What makes a computer a computer?

It is the secret that everybody knows: a story which has rightly become a legend. During the Second World War, Alan Turing and a team at Bletchley Park invented the computer in order to crack the Enigma code, shortening the war and saving millions of lives. Turing was persecuted for his sexuality and died tragically, to be recognised as a national hero decades later.

Except that, while Turing deserves all the acclaim we can give him, part of this story is wrong. Turing did not invent the computer to crack the Enigma code. The difference between the device used for Enigma, the Bombe, and the real first computer, Colossus, as well as the original thinking that Turing did before the war, help us understand what makes a computer a computer, and why they make such a difference to the world.

The Bombe was not a computer because it was a special device designed for one purpose only: to find a valid setting for the Enigma machine used by the German navy to encode messages.. The Bombe accepted inputs (guesses at the contents of the message - the ‘crib’) and produced outputs (valid rotor settings) but could only do one thing. It was a very important thing, but it was only one thing.

Colossus, built by a team led by Tommy Flowers, a less well known hero of wartime decryption, was different. It was designed to crack a harder code, the Lorenz cypher used by the German high command. In order to crack this code, Colossus was programmable: it could be told to do more than one thing. The physical process of programming Colossus looked very different to the process of programming computers today: there were plugs and switches rather than screens, and keyboards. But the logic on which Colossus operated could be changed, and that’s what made the difference.

Turing made a direct contribution to Colossus through his work on the mathematics and statistics of decryption, but he had made an even more important contribution before the war. Turing was working on a complex mathematical question of whether certain propositions can be proven by computation: the iterative exercise of logical operations. In this work he managed to show that some propositions are not provable in this way. But, even more importantly, in the process, he created the idea of the ‘universal computing machine’: a machine which follows instructions and can therefore be used to emulate any other machine which follows logical operations.

It is difficult to overstate the importance of this idea: it makes all the difference between special purpose machines which can only do one thing, and machines which can be programmed to exercise any logic. It is the birth of the digital age, and it has given rise to much of the modern world and the computing revolution which has only just started.

It is why the mobile phone in your hand and the mainframe sitting in your bank are fundamentally examples of the same thing, even though they spend their time in very different environments, they look very different and they serve very different purposes. We could take even more extreme examples: the University of Michigan has created a computer which measures 0.3mm square, while the Fugaku, the world’s most powerful supercomputer, cost $1BN and contains 158,976 CPUs. But from the perspective of their natures as universal computing machines, these are just details.

What makes a computer a computer is its programmable nature. The next article in this series will consider how we get a computer to do what we want: what is programming? And why do programmes seem to go wrong all the time?

The Round Trip Question: Journey Map

This series of articles is driven by a conviction that computing is increasingly important to our lives, but many people don’t understand how they work, and that those of us working? in the industry therefore have a duty to explain. It attempts to answer The Round Trip Question: what happens when you press ‘send’ on the mobile banking app on your phone?

I’ll use this section at the bottom to capture the list of questions which arise as I write each article. If I go wrong, or if you have other questions, please tell me in the comments.

To-do:

What is computer programming?

Why do programmes go wrong all the time?

How does my mobile phone know that I am me?

How does my bank’s computer know that I am me?

Why do action heroes ‘break into the computer room to hack the mainframe’? How realistic is that?

What’s a mainframe?

What’s a computer room?

[From Bradley Safer] Who else can see my data? What are they allowed to do with it?

There will be plenty more questions. For now, though, here’s the very rough picture of what we have covered so far:

(Views in this article are my own.)