The Farranti 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. At least two Pegasuses survive, one in The Science Museum (London) and one in The Manchester Museum of Science and Industry.
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 know 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 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. The Pegasus basic instruction cycle time for add/subtract/move and logical instructions was 128 microseconds. Multiply, divide, justify and shift instructions took a variable time to complete. Transfers to and from magnetic drum were synchronous and had to be optimised where possible. The layout of blocks on the magnetic drum was interleaved to allow some processing between transfers to/from consecutive blocks.
In 1956 the first Pegasus was used to calculate the stresses and strains in the tail plane of the first vertical take off aeroplane, the SR53; the results were used to check the manufacturers figures; the programmer was Anne Robson. Because of the importance of a computer it was housed in the drawing room, complete with an Adam's ceiling, of Ferranti's London office in Portland Place.
In 1957, a Pegasus computer was used to calculate 7480 digits of pi, a record at the time.