I actually have a tool I wrote to automatically derive on x86 decoder from observing hardware execution (based in part on sandsifter, so please don't ask me if I've heard of it), and it turns out to largely be a lot simpler than people make it out to be... if you take a step back and ignore some of what people have said about the role of various instruction prefixes (they're not prefixes, they're extra opcode bits).

(FWIW, this is fairly dated in that it doesn't cover the three-byte opcodes, or the 64-bit prefixes that were added, like the REX and VEX prefixes).

I spent some time last weekend on a small side project which involves JIT encoding ARM64 instructions to run them on Apple Silicon.

I’ve written assembly before, but encoding was always kind of black magic.

How surprised was I to learn how simple instruction encoding is on arm64! Arguably simpler than implementing encoding wasm to byte code, which I played with a while ago.

If you want to play with this, based on my very very limited experience so far, I’d suggest starting with arm - fixed length 4 byte instructions, nice register naming scheme, straightforward encoding of arguments, make it very friendly.