Format of BIOS switch-to-protected-mode Global Descriptor Table:
Offset	Size	Description	)
 00h  8 BYTEs	null descriptor (initialize to zeros)
 08h  8 BYTEs	GDT descriptor (see #00501)
 10h  8 BYTEs	IDT descriptor
 18h  8 BYTEs	DS descriptor
 20h  8 BYTEs	ES
 28h  8 BYTEs	SS
 30h  8 BYTEs	CS
 38h  8 BYTEs	uninitialized, used to build descriptor for BIOS CS