Let me start by saying that I've never really investigated the BIOS before, so I may make some false assumptions below. Please point out the errors of my ways.
Me too!! I've got a pretty good idea how things work, but do x86 asm. I'm also unsure about some of the memory limitations, and what, if anything, does the BIOS do once a protected mode OS starts.
Under linux, bios is not used (well, with exceptions of APM).
Other 'systems' switch into v86 mode to call bios functions.