computer Device that processes data (information) by following a set of instructions called a program. All digital computers work by manipulating data represented as numbers. The tallying principle of the abacus was mechanized in calculating machines, such as those devised by Charles Babbage, in which complicated calculations were processed by means of geared wheels. By the mid-1940s mechanical machines were replaced by electronic versions. Some used groups of electromagnetic switches, called relays, to register binary numbers. At any instant, each switch could be either on or off, corresponding to the digits 1 or 0 in the binary system. Stages in the long-term development of electronic digital computers are termed computer generations. In 1946, engineers at the University of Pennsylvania developed the first generation computer. The 27-tonne machine called ENIAC (Electronic Numerical Indicator and Computer) used electronic valves instead of relays. Programming ENIAC to do a particular task was a lengthy process that consisted of changing wired connections. John Von Neumann helped to develop techniques for storing programs in code to avoid this problem. In 1951, UNIVAC 1 became the first computer offered for general sale. This second generation computer used a transistor to perform the same role as valves. As a result, computers became smaller and more commonplace. In the 1960s, a third generation of computers appeared with the invention of integrated circuits, leading to a further reduction in size. Fourth generation computers, developed in the 1980s, are even smaller, utilizing powerful microprocessors. Microprocessors contain a complete central processing unit (CPU) which controls operations. The latest microprocessors contain more than a million transistors and other components, all in a package little bigger than a postage stamp. Read-Only Memory (ROM) and Random Access Memory (RAM) chips act as permanent and temporary electronic memories for storing data. A typical desktop computer system consists of: a main unit, containing a central processor together with memory chips and storage devices (usually magnetic disks); a monitor, containing a cathode-ray tube; a keyboard; a mouse and printer. Computer programs are usually stored on disks and transferred to the machine's RAM when required. The keyboard and mouse are called input devices, since they allow the user to feed information into the computer. The keyboard enables the user to enter letters, numbers and other symbols. The mouse, a graphical user interface (GUI), is a small device moved by hand, which enables the user to control the computer by positioning a pointer on the monitor screen, to select functions from a list. Fifth generation computers using very large-scale integration (VLSI) chips will utilize the developments of artificial intelligence (AI) and may be controlled by spoken commands. A magnetic disk drive, such as a hard disk, acts as both an input and output device. It can supply programs and data to the computer, and store its output. Most computers have CD-ROM drives; these receive data from an optical storage disk. Many other peripherals are used, such as a scanner which converts images into a digital signal so that they can be stored and displayed by the computer, and other hardware for storing and manipulating sounds. The modern computer market is dominated by PCs – the generic term used to refer to machines based on the original IBM personal computer produced in the early 1980s. All these machines use an operating system (such as DOS or Windows) produced by the giant software corporation, Microsoft. Other popular operating systems include Apple Macintosh and UNIX.