Here’s my solution (it’s the one in the book):
And here’s the link:
http://www.lodev.org/jstumble/?board=1lr1ree0rleellxeer0rreelll0er0rrrelll0lr0rrreril0ll
Description: The two bits at the top left set up a 4 ball cycle. The first three balls each subtract one from register A. The fourth ball adds one to register B. When register A underflows, the computer stops.