Computer, Digital

The digital computer is a programmable electronic device that processes numbers and words accurately and at enormous speed. It comes in a variety of shapes and sizes, ranging from the familiar desktop microcomputer to the minicomputer, mainframe, and supercomputer. The supercomputer is the most powerful in this hierarchy and is used by organizations such as NASA to process upwards of 100 million instructions per second. The impact of the digital computer on society has been tremendous; in its various forms, it is used to run everything from spacecraft to factories, healthcare systems to telecommunications, banks to household budgets. The story of how the digital computer evolved is largely the story of an unending search for labor-saving devices. Its roots go back beyond the calculating machines of the 1600s to the pebbles (in Latin, calculi) that the merchants of Rome used for counting, to the abacus of the fifth century b.c. Although none of these early devices were automatic, they were useful in a world where mathematical calculations, laboriously performed by human beings, were riddled with human error. By the early 1800s, with the Industrial Revolution well underway, errors in mathematical data had assumed new importance; faulty navigational tables, for example, were the cause of frequent shipwrecks. Such errors were also the source of irritation to Charles Babbage, a brilliant young English mathematician. Convinced that a machine could do mathematical calculations faster and more accurately than humans, Babbage in 1822 produced a small working model of his difference engine. The difference engine's arithmetic functioning was limited, but it could compile and print mathematical table with no more human intervention needed than a hand to turn the handles at the top of the model. Although the British government was impressed enough to invest £17,000 in construction of a full-scale difference engine, it was never built; the project came to a halt in 1833 in a dispute over payments between Babbage and his workmen. By that time, Babbage had already started to work on an improved version--the analytical engine, an automated programmable machine that could perform all types of arithmetic functions. The Analytical Engine had all the essential parts of the modern computer: an input device, a memory, a central processing unit, and a printer. For input and programming, Babbage used punched cards, an idea borrowed from Joseph Jacquard, who had used them in his revolutionary weaving loom in 1801. Although the analytical engine has gone down in history as the prototype of the modern computer, a full-scale version was never built. Among the deterrents were lack of funding and a technology that lagged well behind Babbage's vision.

Even if the analytical engine had been built, it would have been powered by a steam engine, and given its purely mechanical components, its computing speed would not have been great. Less than twenty years after Babbage's death in 1871, an American by the name of Herman Hollerith was able to make use of a new technology--electricity--when he submitted to the United States government a plan for a machine that could compute census data. Hollerith's electromechanical device tabulated the results of the 1890 U.S. census in less than six weeks, something of an improvement over the seven years it had taken to tabulate the results of the 1880 census. Hollerith went on to found the company that ultimately emerged as IBM. World War II was the movitation for the next significant stage in the evolution of the digital computer. Out of it came the Colossus, a special-purpose electronic computer built by the British to decipher German codes; the Mark I, a gigantic electromechanical device constructed at Harvard University under the direction of Howard Aiken; and the ENIAC, another huge machine, but one that was fully electronic and thus much faster that the Mark I. Built at the University of Pennsylvania under the direction of John Mauchly and J. Presper Eckert, the ENIAC operated on some 18,000 vacuum tubes. If its electronic components had been laid side by side two inches apart, they would have covered a football field. The ENIAC was a general-purpose computer in theory, but to switch form one program to another meant that a part of the machine had to be disassembled and rewired. To circumvent this tedious process, John von Neumann, a Hungarian-born American mathematician, proposed the concept of the stored program--that is, coding the program in the same way as the stored data and keeping it in the computer for as long as needed. The computer could then be instructed to change programs, and the programs themselves could even be written to interact with each other. For coding, Neumann proposed using the binary numbering system--0 and 1--rather than the 0 to 9 of the decimal system. Because 0 and 1 correspond to the on or off states of electric current, computer design was greatly simplified. Neumann's concepts were incorporated in the British-built EDSAC and the University of Pennsylvania's EDVAC in 1949, and in the UNIVAC and other first-generation computers that followed in the 1950s. All these machines were large, plodding dinosaurs by today's standards. Since then, advances in programming languages and electronics--among them, the transistor, the integrated circuit, and the microprocessor--have brought about computing power in the forms we know it today, ranging from the supercomputer to far more compact models.