Выполнение компьютерных программ

Running of a program by a computer

Execution in computer and software engineering is the process by which a computer or virtual machine interpret and acts on the instructions of a computer program. Each instruction of a program is a description of a particular action which must be carried out, in order for a specific problem to be solved. Execution involves repeatedly following a "fetch–decode–execute" cycle for each instruction done by control unit. As the executing machine follows the instructions, specific effects are produced in accordance with the semantics of those instructions. Programs for a computer may be executed in a batch process without human interaction or a user may type commands in an interactive session of an interpreter. In this case, the "commands" are simply program instructions, whose execution is chained together. The term run is used almost synonymously. A related meaning of both "to run" and "to execute" refers to the specific action of a user starting (or launching or invoking) a program, as in "Please run the application."

The context in which execution takes place is crucial. Very few programs execute on a bare machine. Programs usually contain implicit and explicit assumptions about resources available at the time of execution. Most programs execute within multitasking operating system and run time libraries specific to the source language that provide crucial services not supplied directly by the computer itself. This supportive environment, for instance, usually decouples a program from direct manipulation of the computer peripherals, providing more general, abstract services instead.

In order for programs and interrupt handlers to work without interference and share the same hardware memory and access to the I/O system, in a multitasking operating systems running on a digital system with a single CPU/MCU it is required to have some sort of software and hardware facilities to keep track of an executing processes data (memory page addresses, registers etc.) and to save and recover them back to the state they were in before they were suspended. This is achieved by a context switching. The running programs are often assigned a Process Context IDentifiers (PCID). In Linux based operating systems, a set of data stored in registers is usually saved into a process descriptor in memory to implement switching of context.

A system that executes a program is called an interpreter of the program. Loosely speaking, an interpreter directly executes a program. This contrasts with a language translator that converts a program from one language to another before it is executed.