Design Your Own Microprocessor

YOU’LL NEED THIS

INTERNET BROWSER www.falstad.com/circuit

LOGISIM www.church.com/logisim

MOST LIKELY, the processor in your PC has upwards of a billion transistors. Trying to get your head around the workings of such a complicated electronic circuit might, therefore, seems a daunting task. Yet helping you to understand what goes on inside a CPU is the task we’ve set ourselves here. The latest chips are complicated by many bells and whistles doing way more than straight “processing”, so we should think of a very basic chip and take it on trust that the principles here can indeed be scaled up.

In 1971, the very first microprocessor—the 4-bit Intel 4004—still had 2,250 transistors on board. While that might be a whole lot more manageable than a couple of billion, it might still seem to be no mean feat to understand how that worked from an electronics viewpoint.

But that this isn’t nearly as difficult to understand as you might fear. The fact is that logic circuitry is created in a so-called bottom-up approach. Transistors—the fundamental electronic building block—are used to create logic gates, which are the fundamental logic building blocks. From this point on, we can forget about transistors and see how logic gates can, in turn, be used to create more complicated logic building blocks such as decoders and multiplexers. In the next stage, these are used to create even more complicated logic elements, and this continues until we finally end up with a complete CPU. Nowhere in this process will we see more than a handful of transistors or a similar number of logic gates—what could be easier?

1 LOGIC GATES

The bottom-up approach might make it easier to understand complicated logic circuitry, but learning about it with real hardware would still be a major undertaking. After all, since each transistor has three leads, and because components other than transistors are also needed, building even