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Clocking mode reg.     index 01h  SEQU

Layout  b7,b6       Reserved
        b5     SO   Screen off
        b4     S4   Shift 4
        b3     DC   Dot clock
        b2     SL   Shift load
        b1     BW   (EGA only)
        b0     8/9  8/9 dot clocks

- Screen off (bit 5)
        Turns the screen off along with the picture generating logic.
        Because no time is spent refreshing the screen, the video
        processor has direct control of the video memory, which
        facilitates rapid memory access.

        0 : Screen is turned on.
        1 : Screen is turned off.

- Shift four (bit 4) (see bit 2)
        0 : Load the serializers every character clock cycle.
        1 : Load the serializers every fourth character clock cycle.

- Dot clock (bit 3)
        This is the basic graphics clock for the system and is
        generated from the master clock output. High resolution
        graphics are achieved when the dot clock is triggered every
        input clock, but if the dot clock is triggered by the master
        clock divided by two, then this slows down the system and
        stretches out the other timing signals. This divide-by-two
        mode is used to generate the 320 and 360 horizontal
        resolutions on the VGA, and can also be used to force
        non-standard VGA modes.

        0 : Set the dot clock to the same frequency as the master
            clock.
        1 : Divide the master clock by 2 to derive the dot clock
            (used in some 320- or 360-column modes).

- Shift load (bit 2)
        The display memory is organized in parallel words. The display
        output, however, requires a serial output stream. A device
        called a video serializer converts this parallel data.

                b4 b2   Load serializers
                0  0    every char. clock cycle
                0  1    every other char. clock cycle
                1  ?    every fourth char. clock cycle

- 8/9 dot clocks (bit 0)
        0 : Character clocks 8 dots wide are generated. Used in all
            modes in which there are 320 or 640 horizontal dots.
        1 : Character clocks 9 dots wide are generated. Used in the
            monochrome mode 7 in which there are 720 horizontal dots.