Right. I’ve spent over three hours today trying to understand why you can’t call a bios ISR when in protected mode. I get that once you set and IDT it wont necessarily be in the usual address for the IVT plus segments dont have a fixed size in Protected mode, etc.. But I still don’t get why can’t you jsut create a single 4GB segment, map your IDT segments to the BIOS IVT, set everything in ring 0 and call them. Shouldn’t that work?
Most articles either say: “Remember you cant use BIOS interrupts in protected mode!” without exploring the subject or are extremely descriptive and cite traps, exceptions, pics remmaping, lack of rights and problems with segment registers as the reason behind it.
It would be extremely helpful if someone could come up with a more human-friendly explanation… I’m not doubting what the articles say, I just want to understand why it is such a ‘pain’!
Thanks in advance!
I think the biggest problem is that the BIOS routines were written assuming the processor is in real mode. If you call them from an unsupported context, you can’t be sure the BIOS routines will behave as expected. They could fail on their own, or they could mess up the processor state and kick you out of protected mode.