0040h:0066h           Current setting of the Color Select Register (port 3x9h)

    Length: 1 byte

    Contains the current setting of the Color Select Register (port
    03x9h)

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    Depending upon which CRT controller is currently active, this value
    could reflect either the contents of:

                03B9h for Monochrome, or
                03D9h for Color Graphics Adapter

    Since the Monochrome display adapter reserves port 03B9h, this
    byte will reflect the value of 03D9h for the CGA, and will not be
    used for the monochrome display adapter.