Port 3C5, 04  -  Memory Mode Register

                1111 xxxx  Reserved
                xxxx 1xxx  Chain 4
                             This bit controls the map selected during
                             systen Read operations. When set to 0, this
                             bit enables system addresses to sequentialy
                             access data within a bit map by using the
                             Map Mask register. When set to 1, this bit
                             causes the two low-order bits to select the
                             map accessed as show below.
                xxxx x1xx  Odd/Even
                             When this bit is set to 0, even system addresses
                             access maps 0 and 2, while odd system addresses
                             access maps 1 and 3. When this bit set to 1,
                             system addresses sequentially access data within
                             a bit map, and maps are accessed according to
                             the value in the Map Mask Register (3C5, 02).
                xxxx xx1x  Extended Memory
                             When set to 1, this bit enables the video memory
                             from 64Kb to 256Kb. This bit must be set to 1 to
                             enable the character map selection descriped
                             for register 3C5, 03.
                xxxx xxx1  Reserved