Ferranti Pegasus

Ferranti Pegasus

Pegasus was the computer designed around a programmer's complaint. When Ferranti set out to build a mid-range machine for British industry in the mid-1950s, Christopher Strachey — already the country's most formidable programmer — laid down three design objectives, the first of which was a quiet act of war on the era's culture: minimize the need for optimum programming, he wrote, "because it tended to become a time-wasting intellectual hobby of the programmers." On a drum machine, arranging code so the rotating memory delivered instructions just-in-time was an addictive puzzle; Strachey wanted the machine, not the programmer, to absorb the pain. Add "the needs of the programmer were to be a governing factor" and "cheap and reliable," and you have the industry's first user-experience brief.^[1]

The machine that resulted — built by W. S. "Bill" Elliott's team on modular plug-in packages (it began life named the Ferranti Package Computer) — was generous where 1956 machines were usually miserly: eight accumulators, seven of which doubled as index registers (a first), two 19-bit instructions packed per 39-bit word, and an extra bit in each word usable as a hardware breakpoint for debugging. Above the 128 words of nickel delay-line working store sat a 7,936-word drum spinning at 3,750 rpm. Ferranti sold 38 of them — its most popular valve computer — from 1956 to 1959, at about £50,000 for the base machine.^[2]

A machine that worked for a living

Pegasus machines did real jobs for decades: stresses in the Saunders-Roe SR.53's tailplane (1956), ground-resonance analysis for the helicopter that became the Westland Scout, and — as a party trick with consequences — 7,480 digits of π in 1957, a record at the time. Stafford Beer installed one at United Steel in Sheffield, the first computer installed expressly for management cybernetics. And the reliability numbers vindicated Strachey: the machine at King's College, Durham (Newcastle) logged better than 98% availability in its second year.^[3]

Where it sits

Pegasus is the counterexample to "early computers were programmer-hostile." Strachey — who would go on to co-design CPL and invent denotational semantics — used one machine to argue that hardware should serve the programmer, not audition for him. It sold well, ran forever, and left its design philosophy written into everything that came after.

Check your understanding

• What were Strachey's three design objectives for the Pegasus?
• What was unusual about the Pegasus's accumulators?
• How many were built, and what made the Science Museum's machine famous?
• What mathematical record did a Pegasus set in 1957?