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Ferranti Pegasus Computer
From: Science Museum
| By:
Tony Sale |
EDITOR'S INTRODUCTION |
The Ferranti Pegasus computer is another striking example of the importance of Britain, and especially Manchester, in the creation of a modern computer society. Although it is now considered slow, large and bulky, this electronic computer introduced a new form of 'order code', which is a simple way of instructing a computer to carry out actions in sequence. |
 he Ferranti Pegasus electronic computer is an example of an early computer that predates the use of transistors or modern integrated circuits. It is a tribute to the engineering design skills at Ferranti that this example, built in 1959, is still fully functional. |
The Pegasus is most impressive to see and hear when running. Its valve circuitry consumes eighteen kilowatts of power provided by a noisy generator, while air conditioning units roar to keep it cool; and all this is to achieve the computing power of a modern hand-held calculator. But it should be remembered that when the first model appeared in 1956 it was capable of performing calculations infinitely faster than adding machines powered by human operators, the only alternative form of calculation generally available at that time. |
The Pegasus computer used what was, for the time, a novel form of construction in which the circuitry was built from a number of modular packages. The package concept was first developed by Elliot Bros at Boreham Wood and arose from a Navy requirement for an electronic gun-control computer. Previous computer designs involved assemblies of components wired together as one or two large units. However for shipborne operations it was not possible to carry spares of such large units, and so in the Elliott system the circuitry was built up from a large number of packages of which there were only a small number of different types. Each package within a type was interchangeable, and hence only a small number of spares had to be carried since a faulty package could be replaced and then repaired elsewhere. |
The package concept had many critics. Some said that the plugs and sockets needed at the rear of packages would be unreliable; others that the use of packages would restrict the logical design of a computer. All of these criticisms proved unfounded as Pegasus so successfully demonstrated. |
The designer Christopher Strachey (1916-75) was at this time working at the National Research and Development Corporation (NRDC) and was an enthusiastic supporter of the package concept. Basing his design on experimental work he had done with Elliot Bros, he proposed the basic architecture for the Pegasus. |
Strachey also proposed a new type of logic design, the 'order code' for the computer. This was a major advance in clarity and simplicity. The order code is the way in which a computer is instructed to carry out the very limited actions that it can perform. The computer uses binary information: ones and zeros. Humans cannot easily express themselves in this way; it is the order code which mediates between human intentions and the computer's internal processes. |
In devising his logical structure for the machine, Strachey was following the example of Maurice Wilkes (b. 1913), one of the leading British computer pioneers, who designed and built the EDSAC computer at Cambridge in the late 1940s. Wilkes stressed the importance of the order code and placed great emphasis on ease of programming even at the expense of computing speed. As a result Pegasus was a machine that was easy to programme, logically very 'clean' and much loved by all who worked with it. Strachey's order-code structure persisted into the late 1970s in the ICL 1900 range of computers. |
The Pegasus I evolved into the Pegasus II which, although of the same outward appearance, had improved circuitry, a larger drum store and more peripherals, including a printer and magnetic tape units. In all, 38 Pegasus I and IIs were sold, many going overseas. The Pegasus was one of the first computers to be produced on a production line set up in Ferranti's factory at West Gorton in Manchester. |
The Science Museum's Pegasus, no. 18, was first sold to Scania in Sweden in 1959. In 1963 it came back to Ferranti whose computer division had by then become part of Standard Telephones and Cables Ltd. It then went to the chemistry department of University College London where Dr Judith Milledge used it for the analysis of X-ray crystallography results. In 1983 it was acquired by the Science Museum though it was initially retained by International Computers Ltd at West Gorton where it was put on display to visitors. In 1989 it was restored to the Museum. This coincided with the formation of the Computer Conservation Society, and former Pegasus maintenance engineers were contacted to set up a Pegasus Working Party. The machine was in full working order again by the summer of 1990. |
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