Common CPU Components and Registers

This lesson provides a comprehensive overview of all the key CPU components and registers you need to know for OCR J277 Section 1.1.1. While earlier lessons introduced these individually, this lesson brings everything together and explores how the components interact.

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The Three Main CPU Components

1. The Arithmetic Logic Unit (ALU)

The ALU is the part of the CPU that performs all arithmetic and logical operations.

Arithmetic operations:

• Addition (+)
• Subtraction (-)
• Multiplication (*)
• Division (/)

Logical operations:

• Comparisons: equal to (==), not equal to (!=), greater than (>), less than (<)
• Boolean logic: AND, OR, NOT

Every calculation the computer performs — from adding numbers in a spreadsheet to determining whether a character in a game has collided with an obstacle — passes through the ALU.

The ALU receives data from registers (typically the accumulator) and sends results back to registers after processing.

2. The Control Unit (CU)

The Control Unit is the component that manages and coordinates all CPU activities. It does not process data itself — instead, it tells other components what to do and when.

The CU is responsible for:

Function	Description
Fetching	Sending signals to memory to retrieve the next instruction
Decoding	Interpreting the fetched instruction (splitting it into opcode and operand)
Coordinating	Sending control signals to the ALU, registers, memory, and I/O devices
Timing	Using the system clock to synchronise all operations

Think of the CU as a conductor in an orchestra — it does not play any instruments, but it ensures every musician (component) plays at the right time.

3. The Cache

Cache is a small, ultra-fast memory built into the CPU. It stores copies of the most frequently accessed data and instructions to reduce the time the CPU spends waiting for data from RAM.

Cache operates on the principle of locality of reference:

• Temporal locality: Data that has been accessed recently is likely to be accessed again soon.
• Spatial locality: Data stored near recently accessed data is likely to be needed next.

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Key Registers

Registers are tiny, extremely fast storage locations inside the CPU. They are the fastest type of memory in a computer because they are built directly into the CPU chip.

The Four Registers You Must Know