400  GR : REM  SET COLOR GRAPHICS AREA
420  HOME : REM CLEAR TEXT AREA
440 X = 0: REM  SET STARTING POSITION OF BACK-AND-FORTH VARIABLE
460 Y = 5: REM   SET STARTING POSITION OF UP-AND-DOWN VARIABLE
480 XV = 2: REM SET X VELOCITY
500 YV = 1: REM SET Y VELOCITY
520  REM CALCULATE NEW POSITION
540 NX = X + XV:NY = Y + YV
560  REM IF BALL EXCEEDS SCREEN EDGE, THEN BOUNCE
580  IF NX > 39 THEN NX = 39:XV = - XV
600  IF NX < 0 THEN NX = 0:XV = - XV
620  IF NY > 39 THEN NY = 39:YV = - YV
640  IF NY < 0 THEN NY = 0:YV = - YV
660  REM PLOT NEW POSITION
680  COLOR= 7: PLOT NX,NY
700  REM ERASE OLD POSITION
720  COLOR= 0: PLOT X,Y
740  REM SAVE CURRENT POSITION
760 X = NX:Y = NY
780  GOTO 540