Arithmetic & Logic Unit | Computer Architecture & Organisation (CAO) - Computer Science Engineering (CSE) PDF Download

Arithmetic and Logic Unit
ALU is the combinational circuit of that part of computer that actually performs arithmetic and logical operations on data. All of the other elements of computer system- control unit, registers, memory, I/O are their mainly to bring data into the ALU for it to process and then to take the result back out. An ALU & indeed all electronic components in computer are based on the use of simple digital logic device that can store binary digit and perform simple Boolean logic function. Figure indicates in general in general term how ALU is interconnected with rest of the processor.

Arithmetic & Logic Unit | Computer Architecture & Organisation (CAO) - Computer Science Engineering (CSE)

Data are presented to ALU in register and the result of operation is stored in register. These registers are temporarily storage location within the processor that are connected by signal path to the ALU. The ALU may also set flags as the result of an operation. The flags values are also stored in registers within the processor. The control unit provides signals that control the operation of ALU and the movement of data into an out of ALU.

The design of ALU has three stages.

1. Design the arithmetic section
The basic component of arithmetic circuit is a parallel adder which is constructed with a number of full adder circuits connected in cascade. By controlling the data inputs to the parallel adder, it is possible to obtain different types of arithmetic operations. Below figure shows the arithmetic circuit and its functional table.

Arithmetic & Logic Unit | Computer Architecture & Organisation (CAO) - Computer Science Engineering (CSE)

Arithmetic & Logic Unit | Computer Architecture & Organisation (CAO) - Computer Science Engineering (CSE)

2. Design the logical section
The basic components of logical circuit are AND, OR, XOR and NOT gate circuits connected accordingly. Below figure shows a circuit that generates four basic logic micro-operations. It consists of four gates and a multiplexer. Each of four logic operations is generated through a gate that performs the required logic. The two selection input S1 and S0 choose one of the data inputs of the multiplexer and directs its value to the output. Functional table lists the logic operations.

Arithmetic & Logic Unit | Computer Architecture & Organisation (CAO) - Computer Science Engineering (CSE)

Arithmetic & Logic Unit | Computer Architecture & Organisation (CAO) - Computer Science Engineering (CSE)

3. Combine these 2 sections to form the ALU
Below figure shows a combined circuit of ALU where n data input from A are combined with n data input from B to generate the result of an operation at the G output line. ALU has a number of selection lines used to determine the operation to be performed. The selection lines are decoded with the ALU so that selection lines can specify distinct operations. The mode select S2 differentiate between arithmetic and logical operations. The two functions select S1 and S0 specify the particular arithmetic and logic operations to be performed. With three selection lines, it is possible to specify arithmetic operation with S2 at 0 and logical operation with S2 at 1. 

Arithmetic & Logic Unit | Computer Architecture & Organisation (CAO) - Computer Science Engineering (CSE)

Arithmetic & Logic Unit | Computer Architecture & Organisation (CAO) - Computer Science Engineering (CSE)

The document Arithmetic & Logic Unit | Computer Architecture & Organisation (CAO) - Computer Science Engineering (CSE) is a part of the Computer Science Engineering (CSE) Course Computer Architecture & Organisation (CAO).
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FAQs on Arithmetic & Logic Unit - Computer Architecture & Organisation (CAO) - Computer Science Engineering (CSE)

1. What is an Arithmetic & Logic Unit (ALU)?
Ans. An Arithmetic & Logic Unit (ALU) is a digital circuit that performs arithmetic and logical operations in a computer. It is a crucial component of the central processing unit (CPU) and is responsible for executing operations such as addition, subtraction, multiplication, division, and logical comparisons.
2. How does an ALU perform arithmetic operations?
Ans. An ALU performs arithmetic operations by using basic binary arithmetic techniques. For addition and subtraction, it uses circuits like full adders and subtractors. Multiplication and division are typically performed using algorithms such as Booth's algorithm or non-restoring division. The ALU also includes additional circuits for handling carry and overflow conditions.
3. What are the logical operations performed by an ALU?
Ans. An ALU performs logical operations such as AND, OR, NOT, and XOR. These operations are used to manipulate binary data and perform tasks like bitwise comparisons, data masking, and conditional branching in computer programs. The ALU uses logic gates like AND gates, OR gates, and XOR gates to perform these operations.
4. Can an ALU handle floating-point arithmetic?
Ans. It depends on the design of the specific ALU. Some ALUs are designed to handle floating-point arithmetic, while others are not. Floating-point arithmetic involves operations on numbers with fractional parts and is commonly used in scientific and engineering calculations. ALUs that support floating-point arithmetic have additional circuits and algorithms to handle the complexities of floating-point representation and computation.
5. How does an ALU affect the overall performance of a computer?
Ans. The performance of an ALU directly impacts the overall performance of a computer. A faster and more efficient ALU can execute arithmetic and logical operations quickly, enabling faster processing of instructions and data. The ALU's design, including the number of bits it can handle and the supported operations, also affects the range and accuracy of calculations that can be performed. Therefore, an optimized ALU design is crucial for improving the overall speed and efficiency of a computer system.
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