Reply to post: Re: Quite a challenge

This whopping 16-bit computer processor is being built by hand, transistor by transistor

the spectacularly refined chap

Re: Quite a challenge

Doing a 8-bit design would have been less than half the work.

That is something of a mixed bag. It reduces complexity in terms of components and wiring but significantly increases design effort. Large parts of a 16 bitter are simply the equivalent 8 bit circuit replicated but going in the other direction introduces some significant extra issues. Presumably you would want to address more than 256 bytes memory so that makes an address (at least) two words long. Similarly it's somewhere between difficult and impossible to encode a complete, useful instruction set in eight bits so you have multi-word instructions too. That gives you a large amount of hassle co-ordinating those half quantities and you need multiple cycles to send those values around. That in turn adds complications as you co-ordinate timing in multiple-cycle instructions.

I mentioned somewhere above that 12-bitter I set about designing a few years ago. 12 bits was chosen very deliberately as the simplest option - it's the narrowest width where you can sensibly have arithmetic, addresses and instructions all the same length. All instructions were single cycle so keeping everything in sync was also made a lot simpler, even if multiple cycles would have allowed you to crank up the clock rate a little. It did limit you to a 4096 word address space but I considered that adequate for a demonstration system.

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