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the state machine - or lack of one

A project log for The Blinking Computer

An educational 16-bit CPU made from < 1000 transistors with lots of LEDs that anyone can build and learn from

Tony RobinsonTony Robinson 02/09/2017 at 20:400 Comments

Wow, the state machine of the CPU is hard - really hard. I thought I should start with what took the most real estate, the ALU and the memory cells, now I think I'm (touch wood) on top of both of those (I will update logs with progress as and when) but I'm struggling with the state machine - that is the bit of logic that raises all of the right enable lines at the right time. I shouldn't be at all surprised, there's a few people on home-built computers web-ring who said the same thing - it's the go-to site.

So this isn't really a log entry as I don't have a state machine. What I find fascinating is that a CPU is something that, in my software world, runs compiled code (leaving aside the fact I run Python under Linux in a VM running Windows) but is really something that may execute microcode which may execute itself to fetch instructions (my I = *P++) which executes a state machine which executes a set of control lines which executes a set of logical functions which are expressed as analog signal processing where semiconductor physics (my almost-lost first degree) does the hard work.

So many levels. I really don't like public speaking unless I have something worthwhile to say, but taking all this from a "BASIC" language through FORTH, assembly code, right the way down to the electrons and holes is something I feel confident I can engage and audience with. Teaching technology is not my goal - my goal is to encourage others to explore their dreams, however corny that sounds. But to do that I've got to succeed and that's a lot more time and effort away that I can plan for (even when you take into account Hofstadter's law).

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