This video walks through the disassembly of the Commodore PET Space Invaders 6502 executable. It starts with an explanation of assembly language and how that is converted to machine language that can be executed by the CPU. It then walks through an example disassembly of the Space Invaders Commodore PRG file step by step, resulting in a final disassembly that can be modified and re-assembled to a working game.
You can download the disassembled Space Invaders code here:
http://www.commodore.international/downloads/invaders.asm