Three primary steps are required to open the gameport device:

   *  Create a message port using CreatePort(). Reply messages from the
      device must be directed to a message port.

   *  Create an I/O request structure of type IOStdReq. The IOStdReq
      structure is created by the CreateExtIO() function. CreateExtIO()
      will initialize the I/O request with your reply port.

   *  Open the gameport device.  Call OpenDevice(), passing the I/O request
      and and indicating the unit you wish to use.

   struct MsgPort *GameMP;   /* Message port pointer */
   struct IOStdReq *GameIO;  /* I/O request pointer */

     /* Create port for gameport device communications */
   if (!(GameMP = CreatePort("RKM_game_port",0)))
       cleanexit(" Error: Can't create port\n",RETURN_FAIL);

     /* Create message block for device I/O */
   if (!(GameIO = CreateExtIO(GameMP,sizeof(struct IOStdReq))))
       cleanexit(" Error: Can't create I/O request\n",RETURN_FAIL);

     /* Open the right/back (unit 1, number 2) gameport.device unit */
   if (error=OpenDevice("gameport.device",1,GameIO,0))
       cleanexit(" Error: Can't open gameport.device\n",RETURN_FAIL);

The gameport commands are unit specific.  The unit number specified in the
call to OpenDevice() determines which unit is acted upon.