# `vcmpgtsb` — Vector Compare Greater-Than Signed Byte

> **Category:** [VMX (Altivec)](../categories/vmx.md) · **Form:** [VC](../forms/VC.md) · **Opcode:** `0x10000306`

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## Assembler Mnemonics

| Mnemonic | XML entry | Flags | Description |
| --- | --- | --- | --- |
| `vcmpgtsb` | `vcmpgtsb` | — | Vector Compare Greater-Than Signed Byte |
| `vcmpgtsb.` | `vcmpgtsb` | Rc=1 | Vector Compare Greater-Than Signed Byte |

## Syntax

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

## Encoding

### `vcmpgtsb` — form `VC`

- **Opcode word:** `0x10000306`
- **Primary opcode (bits 0–5):** `4`
- **Extended opcode:** `774`
- **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` | vcmpgtsb: read | Source A vector register. |
| `VB` | vcmpgtsb: read | Source B vector register. |
| `VD` | vcmpgtsb: write | Destination vector register. |
| `CR` | vcmpgtsb: 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

### `vcmpgtsb`

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

## Status-Register Effects

- `vcmpgtsb`: **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

**`vcmpgtsb`**
- xenia-canary XML: [`tools/ppc-instructions.xml` — search for `mnem="vcmpgtsb"`](../../xenia-canary/tools/ppc-instructions.xml)
- xenia-canary emit: [`src/xenia/cpu/ppc/ppc_emit_altivec.cc:735`](../../xenia-canary/src/xenia/cpu/ppc/ppc_emit_altivec.cc#L735)
- xenia-rs opcode: [`crates/xenia-cpu/src/opcode.rs:97`](../../xenia-rs/crates/xenia-cpu/src/opcode.rs#L97)
- xenia-rs decoder: [`crates/xenia-cpu/src/decoder.rs:566`](../../xenia-rs/crates/xenia-cpu/src/decoder.rs#L566)
- xenia-rs interpreter: [`crates/xenia-cpu/src/interpreter.rs:3762-3774`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L3762-L3774)
<details><summary>xenia-rs interpreter body (frozen snapshot)</summary>

```rust
        PpcOpcode::vcmpgtsb => {
            let a = crate::vmx::as_i8x16(ctx.vr[instr.ra()]);
            let b = crate::vmx::as_i8x16(ctx.vr[instr.rb()]);
            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] = (int8(VA[i]) > int8(VB[i])) ? 0xFF : 0x00`. Lane 0 is the most-significant byte after `stvx`.
- **Sign matters.** Identical bit patterns to [`vcmpgtub`](vcmpgtub.md) compare differently because of the signed interpretation: e.g. `0xFF > 0x01` is `true` unsigned but `false` signed (`-1 > 1`).
- **CR6 update when `Rc=1`** (`vcmpgtsb.`). CR6 = `[lt = all-true, gt = 0, eq = all-false, so = 0]` — built by xenia's `crate::vmx::cr6_flags_from_mask`.
- **Compose with `vsel`.** Mask drives [`vsel`](vsel.md) to select per byte.
- **Common usage.** Signed-byte audio thresholding, signed-difference sign extraction (`vsubsbs` then `vcmpgtsb`).
- **No `VSCR` interaction, no XER, no traps.**
- **Aliasing legal.**
- **No VMX128 sibling.**

## Related Instructions

- [`vcmpgtub`](vcmpgtub.md) — same width, unsigned `>`.
- [`vcmpequb`](vcmpequb.md) — equality at byte width.
- [`vcmpgtsh`](vcmpgtsh.md), [`vcmpgtsw`](vcmpgtsw.md) — signed `>` at half / word width.
- [`vsel`](vsel.md), [`vand`](vand.md), [`vandc`](vandc.md), [`vor`](vor.md), [`vxor`](vxor.md) — mask consumers / combinators.
- [`vmaxsb`](vmaxsb.md), [`vminsb`](vminsb.md) — direct signed max / min.

## IBM Reference

- [AIX 7.3 — `vcmpgtsb` (Vector Compare Greater-Than Signed Byte)](https://www.ibm.com/docs/en/aix/7.3.0?topic=set-vcmpgtsb-vector-compare-greater-than-signed-byte-instruction)
- [IBM AltiVec Technology Programmer's Interface Manual, Chapter 6 — Vector Compares](https://www.nxp.com/docs/en/reference-manual/ALTIVECPIM.pdf)