VBlank Interrupt

The VBlank Interrupt (short for "Vertical Blanking Interrupt") is an interrupt signaled by the STIC when it has finished drawing the active display. It signals this interrupt as it begins to draw the solid border at the bottom of the screen. The VBlank Interrupt is important for two reasons:

• It provides a regular "pulse" upon which to base game timing and events.
• It signals the start of the VBlank Interval, which controls access to STIC registers, GRAM and GROM.

On NTSC Intellivisions, this interrupt occurs once every 14394 cycles. On PAL/SECAM Intellivisions, this interrupt occurs once every ?? cycles.