The PET emulator was an interesting case. It relied on the fact that BASIC 2.0 on the C64 was similar enough to that on the PET (and IEC serial devices similar

The PET emulator was an interesting case. It relied on the fact that BASIC 2.0 on the C64 was similar enough to that on the PET (and IEC serial devices similar enough to IEEE-488 ones, from the BASIC side, at least) that many programs ran unmodified. The interesting bits were adding some goop so that many common POKEs and PEEKs from BASIC would "just work" as well (like CB audio, screen memory, etc.). This was even enough for some machine language programs, though many choked on the different memory map.