Ferranti Pegasus

In those days it was common for it to be unclear whether a failure was due to the hardware or the program. As a consequence, Christopher Strachey of NRDC, who was himself a brilliant programmer,[12] recommended the following design objectives:

1. The necessity for optimum programming (favoured by Alan Turing) was to be minimised, "because it tended to become a time-wasting intellectual hobby of the programmers".
2. The needs of the programmer were to be a governing factor in selecting the instruction set.
3. It was to be cheap and reliable.

The first objective was only partially met: because both program and the data on which it was to operate had to be in the 128 words of primary storage contained in 8-word nickel delay lines. The rest of the memory was held on a 7936-word magnetic drum,[13] which rotated at 3750 rpm,[14] so it was often necessary to use ingenuity to reduce the number of transfers between the fast store and the drum.

The front panel of the Pegasus

Pegasus had eight accumulators, seven of which could also be used as index registers, 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. Each word contained 39 bits plus 1 bit for parity checking. Two 19-bit instructions were packed into one word, with the extra bit that could be used to indicate a breakpoint (optional stop), to assist in debugging. In line with Strachey's second objective, 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.

The speed of arithmetic operations was about the same as in the Elliott 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 the drum were synchronous. The layout of blocks on the drum was interleaved to allow some processing between transfers to/from consecutive blocks. The computer was advertised as weighing 2,560 lb (1.2 t).[15]

To what extent Strachey's third objective was reached, depends on how one views a price of £50,000 for Pegasus 1, which did not have magnetic tape drives, line printer or punched card input and output. The modular design with plug-in units of hardware did, however, make it very reliable by the standards of the day, and maintenance was "a doddle of a job".[10]

A printout from a Pegasus computer

The initial version of Pegasus, Pegasus 1 was intended for scientific and engineering applications. Its input was via 5-hole paper tape with output on tape. The variant for business data processing was called Pegasus 2 and could be equipped with punched cards, magnetic tape and line printer.[16]

In 1956 the first Pegasus was used to calculate the stresses and strains in the tail plane of the Saunders-Roe SR.53; 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.

A Pegasus 1 was installed at Cyber House, Sheffield by Stafford Beer for the use of United Steel. It was the first computer installed for management cybernetics.[17] The Pegasus at Southampton University was used for analysis of ground resonance data for the Saro P.531 helicopter, which eventually entered production as the Westland Scout and Westland Wasp.[18]

In 1957 a Pegasus computer was used to calculate 7480 digits of π, a record at the time. In 1959 Handley Page Ltd were advertising for experienced Pegasus programmers to join their aviation design team at Cricklewood, London [19]

The University of Leeds had a Pegasus computer, run by Sandy Douglas. This was used, among other things, for a project to process the University's matriculation records.[20]

Other people who worked on the Pegasus included Hugh McGregor Ross and Donald B. Gillies.

• List of vacuum-tube computers

• Berners-Lee, C. M. (22 May 1969). "Pegasus – A Pioneer in Reliability and Ease of Use" (PDF). Computer Weekly. p. 12. Archived (PDF) from the original on 18 June 2022.
• Felton, G. E. (1962), The Pegasus Programming Manual (PDF), London: Ferranti Ltd.
• Kershaw, Donald, "Experiences with Pegasus 1", Resurrection: The Bulletin of the Computer Conservation Society (14)
• Lavington, Simon (1980). Early British Computers: The Story of Vintage Computers and the People Who Built Them. Manchester University Press. pp. 78–86, 102–105. ISBN 0719008034 – via Internet Archive.
• Lavington, Simon (2000), The Pegasus Story: A history of a vintage British computer, London: Science Museum, ISBN 978-1900747400
• Ross, Hugh McGregor (2012), Pegasus: The Early Seminal Computer, Colleagues, Sandy, Bedfordshire: Authors Online, ISBN 978-0-7552-1482-2
• The Ferranti Pegasus Computer (YouTube). The Science Museum. 13 May 2015.
• Weik, Martin H. (June 1957). A Second Survey of Domestic Electronic Digital Computing Systems (Report). Aberdeen, MD: Ballistic Research Laboratory. pp. 112–114. BRL Report No. 1010 – via HathiTrust.

Wikimedia Commons has media related to Ferranti Pegasus.

• The History of Computing Project: The Industrial Era, 1955–1957 Archived 2012-06-04 at the Wayback Machine
• The 1962 edition of the Pegasus Programming Manual (38 MB PDF)
• The Computer Conservation Society, who helped to maintain the Pegasus installation in the London Science Museum