I understand this BCD sum circuit. The only thing that I'm not understanding is why the last full adder carry out also triggers an 6-sum in the other part of the circuit. I mean, if the nibble is not an valid BCD number, we sum six to the number. Not valid BCD numbers are 10,11,...,15. Using K-maps you simplify this list, use an OR gate and sum six to the nibble. So, my question is: why the carry-out is also a trigger to sum six to the nibble? And in the same manner, why invalid nibbles also represent carry out? In my view, carry-out shouldnt go into that OR gate. It should be an indicator of overflow, and OR gate would be used only by the invalid nibbles. PS: Thread title is small, because I was getting that bug of creating a new thread blank page. I would like the title to be: "Doubt about BCD sum circuit using full adders"