VGA Graphics Controller Data Register

        All registers are read/write.

        3CF, 00  Set/Reset register
        3CF, 01  Enable Set/Reset register
        3CF, 02  Color Compare register
        3CF, 03  Data Rotate Register
        3CF, 04  Read Map Select Register
        3CF, 05  Graphics Mode Register
        3CF, 06  Miscellaneous Register
        3CF, 07  Color Don't Care Register
        3CF, 08  Bit Mask Register