FullAdder is a two bit adder with additional carry in bit which is composed by Gates components.
Its logic behavior is like this:
FullAdder behavior
input a | 
input b | 
input carry c_in | 
sum s | 
output carry c_out | 
| 
 0  | 
 0  | 
 0  | 
 0  | 
 0  | 
| 
 1  | 
 0  | 
 0  | 
 1  | 
 0  | 
| 
 0  | 
 1  | 
 0  | 
 1  | 
 0  | 
| 
 1  | 
 1  | 
 0  | 
 0  | 
 1  | 
| 
 0  | 
 0  | 
 1  | 
 0  | 
 1  | 
| 
 1  | 
 0  | 
 1  | 
 0  | 
 1  | 
| 
 0  | 
 1  | 
 1  | 
 0  | 
 1  | 
| 
 1  | 
 1  | 
 1  | 
 1  | 
 1  |