INT 10,7 - Scroll Window Down

	AH = 07
	AL = number of lines to scroll, previous lines are
	     blanked, if 0 or AL > screen size, window is blanked
	BH = attribute to be used on blank line
	CH = row of upper left corner of scroll window
	CL = column of upper left corner of scroll window
	DH = row of lower right corner of scroll window
	DL = column of lower right corner of scroll window


	returns nothing


	- in video mode 4 (300x200 4 color) on the EGA, MCGA and VGA
	  this function scrolls page 0 regardless of the current page
	- can be used to scroll graphics screens, using character coords
	- on CGA's this function disables video adapter, causing flitter