INT 2F U - Quarterdeck QDPMI.SYS v1.0 - INSTALLATION CHECK
        AX = DE01h
        BX = 4450h ("DP")
        CX = 4D49h ("MI")
        DX = 3039h ("09")
Return: AL = FFh if installed
            BX = 4D42h ("MB")
            CX = 4921h ("I!")
            DX = 8F4Fh
            ES:DI -> filename of DPMI host overlay
Note:   the installation check consists of testing for the existence of the
          character device QDPMI$$$