Dos 4+ XMA2EMS.SYS extension internal - Installation check

        Entry:
          AX = 1B00h
        Return:
          AL = FFh if installed

        Note: XMA2EMS.SYS extension is only installed if DOS has page frames
              to hide. This extension hooks onto INT 67 AH=58h and returns
              from that call data which excludes the physical pages being
              used by DOS.