computer

A mainframe computer. Functionally, it has the same component parts as a microcomputer, but on a much larger scale. The central processing unit is at the hub, and controls all the attached devices.

A hard disk. Data is stored in sectors within cylinders and is read by a head which passes over the spinning surface of each disk.

A floppy disk drive. As the disk is inserted into the drive, its surface is exposed to the read-write head, which moves over the spinning disk surface to locate a specific track.

Programmable electronic device that processes data and performs calculations and other symbol-manipulation tasks. There are three types: the digital computer, which manipulates information coded as binary numbers (see binary number system); the analogue computer, which works with continuously varying quantities; and the hybrid computer, which has characteristics of both analogue and digital computers. In common usage, when someone refers to a ‘computer’, they tend to mean a digital computer.

There are four types of digital computer, corresponding roughly to their size and intended use. Microcomputers (personal computers, or PCs) are the smallest and most common, used at home, in small businesses, and in schools. They are usually single-user machines. Minicomputers (or mid-range computers) are found in medium-sized businesses and university departments. They may support from around 10 to 200 users at once, although these have now largely been replaced by networks of microcomputers. Mainframes (or enterprise servers), which can often service many hundreds of users simultaneously, are found in large organizations, such as national companies and government departments. Supercomputers are mostly used for highly complex scientific tasks, such as analysing the results of nuclear physics experiments and weather forecasting.

In 2004 San Diego, Phoenix, Detroit, New York, and Sacramento were found to be the most wired cities, with broadband penetration of 65% or higher.

Microcomputers now come in a range of sizes, from battery-powered electronic organizers, palmtop and laptop PCs, to floor-standing tower systems that may serve local area networks or work as minicomputers. Large-scale computers built out of multiple microprocessors are starting to challenge traditional mainframe and supercomputer designs.

Binary number code This is the code based on the binary number system that is used to represent instructions and data in all modern digital computers – for example, in the ASCII code system used by most microcomputers, the capital letter A is represented by the binary number 01000001.

Because binary numbers use only the digits 0 and 1, they can be represented by any device that can be set to two different states. In a digital computer several different two-state devices are used to store or transmit binary number codes – circuits, which may or may not carry a voltage; disks or tapes, parts of which may or may not be magnetized; and switches, which may be open or closed. Digital computers are designed in this way for two reasons. Firstly, it is much easier and cheaper to construct two-state devices than devices that can exist in more than two states. Secondly, communication between two-state devices is very reliable because only two different signals, 0 or 1 (on or off), need to be recognized.

Basic components At the heart of a computer is the central processing unit (CPU), which executes individual program instructions and controls the operation of other parts. The CPU has three main components: the arithmetic and logic unit (ALU), where all calculations and logical operations are carried out; a control unit, which decodes, synchronizes, and executes program instructions; and the immediate access memory, which stores the data and programs on which the computer is currently working. All these components contain registers, which are memory locations reserved for specific purposes. A main power supply is needed and a cooling system, as the CPU is more efficient when working at lower temperatures. The computer's interface circuits control the peripheral devices that can be attached. These will normally be keyboards and VDUs (visual display units) for user input and output, disk drive units for mass memory storage, and printers.

Programming Computers are controlled by the input of instructions in various programming languages. In procedural programming, programs are written as lists of instructions for the computer to obey in sequence, and this is by far the most popular form of programming. Declarative programming, as used in the programming language PROLOG, does not describe how to solve a problem, but rather describes the logical structure of the problem. Running such a program is more like proving an assertion than following a procedure. Functional programming is a style based largely on the definition of functions. There are very few functional programming languages, HOPE and ML being the most widely used, though many more conventional languages (for example C) make extensive use of functions. Object-oriented programming, the most recently developed style, involves viewing a program as a collection of objects that behave in certain ways when they are passed certain ‘messages’. For example, an object might be defined to represent a table of figures, which will be displayed on screen when a ‘display’ message is received.

Programming languages may be classified as low-level languages or high-level languages. Low-level languages, such as assembly languages, closely reflect the machine code of specific computers. In contrast, a high-level programming language, such as C++, Visual Basic, or Java, consists of instructions more easily readable by humans, which are then translated into the machine code understood by a particular computer using a compiler or an interpreter. High-level languages are thus both easier to program in and more portable.

History Computers are only one of the many kinds of computing device. The first mechanical computer was conceived by Charles Babbage in 1835. He designed an analytical engine, a general-purpose mechanical computing device for performing different calculations according to a program input on punched cards (an idea borrowed from the Jacquard loom). This device was not built in Babbage's lifetime, but it embodied many of the principles on which digital computers are based. A working example has been completed now that the components can be manufactured to high enough tolerances, something that could not be achieved in Victorian times.

The US inventor Herman Hollerith devised the first device for high-volume data processing, a mechanical tabulating machine. Hollerith's tabulator was widely publicized after being successfully used in the 1890 census. The firm he established, the Tabulating Machine Company, was later one of the founding companies of IBM.

In 1943, more than a century after Babbage's analytical engine, Thomas Flowers (1905–1998) built Colossus, the first electronic computer. Working with him at the time was Alan Turing, a mathematician who seven years earlier had published a paper on the theory of computing machines that had a major impact on subsequent developments. John Von Neumann's computer, EDVAC, built in 1949, was the first to use binary arithmetic and to store its operating instructions internally. This design still forms the basis of today's computers.

In 2001, Israeli scientists announced that they had built the first automatic deoxyribonucleic acid (DNA) computer, which solves problems using DNA strands as software and enzymes as hardware.