Here’s my solution (it’s the one in the book):
And here’s the link:
http://www.lodev.org/jstumble/?board=e0eelleer0fexrf00leerxlf0lfrrfe0lfrreer
Description: A much more efficient way of doing addition. First it adds the 1’s place of register A to the 1’s place of register B. Then it adds the 2’s place of register A to the 2’s place of register B.