Port 3CF, 08  -  Bit Mask Register

        1111 1111  Bit Mask 7-0

        When the bit is set to 1, the corresponding bit position in
        each map can be changes. When the corresponding bit is set
        to 0, the bit position in the map is masked to prevent changes,
        provided that the location being written was the last location
        read by the system microprocessor.

        The bit mask applies to write modes 0 and 2. To preserve
        bits using the bit mask, data must be latched internally by
        reading the location. When data is written to preserve the
        bits, the most current data in the latches is written in those
        positions. The bit mask applies to all mask simultaneously.