Full Subtractor for Binary Subtraction

Explore full subtractor design and implementation by combining two half subtractors

40 Participants 30 Minutes Beginner

In the domain of digital arithmetic, full subtractors play a vital role in subtracting binary numbers, particularly in multi-digit calculations. This guide takes you on a journey through the design and implementation of full subtractors by combining two half subtractors. We will start by understanding what a full subtractor is and how it functions, then delve into the process of simplifying its Boolean expression using Karnaugh Maps (K-Maps). We will explore how to effectively implement full subtractors and demonstrate how these circuits can be constructed by amalgamating two half subtractors. Finally, we'll conclude with a deeper understanding of the significance of full subtractors in multi-digit binary arithmetic.



What is a Full Subtractor?

A full subtractor is a fundamental digital circuit used to subtract three binary numbers: the minuend (A), the subtrahend (B), and a borrow-in (Bin). It computes the difference between A and B while considering the borrow from the previous stage in multi-digit subtraction. The full subtractor not only provides the difference but also a borrow output, ensuring accurate subtraction across binary digits in multi-digit arithmetic.




In full subtractor, eight possible operations are possible with three inputs and produces eight, two digit outputs. The operation is shown in the truth table below.




Simplifying Boolean Expression Using K-Maps:

To design efficient full subtractors, we employ Karnaugh Maps (K-Maps) to simplify their Boolean expressions. K-Maps are graphical tools that facilitate the optimization of circuit design. We will explore how K-Maps can be utilized to streamline the Boolean expressions of full subtractors, enabling more efficient and compact circuit implementations. For difference and borrow outputs, boolean expression has to be derived using Karnaugh map. Since it has three input variables, 8-cells k-map is used to simplify the expression.





How to Implement Full Subtractors:

Implementing full subtractors involves using logic gates, such as XOR, AND, and OR gates, based on the simplified Boolean expressions. We will guide you through the step-by-step process of constructing these circuits, providing insights into their practical applications and their role in multi-digit binary subtraction.




Constructing a Full Subtractor Using Two Half Subtractors:

One of the key aspects of this exploration is the construction of a full subtractor by integrating two half subtractors. We will demonstrate how two half subtractors can be combined to form a full subtractor, showcasing the synergy between these building blocks and their role in handling multi-digit binary subtraction.

1. A full subtractor can be constructed by integrating two half subtractors through a cascading arrangement.

2. In the first half subtractor, the difference output is a result of an XOR operation performed on inputs A and B.

3. In the full subtractor, the difference output is obtained by applying an XOR operation to the "Bin" input and the output of the first half subtractor.

4. Furthermore, the borrow output from the first half subtractor is combined with the borrow output from the second half subtractor using an OR operation, yielding the final borrow output of the full subtractor.




In conclusion, full subtractors are indispensable components in multi-digit binary subtraction, providing not only the difference between two binary numbers but also accommodating the borrow from the previous stage. By simplifying their Boolean expressions using Karnaugh Maps and implementing them with logic gates, we can create efficient full subtractors. Furthermore, understanding how two half subtractors can be seamlessly combined to construct a full subtractor highlights the modularity and versatility of digital circuits. Full subtractors are critical tools in the realm of digital arithmetic, ensuring the accuracy of complex multi-digit binary subtraction operations.

Full Subtractor


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