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4 bit Adder Subtractor

Last Updated on June 29, 2023 by Mayank Dham

The 4-bit adder-subtractor is a digital circuit capable of performing arithmetic operations on binary numbers of four bits in length. By leveraging a clever combination of logic gates and flip-flops, this versatile circuit is capable of adding or subtracting two 4-bit binary numbers with remarkable efficiency and accuracy.

In this article, we delve into the inner workings of the 4-bit adder-subtractor, exploring its architecture, logic implementation, and applications. We’ll explore how this compact circuit design manages to provide both addition and subtraction functionalities in a concise package, making it an invaluable component in a wide range of digital systems.

What is Binary Adder and Subtractor?

In the realm of digital circuits, the binary adder-subtractor emerges as a remarkable component that seamlessly combines the functions of addition and subtraction within a single circuit. Its operational mode is determined by the binary value stored in a control signal, allowing it to perform both arithmetic operations with utmost efficiency. This versatile circuit holds a significant position as a vital element of the Arithmetic Logic Unit (ALU), a core component in various computational systems.
Prior understanding of the Exor Gate, binary addition and subtraction, and full adder is required for this circuit.

As inputs to the digital circuit for the digit operation, let’s take two 4-bit binary values, A and B.
A0 A1 A2 A3 for A
B0 B1 B2 B3 for B

Since we are operating on 4-bit numbers, the circuit consists of 4 complete adders. There is a control line K that has a binary value of either 0 or 1, depending on whether addition or subtraction is being performed.

The input carry Cin of the first full adder, which is depicted in the image, is a control line. The input A0, or the least significant bit of A, is input into the full adder directly. The exor of B0 and K serves as the third input. The two outputs are Carry (C0) and Sum/Difference (S0).

If K (Control line) has a value of 1, then B0(exor)K=B0′(Complement B0). Therefore, the formula would be A+(B0′). A+B’+Cin now equals the 2’s complement subtraction of the two numbers A and B. This shows that the four-bit values are being operated on by subtracting when K=1.
Similarly In the event when K=0, B0 (exor) K=B0 . A+B is a straightforward binary addition procedure. This implies that the operation is also applied to the four-bit numbers when K=0.

The second complete adder then receives C0 in serial form as one of its outputs. The least important piece of the sum/difference is recorded as the sum/difference S0. Direct inputs to the second, third, and fourth complete adders are A1, A2, and A3. Next, the B1, B2, B3, and K are EXORed with the second, third, and fourth full adders, respectively, on the third input. The successive full adder receives the carry C1, C2 in serial as one of its inputs. The entire carry to the sum/difference is changed to C3. S0 is added to S1, S2, S3, and the result is recorded.

We use n complete adders to create an n-bit binary adder-subtractor.

4 Bit Adder and Subtractor Example

Assume that we are going to input two 3-bit values into the Full-Adder, X=100 and Y=011.

X0 = 0 X1 = 0 X2 = 1
Y0 = 1 Y1 = 1 & Y2 = 0

For K=0:
Y0⨁K=Y0 and Cin=K=0
So, from first Full-Adder

S0 = X0+Y0+Cin
S0= 0+1+0


S1 = X1+Y1+C0
S1 = 0+1+0
S1=1 and C1=0


S2 = X2+Y2+C1
S2 = 1+0+0
S2=1 and C2=0


X= 100 =4
Y = 011 = 3
Sum = 0111 = 7

For K=1

Y0⨁K=Y0′ and Cin=k=1


S0 = X0+Y0’+Cin
S0 = 0+0+1
S0=1 and C0=0


S1 = X1+Y1’+C0
S1 = 0+0+0
S1=0 and C1=0


S2 = X2+Y2’+C1
S2 = 1+1+0
S2=0 and C2=0


X = 010 = 4
Y = 011 = 3

Difference = 001 = 1

The 4-bit adder-subtractor stands as a testament to the power of simplicity in digital circuit design. Through its ingenious combination of logic gates and flip-flops, this compact circuit delivers exceptional performance in performing arithmetic operations on 4-bit binary numbers. As we have explored in this article, the 4-bit adder-subtractor offers both addition and subtraction functionalities, providing a versatile solution for a wide range of applications.

Understanding the inner workings of the 4-bit adder-subtractor has revealed the fundamental concepts of binary arithmetic and how they are efficiently implemented in this circuit. By examining its architecture and logic implementation, we have gained insights into the various design methodologies and trade-offs involved in achieving optimal performance.

FAQs: 4-Bit Adder and Subtractor

Q1: What is a 4-bit adder and subtractor?
A 4-bit adder and subtractor is a digital circuit that performs both addition and subtraction operations on binary numbers consisting of four bits. It combines the functionality of an adder and a subtractor in a single circuit, allowing for efficient arithmetic calculations.

Q2: How does a 4-bit adder and subtractor work?
The circuit uses a combination of logic gates and flip-flops to perform addition and subtraction. It takes two 4-bit binary numbers as input, along with a control signal that determines the operation to be performed (addition or subtraction). The circuit performs the corresponding operation based on the control signal, utilizing binary arithmetic principles and carry/borrow mechanisms.

Q3: What are the key components of a 4-bit adder and subtractor?
A 4-bit adder and subtractor typically consists of four full adders, which are interconnected to handle the addition or subtraction of individual bits. Additionally, multiplexers or control gates are included to control the flow of data and select the desired operation.

Q4: What are the applications of a 4-bit adder and subtractor?
The 4-bit adder and subtractor find applications in various digital systems, such as arithmetic calculators, microprocessors, digital signal processors, and control units. They are essential for performing arithmetic calculations in these systems accurately and efficiently.

Q5: Can a 4-bit adder and subtractor handle larger numbers?
No, a 4-bit adder and subtractor can only handle binary numbers consisting of four bits. For larger numbers, cascading multiple 4-bit adders or using higher bit adder-subtractor circuits is necessary.

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