I have a strong love for computing history so the news that Edsac is going to be rebuilt at Bletchley Park is really exciting.

Edsac stands for Electronic Delay Storage Automatic Calculator and dates back to the 1940s – it was one of the first computing machines that could run rudimentary programs.

Mercury Delay lines were a popular form of computing memory in the early days but it meant that memory had to be accessed serially – and these delay lines were used on both the Edsac and LEO computers – its a shame that these delay lines won’t be used for the rebuild as they pose too much of a health and safety risk (who wants 5 foot long tubes of mercury laying around!?).

I have yet to see one but I have read that the memory bits would be converted to sound waves as the bubbles emerged from one end of the mercury tube. The memory system could handle 1,024 locations however only 512 were often used and these weren’t 8-bit multiples – instead they were 18 bits (though only 17 could be used).

The story was recently reported by the BBC (Pioneering Edsac computer to be built at Bletchley Park).

On a side note, the Lyons LEO computer was built on the Edsac (from the John von Neumann design).

If you have an interest there is a fascinating Edsac Simulator developed by Martin Campbell-Kelly of the University of Warwick. Head over to The Edsac Simulator.