Memory addresses -- beginner here.

Forgive me if this question is a little vague. Where can I find a reference work that tells me where certain pieces of information about the status of the system (e.g. caps lock and num lock status, and characters on a plain ASCII screen) are located in memory? I used wrinte in QuickBASIC occasionally in high school, and I'm starting to get back into it. But I never did much PEEKing and POKEing, and I want to learn how. Any help is appreciated.

Thanks,

--B.--