Software

The most important aspect of a digital computer is its programmability. To program ENIAC, engineers had to rewire the machine. This task is now is much less involved and is accomplished by computer programs called software. It is this set of instructions that control the operation of the computer hardware by a step-by-step sequence of commands that instruct the computer to perform various functions. The program is actually a pattern of 0's and 1's (an algorithm) that is stored in the computer's main memory for subsequent execution. Programming is, therefore, the process of designing step-by-step algorithms that are useable by the computer. Today, the computer programmer no longer lists series of 0's and 1's, but instead uses symbolic notations known as "assembly" (machine) language. However, assembly language is still dependent on the machine's hardware design. High level languages that use English-like commands were therefore developed in order to achieve machine independence and to ease the writing of algorithms.

FORTRAN (Formula Translating System), was the first comprehensive, high-level programming language and is still widely used today in engineering and scientific applications. Another popular language that was developed in the 1960's is BASIC (Beginner's All-purpose Symbolic Instruction Code). Also still popular is Pascal, which is a highly structured language that enforces programming techniques, and "C", a language used for operating systems and in many professional and commercial-quality programs. Object-oriented programming has recently gained much attention in such languages as SmallTalk and Visual C++. These object-oriented programming (OOP) languages are based on self-contained modules of code and data, called classes, which can be easily modified and reused in other products.

A computer program written in any of these languages must first be decoded (or "compiled") and loaded into the computer. Loading programs, reading data to and from hard disks, and responding to the user's keypresses is the task of a special program known as the operating system (OS). Other functions of the operating system include defining the way the user accesses data and insuring that data to be printed are in fact sent to the printer. Typically, the OS echoes the user's keystrokes onto the computer screen for visual inspection. Pressing the "Enter" or the "Return" key then prompts the operating system to execute the command that has been entered by the user. Alternatively, in operating systems with a graphical user interface (GUI), a mouse can be moved across a flat surface to position the cursor at a specific place on the computer screen. A button is then clicked to indicate the desired position and some action on the selected object is performed or an associated command is executed. In many microcomputers, the OS is stored partly in ROM and partly loaded from the disk. Some examples of popular operating systems include MS-DOS, OS/2, VMS, Macintosh Operating System, Finder, and UNIX.

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