I didn't do much programming on the c64, but things got fun when I somehow acquired a book full of SYS codes. Apparently you could cheat if you loaded the game, did a soft-reset, so the game was still in memory, and entered a SYS code.
Doing a soft-reset required a cartridge though, like the later popular Action Replays for the Amiga. I didn't have access to these fabled cartridges, so I did the next best thing; A bent paper clip, short circuiting some random holes in the cartridge slot. This caused the desired reset, sometimes, allowing me to pass in the SYS codes, and RUN the game again, various cheats applied.
Today I wish I had access to this "bible" of SYS codes, if only for the nostalgia. It provided me with many hours of fun, and breathed new life into old games. All I remember is that the front of it had a sort of robotic character on it, and some purple colours.
You'd have to look for kernal memory map with entry points or BASIC memory map.
Like, perhaps, this one here: http://unusedino.de/ec64/technical/aay/c64/krnromma.htm