It is difficult to recapture the amazing moment when you finally understand how a computer works. I don’t mean the understanding of a particular computer, its specific peculiarities etc. but the general understanding of how this wonderful trick works and what a simple trick it is.

Now the worrying part here is that some readers will have passed this moment of truth many years ago and will have forgotten the pleasure it gave them. Some readers will think they have passed in, but really they haven’t. And finally of course there are the readers who know they don’t know what I am talking about at all! This all makes for a big communication problem made worse by the fact that, to give you the underlying theory I have no choice but to talk about particular realisations of it. Let me give you a short example of this mismatch of world views.

A long, long time ago, in the days when computers had valves, kept people warm and lived in huge buildings (one computer per building), I went to a “Young Person’s Lecture”. It was all about the new (then) art and science of the computer. I knew as little about this subject as it is possible to know, despite having written my first six line Fortran program only a few weeks earlier. The lecture was interesting and it had lots of pleasing “lantern slides” (a sort of early LCD projection unit) and the man pointed to a blackboard (like a whiteboard but black) with a stick (like a laser pointer but made of wood).

I found the whole thing really interesting but I sat up most in the section where he promised to tell me how computer memory worked. I knew a tiny amount about memory – it was where my six-line Fortran program lived before the computer obeyed it – so this was my chance to find out how it worked. The lecturer showed a slide of a large wooden construction something like a bookcase but divided up into compartments which he called “pigeon holes” and, yes, you could see that a family of pigeons might want to take shelter there. He then went on to describe how each pigeonhole had an “address” and this enabled someone to store a pigeon at a particular “location” and then “retrieve” it. “This is how a computer’s memory works” is the final sentence I remember.

I left the lecture in a state of shock and stumbled home. I don’t think I recovered from the idea for many months and certainly Fortran programming was never the same again – possibly to this very day.

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