Ferranti Pegasus

Pegasus was an early vacuum tube (valve) 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 fourteen copies of the Pegasus 2, making it Ferranti's most popular valve computer.

At least two Pegasus machines survive, one in The Science Museum, London and one in The Manchester Museum of Science and Industry. The Pegasus in The Science Museum ran its first program in December 1959 and was regularly demonstrated until 2009 when it developed a severe electrical fault. In early 2014, the Science Museum decided to retire it permanently, effectively ending the life of one of the world's oldest working computers. The Pegasus officially held the title of the world's oldest computer until 2012, when the restoration of the Harwell computer was completed at the National Museum of Computing.

Christopher Strachey of NRDC recommended these design objectives:

The first objective was only partially met: because both program and the data on which it was to operate had to be in the 56 words of primary storage, it was often necessary to resort to tricks in order to reduce the number of transfers between that store and the drum memory. To what extent the third objective was reached, depends on how one views a price of £50,000 for Pegasus 1 without tape drives, line printer or punched card input and output, which required an hour or more of preventative maintenance by a resident engineer every morning, before a programmer or operator was allowed near it.

Pegasus had eight accumulators, seven of which could also be used as index registers. (It was the first computer to allow this dual use.) Accumulators 6 and 7 were known as p and q and were involved in multiply and divide and some double length shift instructions. 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 (optional stop), to assist in debugging. It had a relatively generous instruction set for a computer of its time, but there was no explicit hardware provision for handling either characters or floating point numbers.

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