Making a CPU from Boolean Gates from Scratch

Have you looked at the current announcement of Apple’s transition from intel processors to ARM processors? Maybe you began reading up on things like instruction sets, RISC and CISC processors. You will quickly hit upon a lot of complicated terminology.

So how about going all the way back to basics, and look at what a CPU looks like if you strip it down to its bare bones. What are the most essential parts, and what is just cool stuff added to make it run faster?

Turns out that there is a treasure trove of web pages explaining simple CPU designs and really teaching you everything from scratch. I don’t really have to recreate what they have written, but instead give an overview of all the cool stuff out there.

Mike, author of this site goes through a whole bunch of ways of making a CPU designed like this:

You can make this on breadboard yourself if you want. Let me give some comments to the diagram of how to read it.

The colored lines referred to as busses are usually an abstraction of 16 separate electric wires carrying digital signals. So e.g. the blue lines represent 16 wires carrying data from memory to the various parts of the CPU.

Every part of the CPU in principle gets the 16-bit number read from memory. However the red lines are used to decide which part is activated so it actually receives the number on the blue lines.

The green lines send 16-bit number representing an address. Notice how the RAM (the memory) contains 8 different memory slots numbered from 0 to 7. The green memory bus selects which slot will send the 16-bit number it contains out onto the blue line.

Please note I may be wrong about the green line being 16-bit as you don’t need 16-bits to represent 8 different memory locations. 3-bits would be enough.

Let us take a brief explanation of the the different parts on the diagram which you may not be familiar with.

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