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        IBM Video Graphics Array

        The IBM Video Graphics Array (VGA) and its associated circuitry
        consists of a video buffer, a video digital-to-analog converter
        (DAC), and test circuitry. Then 256Kb video memory is mapped as
        four planes of 64Kb by 8 bits (maps 0 through 3). The video DAC
        drives the analog output to the display connector. The test
        circuitry is used to test for the type of display attached,
        color or monochrome.

        The video subsystem supports all video modes available on the
        IBM Monochrome Display Adapter, IBM Color/Graphics Adapter and
        Enhanced Graphics Adapter. When a monochrome display is
        attached, the colors for the color modes appear as shades of
        gray.

        The new modes available are:

            640 x 480  16- and 2-color graphics
            720 x 400  16-color and monochrome alphanumeric
            350 x 400  16-color alphanumeric
            320 x 200  256-color graphics

        In the 200-scan-line modes, the data for each scan line is
        scanned twice. The double scanning allows the 200-scan-line
        image to be displayed as 400 scan lines.

        The video subsystem serves as the interface between the system
        microprocessor and video memory. When the system microprocessor
        writes to or reads from video memory, all data passes through
        the video subsystem.

        The video subsystem controls the access to video memory form
        the system and the cathode-ray tube (CRT) controller.
        Therefore, programs do not need to wait for horizontal retrace
        to update the display buffer in order to preserve screen
        appearance. The system performs better when accessing the
        display buffer during nonactive display times because there is
        less interference from the CRT controller.

        The video subsystem also controls the system addresses assigned
        to video memory. Up to three different starting addresses can
        be programmed for compatibility with previous video adapters.

        In the graphics modes, the mode determine the way video
        information is formatted into memory, and the way memory is
        organized.

        In alphanumeric modes, the system writes the ASCII character
        code and attribute data to video memory maps 0 and 1,
        respectively. Memory map 2 contains the character font loaded
        by BIOS during an alphanumeric mode set. The font is used by
        the character generator to create the character image on the
        display.

        Three fonts are contained in ROM. Two of these have dot
        patterns that are the same as previous IBM display adapters.
        The third font is a new 8-by-16 character font. Up to eight
        256-character fonts can be loaded into video memory map 2 at
        one time; two fonts can be active at any one time, allowing a
        512-character font.

        The video subsystem formats the information in video memory and
        sends the output to the video DAC. For color displays, the
        video DAC sends three analog color signals (red, green and
        blue) to the display connector. For monochrome displays, BIOS
        translates the color information in the DAC, and the DAC drives
        the summed signal into the green output.

        The auxiliary video connector allows video data to be passed
        between the video subsystem and an adapter plugged into the
        channel connector. The video subsystem can be disabled through
        the POS registers. When it is disabled, the video subsystem
        will not respond to video memory or I/O reads or writes, and
        the video from the adapter can directly drive the video DAC.

        Note: Compatibility with other hardware is best achieved by
              using the BIOS interface or operating system interface
              whenever possible.