I need help. I would like sb to explain me how the serial adder using shift register functions

 

You can see the following link, it may help you.

Serial Adder and Shift Registers.
http://www.add.ece.ufl.edu/3701/labs/lab5_s11.pdf

serial adder using shift register.
http://books.google.com.tw/books?id...e&q=serial adder using shift register&f=false

Serial Adder.
http://www.cs.ubc.ca/~hilpert/eec/ics/serialadders.html