Pong-like game

[+atari2600land]

I need some help on a Pong-like game I'm making. This is just a demo so far to make the ball go around like it does on Pong. That means that once it hits the top screen, it goes diagonally down, which I can't seem to make it do. I've tried a bunch of stuff. It's probably something really obvious that I overlooked, but nevertheless, can someone modify this code so that when it hits a side of the screen it bounces diagonally in the opposite direction?

 

1 set smartbranching on
10 ballheight=4 : CTRLPF=$20
12 COLUPF=14
16 x=40 : y=40
20 ballx=x : bally=y
23 if x=150 then goto 35
25 x=x+1
 if x=150 then goto 35
30 goto 40
35 x=x-1
 if x=20 then goto 20
37 goto 40
40 y=y-1
 if y=20 then goto 50
45 goto 55
50 y=y+1
 if y=80 then goto 40
55 drawscreen
60 goto 20