Ask HN: Going low-level – what to learn next?

I've been interested in (relatively) low-level programming for quite some time. I know C, I can read assembly (write - not so much) and I have basic understanding of CPU (registers, cache, stack and whatnot). What should I read or learn next to be able to reason about things like cache usage of a C program or possible low-level microoptimizations? It's still just a hobby for me for the most part, so it's not that I'm trying to prematurely optimize some production code. I'm just interested in how it all works under the hood.