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)
BUGS:	some implementations (including the original IBM PC) have a bug which
	  destroys BP
	the Trident TVGA8900CL (BIOS dated 9/8/92) clears DS to 0000h when
	  scrolling in an SVGA mode (800x600 or higher)
SeeAlso: AH=06h,AH=72h,AH=73h,INT 50/AX=0014h