THE ATTEMPT TO REBUILD EDSAC, one of the first general purpose computers created in the UK, has been bolstered by the discovery of a rare part.
The part is a '1A-type' chassis and was donated by US citizen Robert Little from Pennsylvania. The part will be used in reconstructing the room-filling Edsac (or Electronic Delay Storage Automatic Calculator) computer.
Edsac reconstruction began in 2013 and is being undertaken by the National Museum of Computing. The machine is already on display and the plan is to fire it up later this year.
The 1A chassis was designed to contain 28 of the 3,000 Edsac valves. Andrew Herbert, leader of the reconstruction project, explained that "as many as 42 of this type existed in the original machine".
Each chassis provided a logical function to Edsac, some of which were represented only once while others were replicated. The 1A-type chassis was a storage regeneration unit that kept data circulating through a short or long tank.
The part is "quite distressed with corrosion", Herbert said, and much of the wiring has broken away from the tag strips.
The researchers hope to be able to reuse some of the chassis valves, if they still work, as a "tangible connection" between the reconstructed Edsac and the original machine from the late 1940s.
Little obtained the chassis in 1969 from a Dr Robert E. Clark, who lived in Cambridge and acquired the part during an auction at the university when the local Edsac unit was decommissioned and replaced by the faster (and larger) Edsac 2.
"This is the first time any of our project team has heard of the auction and it's tantalising to think that more parts of the original Edsac might be out there," Herbert said.
The thermionic valve and vacuum tube machine was constructed at the end of World War II and originally comprised 12 vertical racks with a maximum of 14 individual horizontal chassis for each rack.
Based on the ideas of John von Neumann for a general purpose computer, Edsac had a total storage capacity of 1,024 words (equivalent to 4KB in modern terms) and could perform 650 instructions per second, 1,500 times faster than the mechanical calculator it replaced.
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