Real mode BIOS routine and Protected Mode

Question

I am doing some OS experiment. Until now, all my code utilized the real mode BIOS interrupt to manipulate hard disk and floppy. But once my code enabled the Protect Mode of the

Answer 1

It seems that your question is not how to talk to hardware (the problem device drivers would solve) because the BIOS interfaces are sufficient for you.

Rather, you need to know how to communicate between protected mode ring 0 (which has unlimited access to BIOS calls and all other privileged instructions) and protected mode ring 3 where application code usually lives. This is a system call. Pretty much all architectures run interrupt handlers in privileged mode so a software interrupt is one way of implementing system calls, x86 also provides a syscall instruction which is optimized for this purpose.

Of course, you could just run everything in ring 0 with a flat memory model where you can access all memory directly.

Ring 0/ring 3 is the x86 terminology, but all systems with an MPU support some concept of a privileged mode which allows access to memory by physical address (and for split memory-I/O architectures, access to all of I/O space).