How BCD Ader shows the correct BCD number?

Question

let's say 9(1001)bcd are given both as Addened and Augend, so Binary sum is 10010, so correction is needed and 6(0110) is added to Binary sum the result is 18(0001 1000) requires 8 bits but output of second four bit binary adder is only four bits i.e S8S4S2S1. How this could happen? Or what I'm lacking in my concept? Best wishes!

Answer 1

Well you said it yourself, 18 in BCD is 00011000. When doing the BCD correction you will find that that first 1 in 000011000 is actually a carry. Ignacio basically said the same thing, but that first 1 is the carry, and the remaining bits (1000) are S8S4S2S1, so you get 11000 as CoutS8S4S2S1. The leading 0's don't matter.