Format of global descriptor table:
Offset	Size	Description	)
 00h 16 BYTEs	zeros (used by BIOS)
 10h	WORD	source segment length in bytes (2*CX-1 or greater)
 12h  3 BYTEs	24-bit linear source address, low byte first
 15h	BYTE	source segment access rights (93h)
 16h	WORD	(286) zero
		(386+) extended access rights and high byte of source address
 18h	WORD	destination segment length in bytes (2*CX-1 or greater)
 1Ah  3 BYTEs	24-bit linear destination address, low byte first
 1Dh	BYTE	destination segment access rights (93h)
 1Eh	WORD	(286) zero
		(386+) extended access rights and high byte of destin. address
 20h 16 BYTEs	zeros (used by BIOS to build CS and SS descriptors)