NAME
        AddVBlankInt -- adds a routine executed every vertical blank. (V40)

   SYNOPSIS
        intHandle = AddVBlankInt(intRoutine, intData);
        D0                       a0       a1

        aptr addvblankint(aptr, aptr);

   FUNCTION
        Lets you attach a routine to the system which will get called
        everytime a vertical blanking interrupt occurs.

        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'.

        If your program is to exit without reboot, you MUST call
        remvblankint() before exiting.

        Only one interrupt routine may be added to the system.  ALWAYS check
        the return value in case some other task has previously used this
        function.

   INPUTS
        intRoutine - the routine to invoke every vblank. 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
        remvblankint()