There are reference manuals with a lot of details. Specifically you'd want to read the "Intel® 64 and IA-32 Architectures Software Developer’s Manual", Volumes 2, which includes specifications of each instruction including pseudo-code of how the processor executes them. You can freely access a PDF of the entire manual here: https://www.intel.com/content/www/us/en/developer/articles/t...

But there's a lot of details there — that volume alone spans about 4000 printed pages in the manual — so you're bound to mess things up.

I've written emulators for simpler instruction sets (RISC-V and the 6502) and found that the best way to ensure they work correctly is to do instruction-by-instruction comparisons against a reference. The good news is that if you've got a PC computer, you've got easy access to a reference machine! So you can do side-by-side comparisons where you set up an initial state on both a real machine and an emulated machine, then execute some code on both, and compare the final states. By using the trap flag you could also single-step through the instructions to extract the intermediate states after each instruction and ensure they match the emulated state.

So that's my guess about how he did his too. It's painstaking work, but also kind of fun when you get into it.

Yes, that's exactly what I did. Most useful thing I did was setting up a unit test framework to test a single instruction, and then generating a huge number of variations of those instructions.

I talked a bit about the Intel SDM in the last post I wrote (linked at the top). The SDM is great for getting the specific details, but isn't always approachable for high level overview things.