There are two buttons on the standard Amiga mouse. However, the control
circuitry and software support up to three buttons.

   *  The left button on the Amiga mouse is connected to  CIAAPRA 
      ($BFE001). Port 1 uses bit 6 and port 2 uses bit 7. A logic
      state of 1 means "switch open." A logic state of 0 means "switch
      closed." (See  Appendix F  for more information.)

   *  Button 2 (right button on Amiga mouse) is connected to pin 9 of
      the  controller ports , one of the proportional pins.  See
       Digital Input/Output on the Controller Port  for details.

   *  Button 3, when used, is connected to pin 5, the other
      proportional  controller input .