Computer programs (also software programs, or just programs) are instructions for a computer. A computer requires programs to function, and a computer program does nothing unless its instructions are executed by a central processor. Computer programs are either executable programs or the source code from which executable programs are derived (e.g., compiled).
Computer source code is often written by professional computer programmers. Source code is written in a programming language that usually follows one of two main paradigms: imperative or declarative programming. Source code may be converted into an executable file (sometimes called an executable program) by a compiler. Computer programs may be executed by a central processing unit, with the aid of an interpreter, or may be embedded directly into hardware.
Computer programs may be categorized along functional lines: system software and application software. And many computer programs may run simultaneously on a single computer, a process known as multitasking.
Paradigms
Computer programs can be categorized by the programming language paradigm used to produce them. Two of the main paradigms are imperative and declarative.
Programs written using an imperative language specify an algorithm using declarations, expressions, and statements. A declaration associates a variable name with a datatype. For example: var x: integer; . An expression yields a value. For example: 2 + 2 yields 4. Finally, a statement might assign an expression to a variable or use the value of a variable to alter the program's control flow. For example: x := 2 + 2; if x = 4 then do_something(); One criticism of imperative languages is the side-effect of an assignment statement on a class of variables called non-local variables.
Programs written using a declarative language specify the properties that have to be met by the output and do not specify any implementation details. Two broad categories of declarative languages are functional languages and logical languages. The principle behind functional languages (like Haskell) is to not allow side-effects, which makes it easier to reason about programs like mathematical functions. The principle behind logical languages (like Prolog) is to define the problem to be solved — the goal — and leave the detailed solution to the Prolog system itself. The goal is defined by providing a list of subgoals. Then each subgoal is defined by further providing a list of its subgoals, etc. If a path of subgoals fails to find a solution, then that subgoal is backtracked and another path is systematically attempted.
The form in which a program is created may be textual or visual. In a visual language program, elements are graphically manipulated rather than textually specified.
Compilation or interpretation
A computer program in the form of a human-readable, computer programming language is called source code. Source code may be converted into an executable image by a compiler or executed immediately with the aid of an interpreter.
Compiled computer programs are commonly referred to as executables, binary images, or simply as binaries — a reference to the binary file format used to store the executable code. Compilers are used to translate source code from a programming language into either object code or machine code. Object code needs further processing to become machine code, and machine code is the Central Processing Unit's native code, ready for execution.
Interpreted computer programs are either decoded and then immediately executed or are decoded into some efficient intermediate representation for future execution. BASIC, Perl, and Python are examples of immediately executed computer programs. Alternatively, Java computer programs are compiled ahead of time and stored as a machine independent code called bytecode. Bytecode is then executed upon request by an interpreter called a virtual machine.
The main disadvantage of interpreters is computer programs run slower than if compiled. Interpreting code is slower than running the compiled version because the interpreter must decode each statement each time it is loaded and then perform the desired action. On the other hand, software development may be quicker using an interpreter because testing is immediate when the compilation step is omitted. Another disadvantage of interpreters is the interpreter must be present on the computer at the time the computer program is executed. Alternatively, compiled computer programs need not have the compiler present at the time of execution.
No properties of a programming language require it to be exclusively compiled or exclusively interpreted. The categorization usually reflects the most popular method of language execution. For example, BASIC is thought of as an interpreted language and C a compiled language, despite the existence of BASIC compilers and C interpreters.
Self-modifying programs
A computer program in execution is normally treated as being different from the data the program operates on. However, in some cases this distinction is blurred when a computer program modifies itself. The modified computer program is subsequently executed as part of the same program. Self-modifying code is possible for programs written in Lisp, COBOL, and Prolog.
Execution and storage
Typically, computer programs are stored in non-volatile memory until requested either directly or indirectly to be executed by the computer user. Upon such a request, the program is loaded into random access memory, by a computer program called an operating system, where it can be accessed directly by the central processor. The central processor then executes ("runs") the program, instruction by instruction, until termination. A program in execution is called a process. Termination is either by normal self-termination or by error — software or hardware error.
