6502 assembly language for the Atari 2600 (which actually uses a 6507). I've always wanted a lower level understanding of computers, but it took childhood nostalgia to finally motivate me. I also enjoy reading about the original engineers from Atari, Activision, Commodore, etc, so it's been rewarding to dabble in that world.
Sounds interesting! What resources are you using to learn? In case you haven't seen it already, http://8bitworkshop.com/ provides an online IDE.
I am currently working through this online course by Gustavo Pezzi called "6502 Assembly Language for the Atari 2600". https://courses.pikuma.com/courses/atari2600 It has been excellent. Just the pace and level of detail that I need. I am lucky to have stumbled upon it.
I am also reading "Making Games for the Atari 2600" by Steven Hugg. Previously I was simply exploring what is available on http://www.6502.org/, working through the Easy 6502 tutorial https://skilldrick.github.io/easy6502/, and checking out YouTube tutorials.