# `vsububm` — Vector Subtract Unsigned Byte Modulo

> **Category:** [VMX (Altivec)](../categories/vmx.md) · **Form:** [VX](../forms/VX.md) · **Opcode:** `0x10000400`

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## Assembler Mnemonics

| Mnemonic | XML entry | Flags | Description |
| --- | --- | --- | --- |
| `vsububm` | `vsububm` | — | Vector Subtract Unsigned Byte Modulo |

## Syntax

```asm
vsububm [VD], [VA], [VB]
```

## Encoding

### `vsububm` — form `VX`

- **Opcode word:** `0x10000400`
- **Primary opcode (bits 0–5):** `4`
- **Extended opcode:** `1024`
- **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) |

## Operands

| Field | Role | Description |
| --- | --- | --- |
| `VA` | vsububm: read | Source A vector register. |
| `VB` | vsububm: read | Source B vector register. |
| `VD` | vsububm: write | Destination vector register. |

## Register Effects

### `vsububm`

- **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)

```
; 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

**`vsububm`**
- xenia-canary XML: [`tools/ppc-instructions.xml` — search for `mnem="vsububm"`](../../xenia-canary/tools/ppc-instructions.xml)
- xenia-canary emit: [`src/xenia/cpu/ppc/ppc_emit_altivec.cc:1720`](../../xenia-canary/src/xenia/cpu/ppc/ppc_emit_altivec.cc#L1720)
- xenia-rs opcode: [`crates/xenia-cpu/src/opcode.rs:126`](../../xenia-rs/crates/xenia-cpu/src/opcode.rs#L126)
- xenia-rs decoder: [`crates/xenia-cpu/src/decoder.rs:519`](../../xenia-rs/crates/xenia-cpu/src/decoder.rs#L519)
- xenia-rs interpreter: [`crates/xenia-cpu/src/interpreter.rs:3206-3213`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L3206-L3213)
<details><summary>xenia-rs interpreter body (frozen snapshot)</summary>

```rust
        PpcOpcode::vsububm => {
            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] = a[i].wrapping_sub(b[i]); }
            ctx.vr[instr.rd()] = xenia_types::Vec128::from_bytes(r);
            ctx.pc += 4;
        }
```
</details>

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## Special Cases & Edge Conditions

- **Unsigned-byte modulo subtract.** `VD.b[i] = VA.b[i] − VB.b[i]` as `u8` wrapping; underflow wraps silently. No saturation, no `VSCR[SAT]` update.
- **Useful as a signed sub too** because 8-bit two's-complement sub is bit-identical to unsigned modulo sub. The `m` suffix signals the modulo/wrap semantics regardless of signed interpretation.
- **Big-endian byte lanes.**
- **No `Rc`, no XER.**
- **No VMX128 sibling.**

## Related Instructions

- [`vadduwm`](vadduwm.md)/[`vadduhm`](vadduhm.md)/[`vaddubm`](vaddubm.md) — modulo adds at various widths.
- [`vsububs`](vsububs.md) — saturating sibling.
- [`vsubsbs`](vsubsbs.md) — signed saturating byte sub.
- [`vsubuhm`](vsubuhm.md), [`vsubuwm`](vsubuwm.md) — half-word / word modulo subs.

## IBM Reference

- [AIX 7.3 — `vsububm` (Vector Subtract Unsigned Byte Modulo)](https://www.ibm.com/docs/en/aix/7.3.0?topic=set-vsububm-vector-subtract-unsigned-byte-modulo-instruction)
- [IBM AltiVec Technology Programmer's Interface Manual, Chapter 4 — Integer Arithmetic](https://www.nxp.com/docs/en/reference-manual/ALTIVECPIM.pdf)