Write up of my homebrew CPU build • Hacker News Board

Re: Write up of my homebrew CPU build

#51

Post by ninalanyon » Wed, Mar 18, 2026, 9:32 PM UTC

fc417fc802 wrote: post #26

ninalanyon wrote: post #24

Earlier quoted context omitted.

Even Babbage's Analytical Engine had microcode.

Would you care to elaborate? That sounds both unlikely (but I assume I'm just naive) and also interesting.

Part of the machinery is a cylinder that orchestrates various very low level operations this means that the Jacquard cards can specify a higher level operation. Exactly how sophisticated it is, or is supposed to be, I'm not sure.

And now that you've challenged me I can't remember where I saw this piece of information. Time for a quick web search.

Found something, I don't think this is where i saw it first but it will do:

"Later drawings (1858) depict a regularised grid layout.[18][19] Like the central processing unit (CPU) in a modern computer, the mill would rely upon its own internal procedures, roughly equivalent to microcode in modern CPUs, to be stored in the form of pegs inserted into rotating drums called "barrels", to carry out some of the more complex instructions the user's program might specify.[7]"

https://en.wikipedia.org/wiki/Analytical_engine

Re: Write up of my homebrew CPU build

#52

Post by Lerc » Wed, Mar 18, 2026, 10:16 PM UTC

wwarren wrote: post #49

Teknoman117 wrote: post #46

The unfortunate reality of building these home-brew CPUs is that almost all of the "medium integration" ICs are long out of production - things like the 74181 ALU slice, carry lookahead adders, 16-way register files, etc. Makes doing things larger than 8/16 bit computers very complicated and usually very slow :(

YES, a modern version of the 74181 is equivalent to about 12-20 chips in the LS or HC series. It would be so useful. Someone should do a tiny tapeout!

The Gigatron uses 5 chips for a 4bit ALU.

It's one of the test layouts that I put in this perfboard layout program that I ened up making because trying to figure out wiring on both sides at once in my head melted my brain.

https://fingswotidun.com/PerfBoard/ (Try the 4-bit ALU example)

Of course I haven't yet verified it works because of getting sidetracked by the editor, https://xkcd.com/974/

Re: Write up of my homebrew CPU build

#53

Post by codebje » Thu, Mar 19, 2026, 1:39 AM UTC

When you design the final board, consider making two boards - one for all your logic, and a separate one for all your user-facing I/O - blinkenlights, toggles, buttons, and so on, with something like a ribbon cable connection for them. Being able to separately mount your I/O from your logic board gives you more freedom when it comes time to put the whole thing into some kind of case.

Nice read, thanks for sharing.

Re: Write up of my homebrew CPU build

#54

Post by wwarren » Thu, Mar 19, 2026, 3:24 AM UTC

codebje wrote: post #53

When you design the final board, consider making two boards - one for all your logic, and a separate one for all your user-facing I/O - blinkenlights, toggles, buttons, and so on, with something like a ribbon cable connection for them. Being able to separately mount your I/O from your logic board gives you more freedom when it comes time to put the whole thing into some kind of case. Nice read, thanks for sharing.

Nice idea!!