# How do I create a 1-bit full adder that outputs a 2-bit sum?

I am trying to build a 1-bit full adder that outputs a 2-bit sum.

I know that the standard 1-bit FA outputs a 1-bit sum and a carry bit, but I was wondering how can I modify the FA such that the carry bit output is discarded and the only output is a 2-bit sum.

• Does the carry bit not just become the highest bit of your 2 bit sum?
– K H
Jan 27, 2021 at 0:33
• @KH Yes, but if the carry bit is the highest bit, it means that if I merge the carry and a 1 sum bit into a 2-bit number, the result is 11, when the result should be 10 (2). Jan 27, 2021 at 0:46
• For input 0 0, output 0 carry 0 > 00. For input 1 0, output 1 carry 0 > 01. For input 0 1, output 1 carry 0 > 01. For input 1 1, output 0 carry 1 > 10. Where is the missing case?
– K H
Jan 27, 2021 at 0:56
• @KH Got it, thanks for the quick response :D Jan 27, 2021 at 1:21
• I'd write you an answer but I don't actually know how a full adder works. I only know how it must work to do what it says it does. And now I've looked up a full adder and it appears you have an additional carry in input, so taking that into account 11 is a possible output as well.
– K H
Jan 27, 2021 at 1:25