A major milestone in the reconstruction of EDSAC has been passed – some of the sub-systems of the computer have run continuously for one million cycles over the course of 20 minutes. These subsystems have been individually created in home workshops scattered across the country and now the task of integrating all the components is the focus of the team of volunteers.
The systems that have been shown to work together for a significant time include:
the main control unit, which incorporates the order (instruction) fetch and execute actions
memory access, including the coincidence unit and delay line recirculation
order decoding and recoding circuits
arithmetic and logic unit (ALU)
clock and digit pulse generation.
That leaves initial orders, transfer unit, the delay line main store and the paper reader and printer yet to be incorporated.
As expected, there are still minor hardware bugs to be resolved and various refinements to be added which will make operation more reliable, and enable us to demonstrate the full order code.
“Earlier this year, we began integrating the machine but it’s no easy task and progress can seem frustratingly slow when success seems so close,” explained volunteer engineer James Barr. “Like others on the project, I was able to build my components at home, in my Edshack, but the integration is the crucial testing ground and sub-teams of volunteers must travel many miles to the Museum to accomplish this.
“Unlike our esteemed predecessors, we have several teams not one and as volunteers we don’t work full time. Nonetheless, we have one very big advantage – we know the machine will work! As computing pioneers, the 1940s Cambridge engineers didn’t have the comfort of that knowledge, so work must have been very tense and testing for them with plenty of nagging doubts. No-one had ever made a machine like this before!
Andrew Herbert, leader of the EDSAC project assessed the future work: “By the end of November, we hope to have cleared three of the outstanding major bugs. Then work will stop for three months when the 1940s roof above EDSAC is replaced! It’s frustrating for us all, but the roof work is essential and urgent. EDSAC will be boxed in situ for safety. It’s a hurdle that our predecessors didn’t have, but in so many other respects we are reliving those remarkable pioneering days of seventy years ago.”
The completion of the EDSAC reconstruction is scheduled for the end of June next year, just in time to celebrate the 70th anniversary of the unveiling of EDSAC to Cambridge University at what must have been the first computer conference weekend in British history.
Videos of the reconstruction process can be seen on the EDSAC playlist on the TNMOC YouTube channel.