Extended memory - Block move (INT 15h, 87h)

        Entry:
          AH = 87h
          CX = number of words to move
          ES:SI = pointer to global descriptor table
                   offset 0h  null descriptor (need not be initialized)
                          8h  uninitialized, will be made into GDT descriptor
                         10h  descriptor for source of move
                         18h  descriptor for destination of move
                         20h  uninitialized, used by BIOS
                         28h  uninitialized, will be made into SS descriptor
        Return:
          Source copied into destination