INT 10h,  07h (7)        Scroll Window Down

    Scrolls a specified window downward a specified number of lines.

       On entry:      AH         07h
                      AL         Number of lines to scroll (if 0, clear
                                 entire window)
                      BH         Display attribute for blank lines
                      CH         Row number of upper left corner
                      CL         Column number of upper left corner
                      DH         Row number of lower right corner
                      DL         Column number of lower right corner

       Returns:       None

       Registers destroyed:      AX, SP, BP, SI, DI

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    Lines are inserted at the top with all lines moving down. The new
    lines are filled with blank characters of a specified display
    attribute (value in BH). The lines that disappear at the bottom of the
    window are lost.

       Notes:         If the number of lines to scroll is specified as 0,
                      the entire window is filled with blanks.

                      Use Service 06h to scroll the window up.

                      This service affects only the currently active
                      display page.