No. 78

THE FIRST PROGRAMMABLE COMPUTER

An experimental prototype of what became the world’s first programmable computer (a primitive ‘Turing machine’) began operating at the end of 1943, and paved the way for several such machines to be fully operational by June 1944. The dates are important, because these machines played a key role in providing intelligence for the planners of the D-Day landings in Normandy in the Second World War.

The intelligence was gathered by code-breakers working at the British secret establishment at Bletchley Park. They had previously achieved great success in breaking the German Enigma code using less sophisticated machines, based on principles developed by Alan Turing. But by 1943 the Germans had improved their codes to the point where it was clear that a new kind of machine would be needed to tackle them.

Thomas Flowers, a Post Office research engineer who had already done some work for Bletchley Park, suggested that the solution to the problem would be to build a computer based on electronic valves (called ‘tubes’ in America), which control the flow of electricity through a system and glow like little light bulbs. Flowers had experience of working with valves because he had been involved in developing a valve-based telephone exchange in the 1930s. He had made the key realization that although these valves tended to break down very quickly when they were turned on and off, they had a much longer life if they were left on all the time, even when not in use.

The authorities at Bletchley Park were not convinced, and in February 1943 Flowers went back to his work at the Post Office’s Dollis Hill research station, where the Director allowed him to work on the project semi-officially. But funds were so limited that Flowers paid for much of the equipment himself. The result was a machine using 1,600 valves, fed by a paper tape containing the message to be broken as a series of punched holes. Because of its size, it was dubbed ‘Colossus’.

Colossus was tested at Dollis Hill in December 1943, then dismantled, taken to Bletchley Park in pieces in a series of lorries and re-assembled. There, it broke its first message on 5 February 1944. The code-breakers were astonished. Just a year after having the idea dismissed as pie in the sky, Flowers was told by the authorities to get at least one improved Colossus up and running by 1 June. He was not told why, but just met the deadline with a machine using 2,400 valves. On 5 June, Colossus II decoded a German message which revealed that the German high command had fallen for a deception plan suggesting that the imminent invasion would take place in the Pas de Calais, where Erwin Rommel was told to concentrate his forces. This intelligence was a key factor in persuading General Dwight D. Eisenhower, the Allied commander, to go ahead with the Normandy invasion on 6 June.

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© Bletchley Park Trust/Science & Society Picture Library
Colossus, the world’s first electronic programmable computer, at Bletchley Park in Buckinghamshire.

Although the ‘Colossi’ (several more were built) were there primarily to crack codes, they had a much greater potential. Flowers had designed them so that they could be adapted (‘programmed’, we would now say) to carry out different roles by flipping various switches, and plugging the leads that connected the different parts of the machine in different configurations. This programming had to be done literally by hand, but it made the Colossi true computers in the modern sense of the word. In the mid1930s, Turing had shown mathematically, in a scientific paper entitled ‘On Computable Numbers’, that it is possible to build a machine that will tackle any problem that can be expressed in numerical (digital) terms. The same piece of machinery (what we now call hardware) could be made to do any possible task in accordance with the appropriate sets of instructions (what we now call software) expressed in binary code as a string of 1s and 0s. Such a machine, or computer, is now known as a universal Turing machine, or just a Turing machine, a term that embraces all modern computers, including all the ones in smart phones as well as the big machines used in tasks such as weather forecasting. The Bletchley Park researchers were well aware that the Colossi had the potential to manipulate all kinds of data, including storing pictures, word-processing, or anything that could be expressed in numbers. They had all the components of a genuine all-purpose calculating machine.

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© Topfoto
Colossus in operation.

All this was kept secret even after the war (on the direct instructions of Winston Churchill). As a result, an American machine dubbed ENIAC (from Electronic Numeral Integrator And Computer), which began operating in 1945, is still sometimes referred to as the world’s first electronic computer. Like Colossus, it could be programmed using a plugboard system and, unlike Colossus, it was developed to provide weather forecasts and analyse wind-tunnel tests, among other tasks. But Colossus, and Flowers, were first.