Int 10h, 12h /BL=20h    Select Alternative Print-Screen        EGA, MCGA, VGA

    Installs a Print-Screen routine from the video adapter's BIOS to
    replace the default Int 05h handler (which is usually hard-coded
    for 25-line screens).

        Entry   AH = 12h
                BL = 20h

        Return  (Int 05h vector-table entry updated)