Step 1:
    Send GPD_ASKCTYPE to the device and check for a GPCT_NOCONTROLLER
    return. Never issue GPD_SETCTYPE without checking whether the desired
    gameport unit is in use.

Step 2:
    If GPCT_NOCONTROLLER is returned, you have access to the gameport.
    Set the allocation flag to GPCT_MOUSE, GPCT_ABSJOYSTICK or
    GPCT_RELJOYSTICK if you use a system supported controller, or
    GPCT_ALLOCATED if you use a custom controller.

    struct IOStdReq *GameIO;  /* Must be initialized prior to using */

    BOOL set_controller_type(type)
    BYTE type;
    {

    BOOL success = FALSE;
    BYTE controller_type = 0;

    Forbid();                           /*critical section start */
    GameIO->io_Command = GPD_ASKCTYPE;  /* inquire current status */
    GameIO->io_Length = 1;
    GameIO->io_Flags = IOF_QUICK;
    GameIO->io_Data = (APTR)&controller_type; /* put answer in here */
    DoIO(GameIO);

    /* No one is using this device unit, let's claim it */
    if (controller_type == GPCT_NOCONTROLLER)
        {
        GameIO->io_Command = GPD_SETCTYPE;/* set controller type */
        GameIO->io_Length = 1;
        GameIO->io_Data = (APTR)&type;  /* set to input param */
        DoIO( GameIO);
        success = TRUE;
        UnitOpened = TRUE;
        }
    Permit(); /* critical section end */

    /* success can be TRUE or FALSE, see above */
    return(success);
    }

Step 3:
    The program must set the controller type back to GPCT_NOCONTROLLER
    upon exiting your program:

    struct IOStdReq *GameIO;  /* Must be initialized prior to using */

    void free_gp_unit()
    {
    BYTE type = GPCT_NOCONTROLLER;
    GameIO->io_Command = GPD_SETCTYPE;  /* set controller type */
    GameIO->io_Length = 1;
    GameIO->io_Data = (APTR)&type;      /* set to unused */
    DoIO( GameIO);
    }

This three step protocol allows applications to share the gameport device
in a system compatible way.

    A Word About The Functions.
    ---------------------------
    The functions shown above are designed to be included in any
    application using the gameport device.  The first function,
    set_controller_type(), would be the first thing done after opening
    the gameport device. The second function, free_gp_unit(), would be
    the last thing done before closing the device.