You'd want to use CP/M for as much I/O as possible. CP/m runs in all-RAM mode. So the main ROM isn't mapped in lower address space.
Though I'm sure Steve or Phil could show you how to do a far call to the main ROM if you really need to. But you probably don't. Of course you also can do direct I/o using the in and out instructions without jumping to the main ROM. No ROM switching code needed. -- John.
