FIGURE 6 shows how subtraction
between two numbers that is A-B can be performed using 74LS83 4 bit Full
Adder. Here the number for B (subtrahend) is represented in 2's complement
form. This is obtained by inverting each bit of the subtrahend and adding
1 to the LSB. The process of adding 1 to the LSB of the subtrahend is accomplished
by setting C0=1.
For example, if we want
to perform the subtraction of 6-4.
Therefore, the inputs A3-A0
are represented by 0110 and the inputs B3-B0 will be represented by the
inverted of 4 that are 1011. The subtraction is done as shown below which
is actually addition in 2's complement.