# `vcmpequb` — Vector Compare Equal-to Unsigned Byte

> **Category:** [VMX (Altivec)](../categories/vmx.md) · **Form:** [VC](../forms/VC.md) · **Opcode:** `0x10000006`

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## Assembler Mnemonics

| Mnemonic | XML entry | Flags | Description |
| --- | --- | --- | --- |
| `vcmpequb` | `vcmpequb` | — | Vector Compare Equal-to Unsigned Byte |
| `vcmpequb.` | `vcmpequb` | Rc=1 | Vector Compare Equal-to Unsigned Byte |

## Syntax

```asm
vcmpequb[Rc] [VD], [VA], [VB]
```

## Encoding

### `vcmpequb` — form `VC`

- **Opcode word:** `0x10000006`
- **Primary opcode (bits 0–5):** `4`
- **Extended opcode:** `6`
- **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 | `Rc` | record-form flag (updates CR6) |
| 22–31 | `XO` | extended opcode (10 bits) |

## Operands

| Field | Role | Description |
| --- | --- | --- |
| `VA` | vcmpequb: read | Source A vector register. |
| `VB` | vcmpequb: read | Source B vector register. |
| `VD` | vcmpequb: write | Destination vector register. |
| `CR` | vcmpequb: write (conditional) | Condition-register update. When `Rc=1`, CR field 0 (or CR6 for vector compares, CR1 for FPU) is updated from the result. |

## Register Effects

### `vcmpequb`

- **Reads (always):** `VA`, `VB`
- **Reads (conditional):** _none_
- **Writes (always):** `VD`
- **Writes (conditional):** `CR`

## Status-Register Effects

- `vcmpequb`: **CR6** ← `[all-true, 0, all-false, 0]` when `Rc=1`.

## 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

**`vcmpequb`**
- xenia-canary XML: [`tools/ppc-instructions.xml` — search for `mnem="vcmpequb"`](../../xenia-canary/tools/ppc-instructions.xml)
- xenia-canary emit: [`src/xenia/cpu/ppc/ppc_emit_altivec.cc:719`](../../xenia-canary/src/xenia/cpu/ppc/ppc_emit_altivec.cc#L719)
- xenia-rs opcode: [`crates/xenia-cpu/src/opcode.rs:95`](../../xenia-rs/crates/xenia-cpu/src/opcode.rs#L95)
- xenia-rs decoder: [`crates/xenia-cpu/src/decoder.rs:557`](../../xenia-rs/crates/xenia-cpu/src/decoder.rs#L557)
- xenia-rs interpreter: [`crates/xenia-cpu/src/interpreter.rs:3723-3735`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L3723-L3735)
<details><summary>xenia-rs interpreter body (frozen snapshot)</summary>

```rust
        PpcOpcode::vcmpequb => {
            let a = ctx.vr[instr.ra()].as_bytes();
            let b = ctx.vr[instr.rb()].as_bytes();
            let mut r = [0u8; 16];
            for i in 0..16 { r[i] = if a[i] == b[i] { 0xFF } else { 0 }; }
            let v = xenia_types::Vec128::from_bytes(r);
            if instr.vc_rc_bit() {
                let (t, f) = crate::vmx::cr6_flags_from_mask(v);
                ctx.cr[6] = crate::context::CrField { lt: t, gt: false, eq: f, so: false };
            }
            ctx.vr[instr.rd()] = v;
            ctx.pc += 4;
        }
```
</details>

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## Special Cases & Edge Conditions

- **Per-byte mask: all-ones / all-zero.** Sixteen byte lanes; `VD[i] = (VA[i] == VB[i]) ? 0xFF : 0x00`. Lane 0 is the most-significant byte after `stvx`.
- **Sign-agnostic.** Equality compare is identical for signed and unsigned bytes; there is no separate `vcmpeqsb`.
- **CR6 update when `Rc=1`** (`vcmpequb.`). CR6 = `[lt = all-true, gt = 0, eq = all-false, so = 0]` — built by xenia's `crate::vmx::cr6_flags_from_mask` ([`crates/xenia-cpu/src/vmx.rs`](../../xenia-rs/crates/xenia-cpu/src/vmx.rs)). Standard SIMD-search idiom: `vcmpequb. vMask, vData, vNeedle` then `bc 12,26` to branch when *no* lane matched.
- **Compose with `vsel`.** Mask drives [`vsel`](vsel.md) to pick per-byte between two source vectors.
- **Common usage.** `memchr` / `strlen` / character classification — compare against a broadcast byte (often built with [`vspltisb`](vspltisb.md)) and inspect CR6 for early-out.
- **No `VSCR` interaction, no XER, no traps.**
- **Aliasing legal.**
- **No VMX128 sibling.**

## Related Instructions

- [`vcmpequh`](vcmpequh.md), [`vcmpequw`](vcmpequw.md) — equality compare at half / word width.
- [`vcmpgtub`](vcmpgtub.md), [`vcmpgtsb`](vcmpgtsb.md) — `>` at byte width, unsigned / signed.
- [`vsel`](vsel.md) — primary mask consumer.
- [`vand`](vand.md), [`vandc`](vandc.md), [`vor`](vor.md), [`vxor`](vxor.md) — mask combinators.
- [`vspltisb`](vspltisb.md), [`vspltb`](vspltb.md) — broadcast sources for needle patterns.

## IBM Reference

- [AIX 7.3 — `vcmpequb` (Vector Compare Equal-to Unsigned Byte)](https://www.ibm.com/docs/en/aix/7.3.0?topic=set-vcmpequb-vector-compare-equal-unsigned-byte-instruction)
- [IBM AltiVec Technology Programmer's Interface Manual, Chapter 6 — Vector Compares](https://www.nxp.com/docs/en/reference-manual/ALTIVECPIM.pdf)