A program stored on disk is passive code. A process is one running instance of a program together with the resources needed for its execution.

Process state

A process has execution state such as current instruction position, register state, memory mappings and operating-system resources. Two processes can run the same program while holding different data and being at different points in execution.

Identity and lifecycle

Operating systems assign a process identifier such as a PID. A process is created, becomes runnable, can alternate between running and waiting for events, and eventually terminates with an exit status.

Address spaces

Each process normally sees its own virtual address space. This isolation prevents ordinary memory accesses in one process from directly reading or overwriting another process's private memory.

Process as an abstraction

A physical CPU only executes instructions. The process abstraction packages an instruction stream with protected memory, resources and resumable execution state so the operating system can manage many independent computations.

Several processes can therefore make progress concurrently: by taking turns on one core or by executing simultaneously on several cores.