# `vsel` — Vector Conditional Select
> **Category:** [VMX (Altivec)](../categories/vmx.md) · **Form:** [VA](../forms/VA.md) · **Opcode:** `0x1000002a`
## Assembler Mnemonics
| Mnemonic | XML entry | Flags | Description |
| --- | --- | --- | --- |
| `vsel` | `vsel` | — | Vector Conditional Select |
| `vsel128` | `vsel128` | — | Vector128 Conditional Select |
## Syntax
```asm
vsel [VD], [VA], [VB], [VC]
vsel128 [VD], [VA], [VB], [VD]
```
## Encoding
### `vsel` — form `VA`
- **Opcode word:** `0x1000002a`
- **Primary opcode (bits 0–5):** `4`
- **Extended opcode:** `42`
- **Synchronising:** no
| Bits | Field | Meaning |
| --- | --- | --- |
| 0–5 | `OPCD` | primary opcode (4) |
| 6–10 | `VRT` | destination vector register |
| 11–15 | `VRA` | source A |
| 16–20 | `VRB` | source B |
| 21–25 | `VRC` | source C / shift |
| 26–31 | `XO` | extended opcode (6 bits) |
### `vsel128` — form `VX128`
- **Opcode word:** `0x14000350`
- **Primary opcode (bits 0–5):** `5`
- **Extended opcode:** `848`
- **Synchronising:** no
| Bits | Field | Meaning |
| --- | --- | --- |
| 0–5 | `OPCD` | primary opcode (4 or 5) |
| 6–10 | `VD128l` | destination low 5 bits |
| 11–15 | `VA128l` | source A low 5 bits |
| 16–20 | `VB128l` | source B low 5 bits |
| 21 | `VA128H` | source A high bit |
| 22 | `—` | reserved |
| 23–25 | `VC` | optional VC / XO sub-field |
| 26 | `VA128h` | source A middle bit |
| 27 | `—` | reserved |
| 28–29 | `VD128h` | destination high 2 bits |
| 30–31 | `VB128h` | source B high 2 bits |
## Operands
| Field | Role | Description |
| --- | --- | --- |
| `VA` | vsel: read; vsel128: read | Source A vector register. |
| `VB` | vsel: read; vsel128: read | Source B vector register. |
| `VC` | vsel: read | Source C vector register / 3-bit selector. |
| `VD` | vsel: write; vsel128: read; vsel128: write | Destination vector register. |
## Register Effects
### `vsel`
- **Reads (always):** `VA`, `VB`, `VC`
- **Reads (conditional):** _none_
- **Writes (always):** `VD`
- **Writes (conditional):** _none_
### `vsel128`
- **Reads (always):** `VA`, `VB`, `VD`
- **Reads (conditional):** _none_
- **Writes (always):** `VD`
- **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
**`vsel`**
- xenia-canary XML: [`tools/ppc-instructions.xml` — search for `mnem="vsel"`](../../xenia-canary/tools/ppc-instructions.xml)
- xenia-canary emit: [`src/xenia/cpu/ppc/ppc_emit_altivec.cc:1386`](../../xenia-canary/src/xenia/cpu/ppc/ppc_emit_altivec.cc#L1386)
- xenia-rs opcode: [`crates/xenia-cpu/src/opcode.rs:121`](../../xenia-rs/crates/xenia-cpu/src/opcode.rs#L121)
- xenia-rs decoder: [`crates/xenia-cpu/src/decoder.rs:585`](../../xenia-rs/crates/xenia-cpu/src/decoder.rs#L585)
- xenia-rs interpreter: [`crates/xenia-cpu/src/interpreter.rs:2253-2275`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L2253-L2275)
xenia-rs interpreter body (frozen snapshot)
```rust
PpcOpcode::vsel | PpcOpcode::vsel128 => {
// vD = (vA & ~vC) | (vB & vC)
let (va, vb, vd);
let vc;
if matches!(instr.opcode, PpcOpcode::vsel128) {
va = instr.va128();
vb = instr.vb128();
vd = instr.vd128();
vc = vd; // for 128, vC is encoded in vD field
} else {
va = instr.ra();
vb = instr.rb();
vd = instr.rd();
vc = instr.rc();
}
let a = ctx.vr[va].as_u32x4();
let b = ctx.vr[vb].as_u32x4();
let c = ctx.vr[vc].as_u32x4();
let mut r = [0u32; 4];
for i in 0..4 { r[i] = (a[i] & !c[i]) | (b[i] & c[i]); }
ctx.vr[vd] = xenia_types::Vec128::from_u32x4_array(r);
ctx.pc += 4;
}
```
xenia-rs interpreter body (frozen snapshot)
```rust
PpcOpcode::vsel | PpcOpcode::vsel128 => {
// vD = (vA & ~vC) | (vB & vC)
let (va, vb, vd);
let vc;
if matches!(instr.opcode, PpcOpcode::vsel128) {
va = instr.va128();
vb = instr.vb128();
vd = instr.vd128();
vc = vd; // for 128, vC is encoded in vD field
} else {
va = instr.ra();
vb = instr.rb();
vd = instr.rd();
vc = instr.rc();
}
let a = ctx.vr[va].as_u32x4();
let b = ctx.vr[vb].as_u32x4();
let c = ctx.vr[vc].as_u32x4();
let mut r = [0u32; 4];
for i in 0..4 { r[i] = (a[i] & !c[i]) | (b[i] & c[i]); }
ctx.vr[vd] = xenia_types::Vec128::from_u32x4_array(r);
ctx.pc += 4;
}
```