Sylpheed/tools/ppc-manual/vmx/vaddubs.md

vaddubs — Vector Add Unsigned Byte Saturate

Category: VMX (Altivec) · Form: VX · Opcode: 0x10000200

Assembler Mnemonics

Mnemonic	XML entry	Flags	Description
vaddubs	vaddubs	—	Vector Add Unsigned Byte Saturate

Syntax

vaddubs [VD], [VA], [VB]

Encoding

vaddubs — form VX

• Opcode word: 0x10000200
• Primary opcode (bits 0–5): 4
• Extended opcode: 512
• 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	vaddubs: read	Source A vector register.
VB	vaddubs: read	Source B vector register.
VD	vaddubs: write	Destination vector register.
VSCR	vaddubs: write	Vector Status and Control Register (NJ/SAT bits).

Register Effects

vaddubs

• Reads (always): VA, VB
• Reads (conditional): none
• Writes (always): VD, VSCR
• Writes (conditional): none

Status-Register Effects

• vaddubs: VSCR[SAT] may be stickied on saturating vector operations.

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

/* 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

vaddubs

• Canary XML: tools/ppc-instructions.xml — search for mnem="vaddubs"
• Canary emitter: src/xenia/cpu/ppc/ppc_emit_altivec.cc:379
• Sylpheed opcode: crates/sylpheed-ppc/src/opcode.rs:302
• Sylpheed decoder: crates/sylpheed-ppc/src/decoder.rs:590

Canary emitter (frozen snapshot @ f21ebd49e9)

int InstrEmit_vaddubs(PPCHIRBuilder& f, const InstrData& i) {
  Value* v = f.VectorAdd(f.LoadVR(i.VX.VA), f.LoadVR(i.VX.VB), INT8_TYPE,
                         ARITHMETIC_UNSIGNED | ARITHMETIC_SATURATE);
  f.StoreSAT(f.DidSaturate(v));
  f.StoreVR(i.VX.VD, v);
  return 0;
}

Special Cases & Edge Conditions

• Sixteen unsigned-byte lanes, saturating. Each VD[i] = min(VA[i] + VB[i], 0xFF) for i = 0..15. Lane 0 is the most-significant byte after stvx.
• VSCR[SAT] is sticky-set if any lane saturates. Once set, it stays set until mtvscr clears it. ⚠️ Canary never records SAT: its StoreSAT(DidSaturate(v)) writes a private vscr_sat byte, the x64 DID_SATURATE is a stub that always yields 0, and mfvscr reads vscr_vec, which only mtvscr changes.
• One-sided clamp. Only the upper bound applies (unsigned add cannot underflow). Distinct from vaddsbs, which clips at both +127 and -128.
• Pixel-blend workhorse. Common usage is to add two unsigned-byte colour vectors with clamp-to-white at 0xFF. Saturation behaves the same way as _mm_adds_epu8 on x86 SSE2 — making it a one-to-one host translation candidate.
• Versus modulo. vaddubm wraps silently and never touches VSCR[SAT]. Use vaddubs when overflow indicates "too bright" / "out of range" and you want to flag it sticky.
• No XER side effects.
• No VMX128 sibling.

Related Instructions

• vaddubm — same width, modulo (non-saturating) add.
• vaddsbs — same width, signed saturating add (range -128..+127).
• vadduhs, vadduws — unsigned saturating add at half / word width.
• vsububs — the matching unsigned saturating subtract (clamps to 0).
• vavgub — rounding average; alternative when you want (a + b + 1) >> 1 without overflow worry.
• mtvscr / mfvscr — read or clear the sticky VSCR[SAT] bit.

IBM Reference

• AIX 7.3 — vaddubs (Vector Add Unsigned Byte Saturate)
• IBM AltiVec Technology Programmer's Interface Manual, Chapter 6 — Saturating Integer Arithmetic