In our previous post, we’ve seen how to add two binary bits using XOR and AND gate. Now, let’s say we want to add not two bits but three bits. How would you go about it? Two numbers are A and B and the third number is a carry bit coming from somewhere else (let’s assume so for now). A and B would go to XOR and AND gate just like previously. The new addition is – the output of first XOR and the third Cin bit would again into another XOR and AND.
Add a bunch of these together (like the below) such that the carry out of one is connected to the carry-in of the other. Then we have a Ripple-Carry Adder. These ripple carry adders are used in micro-processors as n-bit adders.