There was some game (and I think a program in a book) where the border color would be changed at a specific scan line to get a horizon that would span the entir

There was some game (and I think a program in a book) where the border color would be changed at a specific scan line to get a horizon that would span the entire screen. I pretty much knew all the clock cycle counts for the instructions as a teenager, and you would code assembler with them always in mind.