The (RD) control code moves the entire screen display down one line, rolling the bottom line completely off the screen. It also inserts a blank foreground line at the top of the screen and moves the cursor to the beginning of the inserted blank line (i.e., the upper left corner of the screen).
See Roll Up (RU).
a. 100 FORMAT (RD) . PRINT (0,100) b. PRINT (0) (RD)