I understand this BCD sum circuit. The only thing that I'm not understanding is why 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 overflow 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 that be an indicator of overflow, and OR gate would be used only by the invalid nibbles.
The Attempt at a Solution
PS: sorry my english. This is not my native language.