Best part, I still have that Commodore VIC-1213 "Machine Language Monitor" [1] along with my beloved VIC-20. I also had (and still have) a character-editor [2] and with that setup I wanted to write a computer game.
Problem was, I didn't know how to write a game, so with the Machine Language Monitor I disassembled Jeff Minter's Gridrunner [3] and learned really wonderful tricks.
I remember, in Gridrunner there was a main loop with 20 JSRs or so and each JSR handled a piece of the game, querying the joystick, updating the spaceship, producing the sound, etc... (kinda cooperative multitasking)
It was a wonderful journey to learn all this stuff from that game and then write my own one...
Jeff, if you ever read this, thank you for that wonderful Gridrunner, not only was playing it a lot of fun, I learned a lot from you!
I had 3 whole K of RAM (some of which was used by the system), so writing pretty much anything required machine code.