CPU Components & Their Function (OCR GCSE Computer Science): Revision Notes

CPU Components & Their Function

The CPU (Central Processing Unit) is the brain of the computer, responsible for executing instructions through the fetch-execute cycle. It consists of several key components that work together to process data efficiently.

ALU (Arithmetic Logic Unit)

Role/Purpose

• Performs all arithmetic operations (e.g., addition, subtraction) and logical operations (e.g., comparisons, AND, OR).
• Executes calculations and decision-making operations within the CPU.

What it Manages

• Data involved in arithmetic or logical operations.

During the Fetch-Execute Cycle

• The ALU processes data and performs calculations once the instruction is decoded.

CU (Control Unit)

Role/Purpose

• Manages and controls the operations of the CPU and coordinates how data moves between the CPU, memory, and input/output devices.

What it Manages

• The control signals that direct the operations of the CPU and other hardware.

During the Fetch-Execute Cycle

• The CU decodes the fetched instruction and controls the sequence of execution, ensuring the correct data is processed at the right time.

Cache

Role/Purpose

• A small amount of high-speed memory within the CPU that stores frequently used instructions and data.

What it Manages

• Frequently accessed data to avoid delays caused by accessing slower main memory (RAM).

During the Fetch-Execute Cycle

• Cache is checked before fetching data from the main memory, speeding up the process if the needed data is stored in cache.

Registers

Registers are small, fast storage locations inside the CPU that temporarily hold data or addresses. Each register has a specific role in the fetch-execute cycle:

Program Counter (PC)

• Stores the address of the next instruction to be executed.
• Ensures the CPU knows where to fetch the next instruction from.

Memory Address Register (MAR)

• Stores the address of the data or instruction to be fetched from or written to memory.
• Points to the specific location in memory being accessed.

Memory Data Register (MDR)

• Stores the data being fetched from or written to memory.
• Acts as a temporary holding area for data transferred between the CPU and memory.

Accumulator (ACC)

• Stores intermediate data results from arithmetic or logic operations performed by the ALU.
• Holds the result of calculations to be used by other parts of the CPU.