IBM Color/Graphics Monitor Adapter

        The adapter has two basic modes of operation: alphanumeric
        (A/N) and all-points-addressable graphics (APA). Additional
        modes are available within the A/N and APA modes. In the
        A/N mode, ths display can operate in either a 40-coulumn
        by 25-row mode or in a 80-column by 25-row mode. In both
        modes characters are defined in a 8-wide by 8-high character
        box and are 7-wide by 7-high, with one line of descender for
        lowercase characters.

        The monitor adapter contains 16K bytes of storage. As an
        example, a 40-column by 25-row display uses 1000 bytes
        to store character information, and 1000 bytes to store
        attribut/color information. This would mean that up to
        eight display screens can be stored in the adapter memory.
        Similary, in an 80-column by 25-column mode, four display
        screens may be stored in the adapter. The entire 16K bytes
        of storage on the display adapter are directly addressable
        by the processor (at address B8000 through BC000, see note
        below), which allows maximum software flexibility in
        managing the screen.

        In A/N color modes, it is also possible to select the color
        of the screen border. One of sixteen colors can be selected.

        In APA mode, there are two resolutions available: a medium-
        resolution color graphics mode (320x200) and a high-resolu-
        tion B/W mode (640x200).



        Note: In 80x25 A/N mode, "snow" may occur on the screen, if
              a program writes directly into the adapter memory. To
              prevent this, wait until the screen is in vertical-
              retrace-mode (an algoritm may be found at port 3DA).