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• Whiel one program waits for IO, other program can run. This is called time-slicing.

• No Multitasking (e.g. MS DOS)
• Cooperative multitasking (program gives up the CPU by going into wait-state) (MS Windows 3.1) (if not, freezing)
• Pre-emptive multitasking (OS is responsible to switch between running processes) (Unix, Linux, Windows 95/NT and later)