INT 10H 06H: Scroll Up / Clear Screen Rectangle

                                                          Compatibility: All
 Expects: AH    06H
          AL    number of lines to scroll in (0=blank entire rectangle)
          BH    video attribute to be used on blank line(s)
          CH,CL row,clm of lower-right corner of rectangle to scroll/blank
          DH,DL row,clm of upper-left corner of rectangle to scroll/blank
 Returns: (none)
    Info: You may use this to scroll screen text up; that is, move each
          line of text higher on the screen and add one or more blank lines
          at the bottom.

          This fn can be used to scroll a rectangular subset of the screen.
          It may also be used to clear the entire screen or any rectangular

   Notes: ž The standard way to clear the screen is to set DX to 0000H, CL
            to 0040:[004a]-1 (usually 79), CH to 0040:[0084] (usually 24),
            BH to 07H (white-on black video attribute), and AL to 00H (to
            clear the entire screen).

          ž To scroll the text up (as when the user presses the down
            arrow), set CX and DX to cover the entire screen (or a
            rectangular subset), and set AL to 1.

          ž CGAs: When most of the screen is a color other than black, you
            should avoid using this function for scrolling.  Original CGA
            BIOSes display an annoying flicker with each scroll operation.
            It is usually best to write a special routine to move the text
            yourself.  See CGA Video Snow and CLS Flash.

See Also: INT 10H 07H (scroll down)
          INT 10H: Video Services