We take it for granted that “computers = digital electronics”, yet this wasn't always the case. Analog computers were popular for many years - and are still useful for some special tasks. You may have seen an old science fiction film (When Worlds Collide) on TV where the hero rushes off to ‘the computer’ to work out whether the Earth is going to be destroyed. (Again?) At this point the view cuts to a room full of whirling rods, gears, and cams. This is because, during the 1940's, the best commercially available computers were mechanical. (Of course, we now know that people were secretly developing and building ‘real’ computers - i.e. digital electronic ones - during the war to crack codes, but the film-makers didn't know that!) In theory, it doesn't matter how a computer works provided it follows the correct rules. That said, since all modern computers use a digital approach that's what we'll look at here.
Digital computers and electronics work because it's possible to hold and process information as patterns of binary numbers. Digital information processing is built upon Boolean Algebra or Boolean Logic. It's also based on some fundamental theorems of logic and mathematics which say that we can build any logical process by using a few appropriate simple ‘rules’ over and over again in the correct ways. Proving something like that is way beyond this course, so we'll believe the mathematicians for now...
In digital systems the basic rules we use for processing patterns of the “1”s & “0”s which make up binary numbers are:- AND, OR, and NOT. The usual symbols for these are:
In theory, every digital information processing system from a digital watch to a Cray supercomputer can be built using components which follow these rules. (Of course, the supercomputer might need slightly better, faster versions of the circuits!) We'll leave the details of how more complex systems can be built up to later, specialised, courses. Here we'll just look at how the electronic circuits can provide the three logical operations (and/or/not) described above. An electronic circuit which performs a basic logical operation is usually called a logic gate. The term ‘gate’ appears to come from the idea that the circuits act like a gate, either passing or stopping logic ‘bits’ (ones and zeros) from passing in a controlled manner.
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University of St. Andrews, St Andrews, Fife KY16 9SS, Scotland.