# `vmrglw` — Vector Merge Low Word
> **Category:** [VMX (Altivec)](../categories/vmx.md) · **Form:** [VX](../forms/VX.md) · **Opcode:** `0x1000018c`
## Assembler Mnemonics
| Mnemonic | XML entry | Flags | Description |
| --- | --- | --- | --- |
| `vmrglw` | `vmrglw` | — | Vector Merge Low Word |
| `vmrglw128` | `vmrglw128` | — | Vector128 Merge Low Word |
## Syntax
```asm
vmrglw [VD], [VA], [VB]
vmrglw128 [VD], [VA], [VB]
```
## Encoding
### `vmrglw` — form `VX`
- **Opcode word:** `0x1000018c`
- **Primary opcode (bits 0–5):** `4`
- **Extended opcode:** `396`
- **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) |
### `vmrglw128` — form `VX128`
- **Opcode word:** `0x18000340`
- **Primary opcode (bits 0–5):** `6`
- **Extended opcode:** `832`
- **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` | vmrglw: read; vmrglw128: read | Source A vector register. |
| `VB` | vmrglw: read; vmrglw128: read | Source B vector register. |
| `VD` | vmrglw: write; vmrglw128: write | Destination vector register. |
## Register Effects
### `vmrglw`
- **Reads (always):** `VA`, `VB`
- **Reads (conditional):** _none_
- **Writes (always):** `VD`
- **Writes (conditional):** _none_
### `vmrglw128`
- **Reads (always):** `VA`, `VB`
- **Reads (conditional):** _none_
- **Writes (always):** `VD`
- **Writes (conditional):** _none_
## Status-Register Effects
_No condition-register or status-register effects._
## 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
**`vmrglw`**
- Canary XML: [`tools/ppc-instructions.xml` — search for `mnem="vmrglw"`](https://github.com/xenia-canary/xenia-canary/blob/f21ebd49e979e44f081f474df78c3fbfee9cb3f2/tools/ppc-instructions.xml)
- Canary emitter: [`src/xenia/cpu/ppc/ppc_emit_altivec.cc:1017`](https://github.com/xenia-canary/xenia-canary/blob/f21ebd49e979e44f081f474df78c3fbfee9cb3f2/src/xenia/cpu/ppc/ppc_emit_altivec.cc#L1017)
- Sylpheed opcode: [`crates/sylpheed-ppc/src/opcode.rs:375`](../../../crates/sylpheed-ppc/src/opcode.rs#L375)
- Sylpheed decoder: [`crates/sylpheed-ppc/src/decoder.rs:585`](../../../crates/sylpheed-ppc/src/decoder.rs#L585)
Canary emitter (frozen snapshot @ f21ebd49e9)
```cpp
int InstrEmit_vmrglw(PPCHIRBuilder& f, const InstrData& i) {
return InstrEmit_vmrglw_(f, i.VX.VD, i.VX.VA, i.VX.VB);
}
// ── delegates to (src/xenia/cpu/ppc/ppc_emit_altivec.cc:1006) ──
int InstrEmit_vmrglw_(PPCHIRBuilder& f, uint32_t vd, uint32_t va, uint32_t vb) {
// (VD.x) = (VA.z)
// (VD.y) = (VB.z)
// (VD.z) = (VA.w)
// (VD.w) = (VB.w)
Value* v =
f.Permute(f.LoadConstantUint32(MakePermuteMask(0, 2, 1, 2, 0, 3, 1, 3)),
f.LoadVR(va), f.LoadVR(vb), INT32_TYPE);
f.StoreVR(vd, v);
return 0;
}
```
Canary emitter (frozen snapshot @ f21ebd49e9)
```cpp
int InstrEmit_vmrglw128(PPCHIRBuilder& f, const InstrData& i) {
return InstrEmit_vmrglw_(f, VX128_VD128, VX128_VA128, VX128_VB128);
}
// ── delegates to (src/xenia/cpu/ppc/ppc_emit_altivec.cc:1006) ──
int InstrEmit_vmrglw_(PPCHIRBuilder& f, uint32_t vd, uint32_t va, uint32_t vb) {
// (VD.x) = (VA.z)
// (VD.y) = (VB.z)
// (VD.z) = (VA.w)
// (VD.w) = (VB.w)
Value* v =
f.Permute(f.LoadConstantUint32(MakePermuteMask(0, 2, 1, 2, 0, 3, 1, 3)),
f.LoadVR(va), f.LoadVR(vb), INT32_TYPE);
f.StoreVR(vd, v);
return 0;
}
```