Interrupt Descriptions
----------------------

  The following interrupts only occur if they have been
 enabled in the Interrupt Enable register ($FFFF) and
 if the interrupts have actually been enabled using the
 EI instruction.

  V-Blank -

   The V-Blank interrupt occurs ~59.7 times a second
   on a regular GB and ~61.1 times a second on a Super
   GB (SGB). This interrupt occurs at the beginning of
   the V-Blank period. During this period video hardware
   is not using video ram so it may be freely accessed.
   This period lasts approximately 1.1 milliseconds.

  LCDC Status -

   There are various reasons for this interrupt to occur
   as described by the STAT register ($FF40). One very
   popular reason is to indicate to the user when the
   video hardware is about to redraw a given LCD line.
   This can be useful for dynamically controlling the SCX/
   SCY registers ($FF43/$FF42) to perform special video
   effects.

  Timer Overflow -

   This interrupt occurs when the TIMA register ($FF05)
   changes from $FF to $00.

  Serial Transfer Completion -

   This interrupt occurs when a serial transfer has
   completed on the game link port.

  High-to-Low of P10-P13 -

   This interrupt occurs on a transition of any of the
   keypad input lines from high to low. Due to the fact
   that keypad "bounce"* is virtually always present,
   software should expect this interrupt to occur one
   or more times for every button press and one or more
   times for every button release.

   * - Bounce tends to be a side effect of any button
      making or breaking a connection. During these
      periods, it is very common for a small amount of
      oscillation between high & low states to take place.