Port 3C5, 02  -  Map Mask Register
                 1111 xxxx  Reserved
                 xxxx 1xxx  Map 3 enable
                 xxxx x1xx  Map 2 enable
                 xxxx xx1x  Map 1 enable
                 xxxx xxx1  Map 0 enable

                 Bits 3-0: When set to 1, these bits enable the system
                 access to the corresponding map. If all maps are enabled,
                 the system can write its 8-bit value to all four maps in
                 a single memory cycle. This substantialy reduces the system
                 overhead during display updates in graphics modes.

                 Data scrolling operations can be enhanced by enabling all
                 maps and writing the display the display buffer address
                 with the data stored in the system latches. This is a
                 Read-Modify-Write operation.

                 When odd/even modes are selected, maps 0 and 1 anf maps
                 2 and 3 should have the same map mask value.

                 When chain 4 mode is selected, all maps should be enabled.