Ferranti Pegasus

The Ferranti Pegasus was one of the first commercial calculating engines built from thermionic valves. Architecturally, it had a number of similarities to modern machines. Seven of its eight registers could be used as both accumulators and index registers, meaning that they could store the results of arithmetic and logical operations as well as being used for addressing memory. This idea is found in most modern CPUs, where general purpose registers have the same abilities.

Instructions for the Pegasus were referred to as orders. Orders were issued to the Pegasus two at a time, each taking up 19-bits of a 40-bit word. One of the remaining bits was used for parity checking - a feature very important on machines of this era which had comparatively unreliable storage. The machine also featured hardware support for debugging - a feature that didn’t make it into Intel CPUs until the Pentium. The remaining bit in a word was used to indicate a breakpoint, where execution should pause and wait for operator intervention.

Just as modern machines have a fast cache near the CPU and a larger store of slower main memory, the Pegasus had 56 words of fast memory stored in delay lines and 5120 words of slower storage on magnetic drums. In a modern machine, the cache memory is loaded from the main memory in blocks known as cache lines and it is with machines like the Pegasus that this terminology originates. Each of the delay lines in the Pegasus could store eight words and these were moved between the main store and the delay line as a block.

The first Pegasus came online in 1956 and was used to perform stress calculations for the Saunders-Roe SR.53 interceptor, a hybrid jet and rocket propulsion aircraft and the first plane capable of vertical takeoff.

When the Pegasus was built, all computers in use in Britain were from British manufacturers. This would change towards then end of the ’50s, when companies like IBM began to market their machines outside the USA.

David Chisnall

Further Reading: Simon Lavington, The Pegasus Story, Science Museum, London, 2000.