“Apple first introduced PowerMac G4 computer systems using AltiVec — a high performance vector processing expansion to the PowerPC architecture — in the fall of 1999. Architecturally, AltiVec adds 128-bit-wide vector execution units to the PowerPC architecture. Early versions of the G4 processor had a single AltiVec unit, while more recent versions have up to four units (simple, complex, floating, and permute). These vector units operate independently from the traditional integer and floating-point units”. Read the interesting article at O’ReillyNet.