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Show HN: 8-bit CPU simulator in C

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Re: Show HN: 8-bit CPU simulator in C

#11

Post by rzach »

userbinator wrote: post #5

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?

Re: Show HN: 8-bit CPU simulator in C

#12

Post by rzach »

simonblack wrote: post #3

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 an…

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.

Re: Show HN: 8-bit CPU simulator in C

#13

Post by rzach »

ls65536 wrote: post #6

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 reasonabl…

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?

Re: Show HN: 8-bit CPU simulator in C

#14

Post by cellularmitosis »

ls65536 wrote: post #6

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 reasonabl…

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

Re: Show HN: 8-bit CPU simulator in C

#15

Post by simonblack »

rzach wrote: post #12
simonblack wrote: post #3

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 an…

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 util…

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.)

Re: Show HN: 8-bit CPU simulator in C

#16

Post by ls65536 »

rzach wrote: post #13
ls65536 wrote: post #6

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 reasonabl…

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…

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.

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