# 4-Bit Computer

Posted on Feb 5, 2014

If you`ve ever wondered how electronic devices like computers can count, this article gives a simple introduction to binary and logic and shows how they are tied together with electronics to make both simple and complex computers. Please note: There are a couple of mistakes on the slides in the youtube video which are shown corrected below. Thanks to reader [Veini] for taking the

time to review the logic diagrams! To begin with let`s have a look at some fundamentals. Since digital computers can only represent two states, on and off or zero and one, there are only two numbers available; therefore they have to count in base 2, not base 10 as we would do. However, it`s very similar, instead of ones, tens, hundreds and thousands, base 2 counts in ones, twos, fours, eights and so on. So, for example 2 in base 10 is one- zero in base 2. When we add numbers in base ten we carry over any digits which are greater than 9 into the next magnitude of units, so nine plus one equals zero carry one, or ten. It is useful to represent this in what`s known as a truth table. Here you can see the 4 possible values of the inputs A and B, and the four possible outputs represented by Sum and Carry. In order to represent the logic` required to get from the possible range of inputs to the desired outputs we use Boolean operations, or as they are more commonly called in electronics, logic gates. Here are the three basic types of logic gates which I`ve chosen because they are the simplest gates to make from transistors. You can make all other types of gates by combining these three. An AND gate outputs one only when both its inputs are one. An OR gate outputs one when either input is one. Finally a NOT gate (or inverter as it is sometimes called) outputs the opposite of its input, so if the input is one the output is zero and...

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