Here’s my solution (it’s the one in the book):
And here’s the link:
Description: There’s a 4-ball cycle set up by the three bits on top. The first ball subtracts one from register A (the right one). The second ball does nothing (this is wasted in my design). The third ball adds 2 to register B. The fourth ball adds 1 to register B. (The third and fourth ball add a total of 3.) When register A underflows, the computer stops.