Direct Graphics Interface Standard (DGIS) - INQUIRE AVAILABLE DEVICES

        AX = 6A00h
        BX = 0000
        CX = 0000
        DX = buffer length (may be 0)
        ES:DI = address of buffer
Return: BX = number of bytes stored in buffer
        CX = bytes required for all descriptions (0 if no DGIS)
Note:   buffer contains descriptions and addresses of DGIS-compatible
        display(s) and printer(s)