INT 1C - System Timer Tick (User Routine)

	- called by INT 8, approximately 18.206 times per second
	  (unless the program modifies the 8253 PIT
	- this is a user routine which the BIOS defaults to a simple IRET
	- this interrupt vector can be used for TSR popup utilities,
	  animated graphics updates and event polling
	- since many TSR's don't maintain the integrity of the interrupt
	  call chain, relying on this for TSR popup isn't recommended