Multithreading typically refers to sharing a single CPU between multiple tasks (or "threads") in a way designed to minimise the time required to switch threads. This is accomplished by sharing as much as possible of the program execution environment between the different threads so that very little state needs to be saved and restored when changing thread. Multiple threads can be executed in parallel on many computer systems. This multithreading generally occurs by time slicing, wherein a single processor switches between different threads--in which case the processing is not literally "simultaneous", for the single processor is only really doing one thing at a time. On a multiprocessor system, threading can be achieved via multiprocessing, wherein different threads can run simultaneously on different processors.