Making history The project is now in its final stages. All the modules have been built and tested, and the machine can have runs of up to a couple of hours: “That will meet our need for demonstrating it in the museum.“ But integration isn’t simple: “Several parts of the machine are independently clocked from each others, and have to be brought into synchronization,” he says. “You can be held back for a whole cycle - we wouldn’t tolerate that in a modern computer design!” Herbert expects to spend a busy summer getting the machine working properly before opening to the public later in the year. “Our next milestone is when we successfully are able to execute all the arithmetical and general instructions in the instruction set,” he told DCD early in 2021. “We are about halfway through that. We’ve knocked all the big bugs out of the machine.” The group is still learning. “As we commission the machine, we’ve discovered some things are redundant. Sometimes you don’t need amplifier states, for instance. There are some valves that aren’t doing anything - and we suspect that was the case in the original!” The next step is to get the entire instruction set working, and then to operate using the delay lines for memory. “The delay lines have been built and tested standalone, so we anticipate that being fairly straightforward.” Eventually, the machine will operate with its traditional input and output via paper tape and a teleprinter. The “initial orders” have been coded onto tape: “That’s like a boot ROM.” “Then we will be done,” says Herbert. We will have achieved the brief of the replica project.“ Life support But the story won’t stop there, because the Museum will then have the issue, as Herbert puts it, of “how to keep the damn thing going.” The reconstruction team has had an incredible depth of experience to draw on, including experts like Peter Lawrence, and chief designer Chris Burton, who worked for Ferranti in Manchester from the 1950s, designing systems like the Pegasus. The group has had a huge spirit of camaraderie, and drawn in pioneers like the early programmers Margaret Marr, Joyce Wheeler and Liz Howe, all of whom shared memories, checking and approving aspects of the final design. But that team won’t be available once the
machine is handed over to the museum. “At the moment you need experts to maintain it,” says Herbert, but that won’t be feasible in the museum, so the restorers will be handing their recreated system over with its own life support system. A small group is looking at creating monitors using modern gadgets like the Raspberry Pi which will track signals from the system. Ideally, these will be able to help museum staff troubleshoot, issuing instructions like “Change valve five on chassis six.” That’s an intrusion of modern technology but is entirely appropriate, he says. “We’ve chosen not to compromise on how EDSAC itself works. It’s as authentic as we can make it. But we’ve had every luxury we can afford on how we build and monitor it. “If you were keeping an elderly genius alive, you would use every medical technique.” The meaning of it all In the end, what is the significance of this achievement? EDSAC will run at around 650 instructions per second, and consumes around 9kW of power. Today, the average phone goes more than a million times faster than that on a tiny power budget. But EDSAC was revolutionary for its time, Herbert points out: “That’s 1,500 times faster than a post doc with a hand calculator.” And it appeared at a time when computing was developing fast, and no one was sure where it would lead. “Cambridge was building a machine to work in the maths lab. They wanted to evaluate it to see what a proper computer could be like. They were surprised by how useful it was and how long it kept going.” It could have been forgotten completely, he says: “Once EDSAC was working they were fed up with it. It was built to be a service machine.
Alan Clarke / TNMOC
"We’ve discovered some things are redundant There are some valves that aren’t doing anything and we suspect that was the case in the original!”
It was handed over as a working service, with an engineering team around it.” The team who built it were excited to move on to EDSAC 2, which made a big step, using a bit slice design instead of EDSAC’s monolithic architecture. “EDSAC 2 used several identical chassis to handle one bit on each one. EDSAC 1 was monolithic. If one chassis blew up, the system was broken. With EDSAC 2, you could pull in a spare one.” EDSAC 2 also took software deeper. “EDSAC 1 was hardwired, but EDSAC 2 was microprogrammed so they could try out new instructions. “ But many of the founding ideas in practical computing flowed from EDSAC. It allowed libraries of subroutines: “That was well done, it was a long while before other machines had such a well designed interface. Others needed machine code, but Cambridge programmers didn’t have to know machine code.“ “It wasn’t the biggest, the fastest or the best engineered,” Herbert sums up, “but it changed the world more than the others.” It kicked off commercial computing in a big way, through the involvement of the J Lyons and Co restaurant chain. Lyons senior management recognized that admin costs were eating away at their margins, and wanted to automate receipts and inventory. They heard that computing was happening in America, and visited the team building Univac, who suggested that EDSAC might be a better fit for them. Lyons provided an engineer, and funding which helped get EDSAC built, and then built its own machine, LEO. It was the first computer in the world to work on commercial applications, and was a clone developed from EDSAC. Herbert hopes the project will revive disappearing expertise and inspire students to learn about the technical challenges of the pioneers, creating a living educational resource for students. “As we have our struggle to work with EDSAC and to get it working, I think back to the pioneers led by Wilkes. They didn’t have modern laptops signal analyzers and Zoom to help them do the work. They had quite simple early oscilloscopes and voltmeters, and we wonder how they went about debugging the machine. "They didn’t even have the knowledge that we had - that it would eventually work. For them at times it must have appeared to be very, very daunting. And, of course, they had no previous experience of building a computer - the team had come from various backgrounds of radar, instrument making, radio, and so forth.” For Herbert, it’s been exciting: “We are privileged that we know how EDSAC works.”
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