Colossus and Programmability

We analyze the capabilities of the Colossus codebreaking devices, built in 1943–1945 under the direction of Tommy Flowers of the UK General Post Office. Colossus is often described as a programmable computer, a misconception we trace to old battles about the “first computer” and to former secrecy about its actual capabilities. In fact, Colossus was not called a computer at the time, and does not meet later definitions because it carried out no mathematical operation other than counting. Colossus automatically executed a program, i.e., performed series of discrete operations, but it was not programmable because that program could not be fundamentally modified by its users. Instead of the old focus on allocating “firsts,” we argue for new perspectives on Colossus focused on its use and impact, its relationship to other early machines, and its place in the history of digital communications engineering.