Sometimes original codepaths (such as BIOS) are still present in modern machines, but basically forgotten. In those cases you may be able to execute code that hasn't been tested/expected in quite awhile.

However, I think the raw BIOS functions don't work in extended mode, but I may be wrong.

Thanks, I know nothing about the BIOS and other related hardware topics. Does "extended mode" mean UEFI? I guess an introductory OS lecture is going to be helpful.

Extended mode means - 32 or 64 bit mode, once "thunked" into that I believe you can no longer access the BIOS directly.

So even if there are exploits lurking, they wouldn't affect modern OSes which are all at least 32 bit now.

This is specifically to the x86 family, it's possible that some other chip still has original code ready to go even in 32 or 64 bit mode.

Thanks for the explanation! I Googled a bit and found the topic fascinating (especially the ones regarding malwares that are particularly difficult to remove)