All operating systems use one fundamental abstraction: the process. A process can be defined either as "an instance of a program in execution" or as the "execution context" of a running program. In traditional operating systems, a process executes a single sequence of instructions in an address space ; the address space is the set of memory addresses that the process is allowed to reference. Modern operating systems allow processes with multiple execution flows — that is, multiple sequences of instructions executed in the same address space.

Multiuser systems must enforce an execution environment in which several processes can be active concurrently and contend for system resources, mainly the CPU. Systems that allow concurrent active processes are said to be multiprogramming or multiprocessing. 161 It is important to distinguish programs from processes; several processes can execute the same program concurrently, while the same process can execute several programs sequentially.

Continue reading here: [61 Some multiprocessing operating systems are not multiuser an example is Microsofts Windows

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