AL number of lines to scroll in (0=blank entire rectangle)
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
Info: You may use this to scroll screen text down; that is, move each
line of text lower on the screen and add one or more blank lines
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
See INT 10H 06H (scroll up / cls) for notes and related info.