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Show HN: 8-bit CPU simulator in C (github.com)
67 points by rzach 11 days ago | hide | past | favorite | 15 comments

Judging by the files and their contents, this is actually a hardcoded gate-level simulator, which is very different in structure from the traditional loop-with-a-switch emulation that most people are probably expecting.

Advice for the author: if you use a netlist-based abstraction instead of hardcoding the circuit in code, you will be able to simplify the code, and make it much easier to modify and inspect.

Yeah, I know since it's gate-level it will be slow (and it's already been tedious to code) but I wanted to get a better sense of how certain components actually worked. Multibyte RAM, for example, is not demonstrated in the NandGame which this is based off of. It has you put two registers together to make a two-byte RAM and then it magically extends it to a 16KB RAM, only saying that the design could be repeated recursively to get bigger RAM. Figuring out how to do that was interesting and enlightening.

Having no experience with netlists other than briefly reading through the Wikipedia page, it seems they are basically just a struct of subcomponents which is what I'm using. How would I go about abstracting that?

What is a netlist?

It's not quite done. I'm still working on the memory components and the instruction set. Feel free to uncomment the testing functions in test.c to see the other components. Also feel free to give advice on ALU and instruction set design.

Is this a simulation of some theoretical 8-bit CPU, or is it a simulation of a real, existing 8-bit CPU?

For what it's worth, there are many existing C-code emulations of actual 1970s 8-bit CPUs on the Net. (6800, 8080, Z80, 6502, etc, etc, etc.) Studying those may assist you in managing to get past the tricky bits.

In my own Z80 emulation work, I have leaned heavily on 'Yaze' and 'Dasm', with a few extra features and debugging of my own.

He mentions it's based on the NandGame, so it's theoretical, but somewhat well defined. Some background here: https://www.hackster.io/news/learn-how-to-build-a-simple-cpu...

Thanks for the recommendations. I've briefly looked at the 6502 instruction set. Part of the problem is that most of those real-world CPUs use multibyte instructions, which I'm not opposed to, but I'm not sure how to actually incorporate them into my current design (It would be a lot easier if I just did a 6502 emulator without any of the logic gate stuff). Thus I'm trying to come up with an instruction set that utilizes only 8 bits per instruction.

I'm fairly rusty on 1970s CPUs, but digging back into the recesses of my memory I have a feeling that the PDP-8 used single-byte instructions. (or maybe I am remembering the EDUC-8 project that was based on the PDP-8.)

This is neat! I had some thoughts recently about creating something similar as well.

For a simple but still practical instruction set, you can probably sanely get away with about two dozen or so different instructions, but at the expense of having larger programs (by number of instructions) to accomplish the same tasks compared to a more featureful instruction set.

Here's a fairly simple set that might be a reasonable place to start...

ALU: and, or, xor, complement, shift left/right, rotate left/right, add, subtract, multiply, divide, compare (or just use subtract and set flags for this one)

Memory: move (copy between registers), load and store (between memory and a register), maybe also push and pop for sanity if you want to provide explicit stack instructions.

Control flow: jump, branch (maybe depending on flags from ALU), call/return

It might also be helpful to look at the 8-bit AVR instruction set (quite popular for small microcontrollers, including what you might find on many Arduino boards). It contains about a hundred or so instructions, with all of the above list present in one form or another (although many are just aliases for some other instruction with the same opcode but otherwise fixed parameters). In the case of AVR, where code size is often the limiting constraint, having a larger vocabulary of instructions is useful for expressing a program in the smallest amount of space possible, but it certainly isn't necessary to have all of them just for completeness.

Hey, thanks for the recommendations! I'll definitely look into the AVR instruction set. I'm not opposed to multibyte instructions but I'm trying for 8-bit instructions. I'm actually planning to use a special register that holds the ALU input so that I have more space for instructions in the actual instruction. It'll take more instructions since you'll have to set the ALU each time you need it changed, but as it's not a physical microcontroller, I'm not worried about running out of room. Do you think this is a feasible plan?

It's certainly possible, but remember that with 8 bits you have just 256 different possible representations, and every instruction opcode with its set of all distinct valid parameters will map to a number of those limited 256 states. For example, this makes it a big challenge to encode anything other than very small ranges of immediate values inside an instruction.

Typically whenever there's a reference to an N-bit instruction set, it's referring to the widths of the registers and data paths. Very often this is not the same as the instruction size, and in many cases instructions may even be of variable length.

So while it's much more restricting to commit yourself to only an 8-bit instruction size, it should still be possible to build something with it.

You might be interested in the LC-3, which is 15 instructions. I just recently wrote an assembler for it in Python https://github.com/pepaslabs/lc3as.py

Cool project! You might already know about this, but if you don't, this is an excellent course on implementing an 8-bit computer, and might answer your questions about the instruction set. https://eater.net/8bit

I did something similar a year or so ago, a really fun and rewarding project https://djhworld.github.io/post/2019/05/21/i-dont-know-how-c...

I didn't go as far as writing my own OS, but I did develop some simple functions to render characters on a screen and accept keyboard input.

This is pretty cool. I actually remember reading your article a few months back. Once I'm finished with the actual computer I might try to write something with Xlib render a terminal and maybe even a GUI for the OS (unless anyone has a better, higher level alternative to Xlib?)

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