anchor.diff:  0 VALUE screen-height
   0 VALUE screen-width
   0 VALUE screen-depth
  +0 VALUE screen-line-bytes
   0 VALUE window-top
   0 VALUE window-left
   
  
positive.code: 		fb8-columns2bytes swap fb8-columns2bytes tuck -
  		over r@ + 2dup + r> swap >r rot char-height 0 ?DO
  			3dup rmove
  			-rot screen-width screen-depth * tuck + -rot + swap rot
  		LOOP
  		3drop r> over -
  	THEN
  	char-height 0 ?DO
  		dup 2 pick fb8-erase-block screen-width screen-depth * +
  	LOOP
  	2drop
  ;
  
negative.code: : fb8-background inverse? ;
  : fb8-foreground inverse? invert ;
  
  : fb8-lines2bytes ( #lines -- #bytes ) char-height * screen-line-bytes * ;
  : fb8-columns2bytes ( #columns -- #bytes ) char-width * screen-depth * ;
  : fb8-line2addr ( line# -- addr )
  	char-height * window-top + screen-line-bytes *
  	frame-buffer-adr + window-left screen-depth * +
  ;
  
  