The only computational element described here is a full adder. It is the basic building block of multi-bit adders and subtracters.
A full adder performs a single column of a binary addition.
It has two inputs, X and Y for bits from the
summands.
It has another input, Ci for the carry from the column to
the right.
The S output is the column sum.
This output should be 1 whenever an odd number of the inputs are 1.
The Co output is the carry output, which is sent to the
column to the left.
This output should be 1 when at least two of the inputs are 1.