SPOTLIGHT ON EDSAC

While the University of Manchester is proud of its development of the first stored program computer, the University of Cambridge makes claim to the first practical stored program computer. For while the SSEM was experimental, the machine at Cambridge, that came less than a year later, was created to solve real-world problems.

More complicated than the SSEM in supporting 14 instructions, one of the main differences, as the name EDSAC (Electronic Delay Storage Automatic Calculator) suggests, was its memory technology. Magnetic core memory wouldn’t appear for another five years but, while the Manchester Baby used a CRT, the EDSAC used a so-called mercury delay line in which ultrasonic waves were continually circulated through a mercury-filled tube.

An interesting episode in the early days of EDSAC will be of interest to all those who have had to debug code, as EDSAC creator Maurice Wilkes later explained. “By June 1949 people had begun to realise that it was not so easy to get programs right as at one time appeared. I well remember when this realisation first came on me with full force. The EDSAC was on the top floor of the building and the tapepunching and editing equipment one floor below…. It was on one of my journeys between the EDSAC room and the punching equipment that, hesitating at the angles of stairs, the realisation came over me with full force that a good part of the remainder of my life was going to be spent in finding errors in my own programs.”