# `vupklsb` — Vector Unpack Low Signed Byte
> **Category:** [VMX (Altivec)](../categories/vmx.md) · **Form:** [VX](../forms/VX.md) · **Opcode:** `0x1000028e`
## Assembler Mnemonics
| Mnemonic | XML entry | Flags | Description |
| --- | --- | --- | --- |
| `vupklsb` | `vupklsb` | — | Vector Unpack Low Signed Byte |
| `vupklsb128` | `vupklsb128` | — | Vector128 Unpack Low Signed Byte |
## Syntax
```asm
vupklsb [VD], [VB]
vupklsb128 [VD], [VB]
```
## Encoding
### `vupklsb` — form `VX`
- **Opcode word:** `0x1000028e`
- **Primary opcode (bits 0–5):** `4`
- **Extended opcode:** `654`
- **Synchronising:** no
| Bits | Field | Meaning |
| --- | --- | --- |
| 0–5 | `OPCD` | primary opcode (4) |
| 6–10 | `VRT/VD` | destination vector register |
| 11–15 | `VRA/VA` | source A vector register |
| 16–20 | `VRB/VB` | source B vector register |
| 21–31 | `XO` | extended opcode (11 bits) |
### `vupklsb128` — form `VX128`
- **Opcode word:** `0x180003c0`
- **Primary opcode (bits 0–5):** `6`
- **Extended opcode:** `960`
- **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 |
| --- | --- | --- |
| `VB` | vupklsb: read; vupklsb128: read | Source B vector register. |
| `VD` | vupklsb: write; vupklsb128: write | Destination vector register. |
## Register Effects
### `vupklsb`
- **Reads (always):** `VB`
- **Reads (conditional):** _none_
- **Writes (always):** `VD`
- **Writes (conditional):** _none_
### `vupklsb128`
- **Reads (always):** `VB`
- **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
**`vupklsb`**
- xenia-canary XML: [`tools/ppc-instructions.xml` — search for `mnem="vupklsb"`](../../xenia-canary/tools/ppc-instructions.xml)
- xenia-canary emit: [`src/xenia/cpu/ppc/ppc_emit_altivec.cc:2076`](../../xenia-canary/src/xenia/cpu/ppc/ppc_emit_altivec.cc#L2076)
- xenia-rs opcode: [`crates/xenia-cpu/src/opcode.rs:129`](../../xenia-rs/crates/xenia-cpu/src/opcode.rs#L129)
- xenia-rs decoder: [`crates/xenia-cpu/src/decoder.rs:494`](../../xenia-rs/crates/xenia-cpu/src/decoder.rs#L494)
- xenia-rs interpreter: [`crates/xenia-cpu/src/interpreter.rs:4144-4153`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L4144-L4153)
xenia-rs interpreter body (frozen snapshot)
```rust
PpcOpcode::vupklsb | PpcOpcode::vupklsb128 => {
let is_128 = matches!(instr.opcode, PpcOpcode::vupklsb128);
let (rb, rd) = if is_128 { (instr.vb128(), instr.vd128()) }
else { (instr.rb(), instr.rd()) };
let b = crate::vmx::as_i8x16(ctx.vr[rb]);
let mut r = [0i16; 8];
for i in 0..8 { r[i] = b[8 + i] as i16; }
ctx.vr[rd] = crate::vmx::from_i16x8(r);
ctx.pc += 4;
}
```
xenia-rs interpreter body (frozen snapshot)
```rust
PpcOpcode::vupklsb | PpcOpcode::vupklsb128 => {
let is_128 = matches!(instr.opcode, PpcOpcode::vupklsb128);
let (rb, rd) = if is_128 { (instr.vb128(), instr.vd128()) }
else { (instr.rb(), instr.rd()) };
let b = crate::vmx::as_i8x16(ctx.vr[rb]);
let mut r = [0i16; 8];
for i in 0..8 { r[i] = b[8 + i] as i16; }
ctx.vr[rd] = crate::vmx::from_i16x8(r);
ctx.pc += 4;
}
```