
            // 16-byte-aligned block). Two-call form is kept for defensive correctness.
            // stvrx at shift==0 is a no-op; the guard fires unconditionally (cheap).
            if let Some(t) = ctx.reservation_table.as_ref().filter(|t| t.is_enabled()) {
                if t.has_active_reservers() {
                    let first_line = ea & !RESERVATION_MASK;
                    let last_line = ea.wrapping_add(15) & !RESERVATION_MASK;
                    t.invalidate_for_write(first_line);
                    if last_line != first_line { t.invalidate_for_write(last_line); }
                }
            }
            crate::vmx::store_vector_right(mem, ea, ctx.vr[instr.rs()]);
            ctx.pc += 4;
        }
```
</details>

**`stvrx128`**
- xenia-canary XML: [`tools/ppc-instructions.xml` — search for `mnem="stvrx128"`](../../xenia-canary/tools/ppc-instructions.xml)
- xenia-canary emit: [`src/xenia/cpu/ppc/ppc_emit_altivec.cc:293`](../../xenia-canary/src/xenia/cpu/ppc/ppc_emit_altivec.cc#L293)
- xenia-rs opcode: [`crates/xenia-cpu/src/opcode.rs:78`](../../xenia-rs/crates/xenia-cpu/src/opcode.rs#L78)
- xenia-rs decoder: [`crates/xenia-cpu/src/decoder.rs:423`](../../xenia-rs/crates/xenia-cpu/src/decoder.rs#L423)
- xenia-rs interpreter: [`crates/xenia-cpu/src/interpreter.rs:3151-3164`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L3151-L3164)
<details><summary>xenia-rs interpreter body (frozen snapshot)</summary>

```rust
        PpcOpcode::stvrx128 | PpcOpcode::stvrxl128 => {
            let ea = ea_indexed(ctx, instr);
            // PPCBUG-514: stvrx128/stvrxl128 were missing invalidate_for_write.
            if let Some(t) = ctx.reservation_table.as_ref().filter(|t| t.is_enabled()) {
                if t.has_active_reservers() {
                    let first_line = ea & !RESERVATION_MASK;
                    let last_line = ea.wrapping_add(15) & !RESERVATION_MASK;
                    t.invalidate_for_write(first_line);
                    if last_line != first_line { t.invalidate_for_write(last_line); }
                }
            }
            crate::vmx::store_vector_right(mem, ea, ctx.vr[instr.vs128()]);
            ctx.pc += 4;
        }
```
</details>

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## Special Cases & Edge Conditions

- **Store-right half of an unaligned vector.** `stvrx` writes `(EA mod 16)` bytes from the **right** (high-lane) half of `VS` to the addresses *just below* `EA & ~0xF` (the bytes from the previous aligned line that fall on the right side of the unaligned vector). The left half of `VS` is not stored.
- **Standard pair-mate of [`stvlx`](stvlx.md).** `stvlx VS, RA, RB ; stvrx VS, RA, RB+16` (or analogous addressing) commits the 16 bytes of `VS` to address `EA` regardless of alignment. The two halves are byte-disjoint, so order is irrelevant for correctness.
- **No alignment masking.** Unlike `stvx`, the exact `EA` is used; `EA mod 16` controls how `VS` splits.
- **`RA0` semantics.** `RA = 0` selects literal zero.
- **Microsoft Xbox 360 specific.** Part of the VMX128 / Cell BE extended set.
- **Implementation in xenia.** The shared snapshot calls `vmx::store_vector_right(mem, ea, vs)`, performing the unaligned partial-byte write of the right side.
- **VMX128 sibling (`stvrx128`).** Identical semantics; alternative operand encoding addressing `v0..v127`.
- **`stvrxl` is the LRU-hint variant.**

## Related Instructions

- [`stvlx`](stvlx.md), [`stvlx128`](stvlx.md) — store-left partner.
- [`stvrxl`](stvrxl.md), [`stvrxl128`](stvrxl.md) — LRU-hint variants.
- [`stvx`](stvx.md), [`stvx128`](stvx.md) — aligned vector store.
- [`lvrx`](lvrx.md), [`lvlx`](lvlx.md) — symmetric unaligned loads.

## IBM Reference

- [AIX 7.3 — `stvrx` (Store Vector Right Indexed)](https://www.ibm.com/docs/en/aix/7.3.0?topic=set-stvrx-store-vector-right-indexed-instruction)
- `PowerISA v2.07B Book I` "Vector Facility"; Microsoft Xbox 360 XDK for VMX128 unaligned stores.