INT 10 - VIDEO - SCROLL DOWN WINDOW
        AH = 07h
        AL = number of lines by which to scroll down (00h=clear entire window)
        BH = attribute used to write blank lines at top of window
        CH,CL = row,column of window's upper left corner
        DH,DL = row,column of window's lower right corner
Note:   affects only the currently active page (see AH=05h)
Warning: some implementations have a bug which destroys BP
SeeAlso: AH=06h,AH=72h,AH=73h,INT 50/AX=0014h

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