NAME
        AddTimerInt -- adds an interrupt that is executed at regular
                       intervals. (V40)

   SYNOPSIS
        intHandle = AddTimerInt(intRoutine, intData);
        D0                      A0          A1

        aptr addtimerint(aptr, aptr);

   FUNCTION
        Calling this routine causes the system to allocate a cia timer
        and set up 'intRoutine' to service any interrupts caused by the timer.
        Although the timer is allocated it is neither running, nor enabled.
        StartIntTimer() must be called to establish the time interval and
        start the timer.

        The routine is called from within an interrupt, so normal
        restrictions apply. The routine must preserve the following
        registers: A2, A3, A4, A7, D2-D7. Other registers are
        scratch, except for D0, which MUST BE SET TO 0 upon
        exit. On entry to the routine, A1 holds 'intData' and A5
        holds 'intRoutine'.

        Only a single cia timer will be allocated by this routine. so this
        routine may only be called once without an intervening call to
        remtimerint().

        The cia timer used by this routine is not guaranteed to always be
        the same. This routine utilizes the cia resource and uses an
        unallocated cia timer.

        If your program is to exit without reboot, you MUST match all
        calls to this function with calls to remtimerint() before exiting.

        Even if you only use the function once in your program; checking
        the return value will make your program more tolerant for
        mulititasking on the Amiga computer platforms.

   INPUTS
        intRoutine - the routine to invoke upon timer interrupts. This routine
                     should be as short as possible to minimize its effect on
                     overall system performance.
        intData - data passed to the routine in register A1. If more than one
                  long word of data is required this should be a pointer to
                  a structure that contains the required data.

   RESULT
        intHandle - a handle used to manipulate the interrupt, or NULL
                    if it was not possible to attach the routine.

   SEE ALSO
        remtimerint(), stoptimerint(), starttimerint()