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Comment by rwmj | Hacker News Reader

QEMU TCG is not that difficult to understand, and has quite comprehensive support for RV64 already, so you could add a new instruction there. As for compiler/kernel support, that's probably a lot more difficult, but it's hard to say what would be involved (eg. deep changes, or simple __asm__ statements) without knowing what kind of feature you'd be trying to add.

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what if i said risc-v isn't as important (to start), but rather just a dead simple isa with a dead simple emulator, compiler and operating system (like something that can take cross-compiled binaries and run them with basic input and output).

like i'd like to experiment with novel ideas in the area, but ideally would like to avoid getting into the complexity of real compilers, operating systems and emulators until the ideas are looking proven...

i can go looking more for stuff myself (maybe toy mips stuff used in schools or something) but this seems like something that people would do and therefore there'd be known tooling?

If you're asking "do simple teaching OSes/toolchains exist?", of course they do (Minix is the most famous one). If you want risc-v specifically, with a quick search I found xv6, which seems to have been put together for https://pdos.csail.mit.edu/6.S081/2021/ .

If you're into the open ISA idea but find the big guys a bit intimidating, you might have fun with OpenRISC. At least lately I've had a blast hacking on it. The kernel and QEMU implementations are very simple, and Stafford Horne is fun to talk with.

I built a RISC-V emulator in python focused on extensibility and debugability for this exact purpose. It should be dead simple to use with plain RISC-V Assembly files. It has a userspace-only mode and a minimal privileged mode: https://github.com/AntonLydike/riscemu

Adding new instructions is as simple as adding a method to a class, as long as you are reading straight assembly files. If you want to parse ELF-files, you also have to add the opcodes for your command to the decoder (which is currently not very well documented). If you'd like to do that, feel free to open an issue and I'll work you through it.

I also built a minimal kernel, which works on the emulator, but it doesn't support IO and is very rough around the edges in general.

Another alternative is to look at something like PicoRV32. There's no "OS" as such that can run on PicoRV32, there is only a few kb of RAM, so single C files that you write from scratch. But it's a simple project and fun for hacking. You can put it on a cheap Lattice FPGA using a completely open source toolchain. A fun way to get into hardware / FPGAs for sure.

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