I can't really give a great answer to that, pretty sure it was all just electrical hardware. I'm curious now, I'll look around and let you know if I find something.

Edit: yep, no code, just circuits. Here is the original circuit diagram from Atari

[–] f4f4f4f4f4f4f4f4@sopuli.xyz 11 points 22 hours ago

Many 80s games have an amusing hybrid design. Galaga has its game code and sprites in ROM, but has an entire custom chip that is a dedicated circuit for generating the ship explosion sound, another custom chip that only makes the scrolling starfield, etc.

[–] who@feddit.org 7 points 1 day ago

This is also how some old glass terminals worked.