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()

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