# Confusion about 2-bit adders

I am a little confused about how a 2-bit adder works. For 1-bit adder, my understanding of it is fine and I am able to draw out the truth table for it. However, if lets say I have a 2-bit adder, I can't seem to work out how the display shows the sum of both binary.

For example,
1st full adder - A1 = 1, B1 = 0, Cin = 0 => sum = 1 and carry out = 0. The carry out then goes to another full adder hence, for the 2nd full adder A2 = 1, B2 = 0, Cin = 0 => sum = 1 and carry out = 0.

Does this mean that my binary representation is 11? Based on looking at the both the sums, however I know that 01 + 01 = 10. I am pretty sure I am understanding something wrongly here and hopefully someone can help me out.

• Yes, in your description, you are adding 11 + 00 and getting 11. A1 is 1st lsb of number A, B1 is first lsb of number B, A2 is second lsb of A, etc. To add 01 and 01, A1 = 1, B1 = 1 yeilding 0 + C2 =1. Next, A2 = 0, B2 = 0 C2 =1, yielding 1. concatenating gets you 10. – Phil Sweet Sep 29 '19 at 21:44
• @PhilSweet Thanks for the reply. So if I am not wrong, I group the As together and Bs together?Like for example A1A2..etc and B1B2..etc instead of doing it like A1B1, A2B2? – Axois Sep 30 '19 at 13:05