I’d rather Have That Much Mars


A Computer Called Leo is a wonderful book, full of background but not skimping on the detail of very early computers. The memory on the first British computers was in the form of a 5 foot long tube of mercury which was fed with a series of pulses representing the bytes to be stored. These pulses in the form of sound, took a defined length of time to pass down the entire tube and could then be fed back in to the start. In the EDSAC computer, these tubes gave it the equivalent of 2 KB which for those of us who first cut our teeth on the ZX81 seems quite generous. (Actually, that is not true. My first computer program was written out on coding sheets and typed in by nice people at the council offices. We received a print out of the results and a nice roll of paper tape with the code on it. I did have that first roll for ages but it seems to have gone missing). The tubes were originally used to filter out stationary objects in radar returns. If you take an incoming radar signal, delay it by a short time, feed it back into it’s own input, you can work out what part of the signal was there in the previous cycle and take it out. This removes things like trees, hills and large assemblages of nougat, leaving only moving stuff like falling whales and Messerschmitts. Of my two most intrepid aunts, the one who was in the WRAF and looked after teams of Radar operators and battle-plotters, actually worked with the guys who invented the mercury delay tubes. I never really asked her about her history but various things have come together. Freddie Williams who was instrumental in building the tubes for use in radar, worked at the Telecommunications Research Establishment in Malvern (where I come from) which possibly explains why my aunt came to be in Malvern as well. She also talked a lot about the Manchester computer experiments which by any definition were the first stored-program computers. The British obsession with secrecy probably gave the original award for this to the Americans. Both the radar stuff and the Colossus computers at Bletchley Park were lost in the OSA and indeed in the case of colossus, destroyed along with all their design notes. The last two Colossus machines were kept at GCHQ and were finally destroyed in the 60s. Interestingly, Public Key Encryption was invented at GCHQ years before Diffie-Hellman but was kept secret.

For some time now I have tried to follow some sort of basic course about the hardware of computers but never get the time. The fantastic thing about the first machines is that it is actually possible for one person to understand the whole thing. It’s a bit like a fractal; you might not be able to handle all or even more than a few of the numbers but the process by which the results are produced is simple enough to code in one page. The architecture of a basic computer is quite simple; it just brings together lots of simple parts to make a whole which can handle complex operations.