Ferranti Pegasus (computer)

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A typical Pegasus computer installation, on view at the Science Museum, London.

The front panel of the Pegasus.

Pegasus was an early thermionic valve (vacuum tube) computer built by Ferranti, Ltd of Great Britain.

The Pegasus 1 was first delivered in 1956 and the Pegasus 2 was delivered in 1959. Ferranti sold twenty-six copies of the Pegasus 1 and twelve copies of the Pegasus 2, making it Ferranti's most popular valve (vacuum tube) computer.

Christopher Strachey recommended these design objectives:

optimum programming (favored by Alan Turing) was to be avoided "because it tended to become a time-wasting intellectual hobby of the programmers";
the needs of the programmer were to be a governing factor in selecting the order code (instruction set); and
it was to be cheap and reliable. (See also Optimum programming for further information.)

Pegasus had eight accumulators, seven of which could also be used as index registers. (It was the first computer to allow this dual use.) It had 56 words of fast memory stored in nickel delay lines, which was supplemented by a magnetic drum holding 5120 words. A word was 40 bits, of which one bit was for parity checking. Two 19-bit instructions were packed into one word and the extra bit (not counting the parity bit) could be used to indicate a breakpoint, to assist in debugging. It had a relatively large instruction set. It was about the same speed as the Elliot 402 computer, which could add in 204 microseconds and multiply in 3366 microseconds.

In 1957, a Pegasus computer was used to calculate 7480 digits of pi, a record at the time.

[edit] References

Early British Computers, by Simon Lavington, Digital Press (US) and Manchester University Press (UK), 1980, ISBN 0-932376-08-8.
The Pegasus Story: A history of a vintage British computer, by Simon Lavington, Michigan State University Press, 2000. ISBN 1-900747-40-5.