Operating system decides how to share the CPU among programs running concurrently.
A process is a program in execution. It is composed of the program, its data, and all other needed information such as the memory address of the first program instruction. Multiprogramming is the concurrent execution of several programs. Some operating systems such as the Windows family are designed to run on single-user computers while others as UNIX support multi-user computers. Multiprogramming on single-user computers is referred to as multitasking. For example, a Windows 98 user can be writing a letter using a word processor while at the same time printing one file and downloading another from the Internet. For that case, Windows 98 has to decide how to share the CPU among these three processes.
