Building the TD4 4-Bit CPU(jayakody2000lk.blogspot.com)
jayakody2000lk.blogspot.com
Building the TD4 4-Bit CPU
https://jayakody2000lk.blogspot.com/2026/05/building-td4-4-bit-cpu.html
20 comments
I just translated the entire book into English via multimodal LLM, which with my (rusty) Japanese language knowledge looks largely correct (including diagrams and manga text.)
I'm not sure of the legal ramifications of this but if anyone shows me their original PDF receipt from the Japanese publisher's website I'd be willing to share it under the conditions you don't publish or share it anywhere publicly...
I'm not sure of the legal ramifications of this but if anyone shows me their original PDF receipt from the Japanese publisher's website I'd be willing to share it under the conditions you don't publish or share it anywhere publicly...
My contact info is just my user name here + the common Google provided domain name
Huh, so its isa is just "Dst := Src+Imm", with Dst one of [A,B,Out,PC] and Src one of [A,B,In,Zero]
Plus just a tiny bit of logic that wires in "Carry Flag" when Src=Zero to control Dst=PC
I guess "MOV A,B" sets Imm to zero for simplicity, because I don't see anything stopping it
Plus just a tiny bit of logic that wires in "Carry Flag" when Src=Zero to control Dst=PC
I guess "MOV A,B" sets Imm to zero for simplicity, because I don't see anything stopping it
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..
Update #2
Ok now I cheated and had Claude translate the entire book into English. It even translated the manga images with hand written hiragana/katana in speech bubbles (using comic sans) plus all the diagrams!
I decided to not take on too much in one go, so having the English text takes a lot of variables out of the equation for me since my Japanese is very rusty after not using it for 20 years..
Ok now I cheated and had Claude translate the entire book into English. It even translated the manga images with hand written hiragana/katana in speech bubbles (using comic sans) plus all the diagrams!
I decided to not take on too much in one go, so having the English text takes a lot of variables out of the equation for me since my Japanese is very rusty after not using it for 20 years..
Update: I broke down and bought the PDF version, now I need to brush up on my Japanese again. Should be a fun project over the coming rainy days..
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.
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)
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/)
But why is this always the first comment on custom CPU builds? Can't there also be other designs out there?
Noone ever mentions this one, but I always found it pretty cool: https://www.donnamaie.com/AMD_2900.html
Because they are both well-known and really well documented in a way that's easy for beginners.
I've been following this series for a while: https://www.youtube.com/playlist?list=PLyR4neQXqQo5nPdEiMbaE...
i look forward to other resources as accessible to those pointed out. by all means, go for it.
where can I find the full list of instructions?
Nice!
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