JOYTEST

                    Mouse counter write test data:
     BIT#  15  14  13  12  11  10  09  08     07  06  05  04  03  02  01  00
  JOY0DAT  Y7  Y6  Y5  Y4  Y3  Y2  xx  xx     X7  X6  X5  X4  X3  X2  xx  xx
  JOY1DAT  Y7  Y6  Y5  Y4  Y3  Y2  xx  xx     X7  X6  X5  X4  X3  X2  xx  xx