Three primary steps are required to open the timer 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 timerequest using
      CreateExtIO().

   *  Open the timer device with one of the five timer device units. Call
      OpenDevice() passing a pointer to the  timerequest.

   struct MsgPort *TimerMP;   /* Message port pointer */
   struct timerequest *TimerIO;  /* I/O structure pointer */

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

     /* Create message block for device IO */
   if (!(TimerIO = (struct timerequest *)
                   CreateExtIO(TimerMP)(sizeof timerequest)) )
       cleanexit(" Error: Can't create IO request\n",RETURN_FAIL);

     /* Open the timer device with UNIT_MICROHZ */
   if (error=OpenDevice(TIMERNAME,UNIT_MICROHZ,TimerIO,0))
       cleanexit(" Error: Can't open Timer.device\n",RETURN_FAIL);

The procedure for applications which only use the timer device functions
is slightly different:

   *  Declare the timer device base address variable TimerBase in the
      global data area.

   *  Allocate memory for a timerequest structure and a  timeval structure
      using AllocMem().

   *  Call OpenDevice(), passing the allocated timerequest structure.

   *  Set the timer device base address variable to point to the timer
      device base.

   struct Library *TimerBase;  /* global library pointer */

   struct timerequest *TimerIO;
   struct timeval *time1;

     /* Allocate memory for timerequest and timeval structures */
   TimerIO=(struct timerequest *)AllocMem(sizeof(struct timerequest),
                                  MEMF_PUBLIC | MEMF_CLEAR);
   time1=(struct timeval *)AllocMem(sizeof(struct timeval),
                                  MEMF_PUBLIC | MEMF_CLEAR);
   if (!TimerIO | !time1)
       cleanexit(" Error: Can't allocate memory for I/O structures\n",
                          RETURN_FAIL);

   if (error=OpenDevice(TIMERNAME,UNIT_MICROHZ,TimerIO,0))
       cleanexit(" Error: Can't open Timer.device\n",RETURN_FAIL);

     /* Set up pointer for timer functions */
   TimerBase = (struct Library *)TimerIO->tr_node.io_Device;