AltiVec is a vector processing unit that is new with the G4 microprocessor. Some system software has been modified to take advantage of the accelerated processing that is possible with AltiVec. System software has also been modified to support low-level AltiVec operations.
The software areas that have been modified to take advantage of AltiVec acceleration are
QuickTime: key codecs, including DV, photo JPEG, MPEG, and MPEG-2
QuickDraw: the 2D acceleration API, which has many blit loops
The software areas that have been added or modified for low-level AltiVec support are
Nanokernel: exception handling and context switching