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                       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