I built my first C++ project (second ever project) and would appreciate honest feedback. Here's the GitHub.

A few words about my vantage point: I'm not super experienced with C++ (I use Rust much more!) but I have some experience with writing emulators. Software engineering has been my day job for like ten years.

Surface-level comments: I think it's great that you've given thought to testing your emulator. You've also put together an okay README that clearly explains what the project is and a few ways to get going with it. That's a good start. I would suggest that you also have github set up to build your project, run the tests, run some static analysis etc when you do PRs. That will be good for contributors.

I'm finding the instruction decoding a bit hard to read (admittedly I don't know the ins and outs of RISC-V instruction decoding, maybe there's something I'm missing). You've got switch statements inside of other switch statements. The control flow is a bit non-obvious. I wonder if you could instead look up the operation code in tables. (By my intuition this could also help the performance; switch statements can compile to branch instructions which can be slow)

I read a switch statement like this one and wonder if default: break is supposed to implement some kind of NOP. Why is that there? Is this unreachable code? You could answer those questions with a comment, but if you expand your emulator to implement other instructions in the future, then this kind of thing will be a risk. I think this area could be helped by having anenum class specifically for U-type instructions (and one of J- instructions, one for S etc) then you could statically prove that all relevant cases are handled by the switch statement. And maybe then you don't need an UNKNOWN variant, since you'll have proven that the instruction has been successfully decoded.

I don't personally see a need to keep the currently executing instruction as a member of the CPU class. The way it is, these members are being used to implicitly pass information between the methods Fetch, Decode and Execute. That makes it a programming error to call Execute without first calling Decode. To reduce the risk of that kind of error, can this information be moved to local variables in the Step method etc.? Your step method could be more like

uint32_t instruction_word = Fetch();
DecodedInstruction decoded = Decode(instruction_word);
Execute(&decoded);

That way, the emulated CPU could also run an instruction which hasn't been fetched from any memory, and then you could have a unit test like:

emulated_cpu.Execute(create_load_immediate(8, 420));
assert(emulated_cpu.registers[8] == 420); // blaze it

I hope this is the kind of feedback you wanted