An EDSAC Simulator

Posted 07 Jun 2024

I’ve wanted to get a simulator for these classic computers, because it’s sometimes hard to understand how they work from descriptions in books.

EDSAC Replica Project

EDSAC was shut down in 1958, then torn down. In 2011, a group started a project to recreate EDSAC at the UK’s National Museum of Computing, at Bletchley Park.

As part of that work, Martin Campbell-Kelly at the University of Warwick wrote a faithful simulator of the machine. You can get that here. It runs on Windows 10, most flavors of Linux, and on the Raspberry Pi.

This program is not FOSS. But a different simulator with source is available here. This one has a permissive license, but is restricted to non-commercial use. So not really FOSS either.

Run The Original Demo Programs

EDSAC ran its first programs on 6 May 1949. The first program computed the square of the integers from 1 to 99. The second program computed prime numbers.

These programs are available for the Warwick simulator (I haven’t tried running the other one yet.)

Simulator Running The Squares    Program

The Squares Program

Simulator Running The Primes    Program

The Primes Program

A Matter of Time

Did you notice the clock on that virtual EDSAC console? That’s there because the simulator will allow you to run at the original speed of EDSAC. If you want to really appreciate how primitive and slow these machines were, use the “real time” checkbox in the UI.

If you do that, you’ll find that it takes EDSAC 7 minutes to calculate the fist 100 squares. For comparison, a quick Perl script to do the same thing takes 8 milliseconds.

Have fun!