It's common in the C64 world to use freezer cartridges that allow you to interrupt normal operation and invoke a monitor program. These typically include rudimentary assemblers and disassemblers as well. Fun and useful for patching and reverse engineering!

Btw, I've only had a rough idea of how these are actually implemented, non-maskable interrupt something-something, and never knew how they make sure that the interrupt vector points to some persistent routine that can invoke the monitor. I found this great article explaining at least how The Final Cartridge 3 does it (in the first two sections): https://www.pagetable.com/282