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A project log for 4 bit computer built from discrete transistors

This project is an attempt to teach myself about the inner workings of computers. Naturally I started from the bottom up.

zaphod 04/16/2015 at 02:410 Comments

last post I mentioned some problems. I seem to have them under control (read as: successfully swept under the rug) if you're interested in what was going on there's an explanation in the comments on the last post.

now for the big news.

for a while I have had a feeling that this thing was not designed as well as it could be. those of you familiar with processor layout probably noted some of the odd design choices that can be observed in the architecture diagram. notably the ALU is sort of smeared all over the place and there is not a main data bus. this leads to some weird behaviour such as an inability to load the @ register from RAM, another problem the architecture creates is a relatively ineffective conditional branching mechanism. for a while I figured I would just finish this project and build the next one right. no longer. I have decided to rework the computer's architecture. this will have a few advantages, first it will make the computer simpler, and second it will make the computer more universal. this move also comes with some setbacks, namely I'll have to take apart some of the stuff I've all ready built. fortunately all of the major logical blocks remain the same, most of the work should be in pulling them apart, not in actually rebuilding anything. all of the boards I've built will be used (except some multiplexers), but the will be connected in a new way.

here's the old architecture diagram:

and here's the new architecture diagram:

what's new?

as you can see the new design is organized around a central bus removing the need for multiplexers everywhere. an indirect effect of the bus centered design is that the control logic is simpler. you'll notice that the first diagram says that the control logic is simplified, that was my way of saying I had no idea what I was doing (as opposed to now when I have almost no idea what I'm doing). the new diagram shows control logic that is basically one level above individual gates. the complexity of the control logic is the major reason for the redesign. for the old architecture to work I would have needed to design specific control logic for each command, now I need to design 3 sets of control logic and then implement it a few times. a side effect of the revised control logic is that there is also a revised command set, one that is hopefully more universal. later this week I will post a more in depth look at the commands, what they do ,and how they work, but this is it for now.

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