draw_ball            ;   this routine draws the ball on the screen
   movf ball_y, w      ;   copy our ball_y co-ordinate to w
   movwf PORTB         ;   and then to portb (our data bus)
   bsf PORTA, 2      ;   now latch this 8-bits to the green 74373
   bcf PORTA, 2      ;   and now close the latches
   movf ball_x, w      ;   copy ball_x co-ordinate to w
   movwf PORTB         ;   then to port b (our data bus)
   bsf PORTA, 4      ;   now latch this 8-bits to the COLUMN data 74373
   bcf PORTA, 4      ;   and close the latches
   bcf PORTA, 3      ;   enable both the green and column 74373 outputs
   bcf PORTA, 6      ;   3 is green and 6 is column data
   call delay         ;   hold this data on the screen for a split second
   bsf PORTA, 3      ;   close both green and column outputs
   bsf PORTA, 6      ;   3 is green and 6 is column data
   return            ;   and return to the main program