# How Computers Calculate - the ALU: Crash Course Computer Science #5 | Summary and Q&A

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March 22, 2017
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CrashCourse
How Computers Calculate - the ALU: Crash Course Computer Science #5

## TL;DR

Learn about the Arithmetic and Logic Unit (ALU) in a computer, its design and function, and how it performs mathematical operations and logical operations.

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### Q: What is the function of the Arithmetic and Logic Unit (ALU) in a computer?

The ALU in a computer is responsible for performing numerical operations, such as addition and subtraction, and logical operations, such as AND, OR, and NOT.

### Q: How is the ALU designed and built using logic gates?

The ALU is built using logic gates, such as AND, OR, NOT, and XOR gates. By combining these gates in clever ways, the ALU can perform complex mathematical and logical operations.

### Q: What mathematical operations can the ALU perform?

The ALU can perform addition, subtraction, and other math operations. More complex operations, like multiplication and division, are usually implemented using a series of additions.

### Q: Are multiply and divide operations supported by the ALU?

Simple ALUs don't have dedicated circuits for multiplication and division. Instead, they perform a series of additions to simulate these operations. More advanced processors may have dedicated circuits for multiplication and division.

### Q: How does the ALU handle logical operations?

The ALU has a logic unit that handles logical operations like AND, OR, and NOT. It uses logic gates to perform these operations on binary inputs.

### Q: What are the flags output by the ALU, and how are they used?

The ALU outputs flags, such as the Zero Flag, Negative Flag, and Overflow Flag. These flags indicate specific states or statuses. They are often used in conditional statements and to detect errors or special cases.

### Q: How does the ALU's design impact the performance of modern computers?

The design of the ALU can impact the speed and efficiency of a computer. Modern computers often use optimized designs, such as carry-look-ahead adders, to improve the speed of mathematical operations performed by the ALU.

### Q: What is the significance of the Intel 74181 ALU in computer history?

The Intel 74181 was a groundbreaking ALU that fit entirely inside a single chip. It played a crucial role in miniaturization and paved the way for more capable and affordable computers. However, its limited capabilities, such as not supporting multiplication and division, made it less versatile than modern ALUs.

## Summary & Key Takeaways

• The ALU is the mathematical brain of a computer and handles numerical operations and logical operations.

• The ALU can be built using logic gates, such as AND, OR, NOT, and XOR gates.

• The ALU's arithmetic unit performs addition, subtraction, and other math operations, while the logic unit performs logical operations like AND, OR, and NOT.

Questions:

1. What is the function of the Arithmetic and Logic Unit (ALU) in a computer?

2. How is the ALU designed and built using logic gates?

3. What mathematical operations can the ALU perform?

4. Are multiply and divide operations supported by the ALU?

5. How does the ALU handle logical operations?

6. What are the flags output by the ALU, and how are they used?

7. How does the ALU's design impact the performance of modern computers?

8. What is the significance of the Intel 74181 ALU in computer history?

### Q: What is the function of the Arithmetic and Logic Unit (ALU) in a computer?

The ALU in a computer is responsible for performing numerical operations, such as addition and subtraction, and logical operations, such as AND, OR, and NOT.