Building the TD4 4-Bit CPU

It's a really neat board. You can get kits from aliexpress etc. I wrote up some notes here https://www.philipzucker.com/td4-4bit-cpu/ . English descriptions are not so readily available.

I also had some fun modelling the chips in verilog and model checking a verilog interpreter against them. https://www.philipzucker.com/td4_ebmc/ George Rennie got a similar thing working using the yosys toolchain https://github.com/georgerennie/philip_zucker_sby_demo

I love this 4bit "isa" with 12 simple instructions. It makes me want to dig out my old 74x chips from storage and make one. That said, thank God for FireFox Reader mode, on mobile at least the add popups make this excellent blog post unreadable..

These little projects are such a good way to illustrate principles in a concrete way, even if they aren't "useful" in any other regard.

Back in 1983 or so, I had a TI/99 computer and found a BASIC program called "PicoProcessor" in one of the home computing magazines. It emulated a 4-bit microprocessor. It only had a handful of instructions, 16 bytes of memory, and a couple of registers but it was enough to illustrate the concepts of how a processor runs machine code, in a way that was much more understandable than just reading about it.

Could I write any useful programs with it? No. But could I see how a CPU adds two numbers? Yes. And that was enough of an introduction that assembly language was suddenly not so mysterious.

Making a 74 series CPU is on my infinite to-do list.

Last time I tried I ended up getting sidetracked by making a tool to help me. https://fingswotidun.com/PerfBoard/

But at least that's made the job of building a nice compact ALU module much easier. (One of the test boards in the app shows it)

it would be remiss to not mention the most excellent ben-eater's 8bit-computer https://eater.net/8bit and ofcourse the nand-to-tetris book + resources (https://www.nand2tetris.org/)

where can I find the full list of instructions?

Nice!