vavgub

Vector Average Unsigned Byte

vavgub vD, vA, vB

Computes (a+b+1)/2 for bytes.

Encoding

Binary Layout

0:5

6:10

11:15

16:20

1026

21:31

Format VA-form

Opcode 0x10000402

Extension VMX (AltiVec)

Operands

vD
Target
vA
Src A
vB
Src B

Example

vavgub vd, va, vb

Registers Altered

MSR

Reference

View in PowerISA v3.1C Specification ↗ OPF Power ISA v3.1C

Description

Computes the average of corresponding unsigned bytes in vA and vB using rounding (vA + vB + 1) >> 1, storing the result in vD. This is a vector operation that processes 16 independent byte elements in parallel. No condition flags or status fields are affected.

Operation

for i in 0 to 15 do
  vD[8*i:8*i+7] ← ((vA[8*i:8*i+7] + vB[8*i:8*i+7] + 1) >> 1)[7:0]
end for

Programming Note

The vavgub instruction is commonly used for averaging pixel values in image processing tasks. Ensure that the source vectors are properly aligned to avoid performance penalties. This instruction operates at user privilege level and will raise an exception if the vector facility is not enabled (MSR.VEC=0).