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