Agnus/
                  Read/   Denise/
Register Address  Write   Paula         Function
-------- -------  -----   -------       --------
Wheel      212      R      P( E )     Wheel events

                 BIT  15-10 unused
                 BIT  9     Mouse Button 5
                 BIT  8     Mouse Button 4


                 signed 8 bit counter counts USB mouse Wheel events

                 BIT#  07,06,05,04,03,02,01,00
                 ----- ------------------------ -----------------------
                       X7 X6 X5 X4 X3 X2 X1 X0