Port 3CF, 02  -  Color Compare register

        1111 xxxx  Reserved
        xxxx 1xxx  Color Compare Map 3
        xxxx x1xx  Color Compare Map 2
        xxxx xx1x  Color Compare Map 1
        xxxx xxx1  Color Compare Map 0

        Bits 3-0:  These bits are the 4-bit color value to be compared
                   when the read mode bit in the Grahpics Mode register
                   is set to 1. When the system does a memory read, the
                   data returned from the memory cycle will be a 1 in
                   each bit position where the four maps equal the
                   Color Compare register. If the read mode bit is 0,
                   the date is returned without comparation.

                   The color compare bit is the value that all bits of
                   the corresponding map's byte are compared across the
                   four maps, and a 1 is returned in each position
                   where the bits of all four maps equal their
                   respective color compare values.