Full Subtractor

·         Full Subtractor is a combinational logic circuit.

·         It is used for the purpose of subtracting two single bit numbers.

·         It also takes into consideration borrow of the lower significant stage.

·         Thus, full subtractor has the ability to perform the subtraction of three bits.

·         Full subtractor contains 3 inputs and 2 outputs (Difference and Borrow) as shown-

Designing a Full Subtractor-

Full subtractor is designed in the following steps-

Step-01:

Identify the input and output variables-

·         Input variables = A, B, B_in (either 0 or 1)

·         Output variables = D, B_out where D = Difference and B_out = Borrow

Step-02:

Draw the truth table-

Step-03:

Draw K-maps using the above truth table and determine the simplified Boolean expressions-

Step-04:

Draw the logic diagram.

The implementation of full subtractor using 1 XOR gate, 3 AND gates, 1 NOT gate and 1 OR gate is as shown below-