GDT - Global Descriptor Table

     Offset Size		Description

	00  8bytes	dummy, set to 0
	08  8bytes	GDT data segment location, set to 0
	10  8bytes	source GDT pointer
	18  8bytes	target GDT pointer
	20  8bytes	pointer to BIOS code segment, set to 0.	used by
			the BIOS to create protected mode code segment
	28  8bytes	pointer to BIOS stack segment, set to 0.  Used by
			the BIOS to create protected mode stack segment
	30  8bytes	user code segment
	38  8bytes	temporary BIOS code segment

	Source/Target GDT use the following format (offsets 10h and 18h):

     Offset Size		Description

	00   word	Segment limit
	02  3bytes	24 bit segment physical address
	05   byte	data access rights, set to 93h
	06   word	reserved word, must be 0

	- see	LGDT  SGDT