Port 3CF, 04  -  Read Map Select Register

        1111 11xx  Reserved
        xxxx xx11  Map select

        Bits 1-0:  These bits select the memory map for system Read
                   operations. This register has no effect on the color
                   compared read mode. In odd/even modes, the value can
                   be a binary 00 or 01 to select the chained maps 0, 1
                   and the value can be a binary 10 or 11 to select the
                   chained maps 2, 3.