The only thing that is definitly – that protected mode with 32 bit stack is not suppoprted in some UNDI functions. But it’s not said e.g., when PXE! structure fields related to segments selectors are filled if system starts in real mode and my code will start PM. It’s said “filles in by UNDI before any calls are made”. But, how it’ll find selector in my space? Or I must use this selector and load GDT uusing provided values. And etc. It’s a little bit hazy for me.

Thanks for you help suggestion, but as I see in Etherboot and other PXE related projects (pxeboot in FreeBSD), main trick is to go in real mode (to be more correct: virtual 86 mode) before calling PXE and UNDI functions, so I’ll do the same and thus it will not be protected mode problems in working with PXE. At least, I hope for that.