This is because in modern operating systems, only a portion of a program is loaded at any one time. The rest of the program sits waiting on a disk unit till it is needed. This saves memory space. Computer programs are executed by processors . A processor is a chip in the computer which executes program instructions.
Processors execute millions of instructions per second.
A Process A process or task is a portion of a program in some stage of execution. A Thread A thread is a separate part of a process. A process can consist of several threads, each of which execute separately. This gives good response times for complex programs. Windows NT is an example of an operating system which supports multi-threading.