Computer System Architecture

This lesson covers the internal architecture of a CPU (Central Processing Unit) as required by the AQA and OCR GCSE Computer Science specifications. You need to understand what the CPU is, why it matters, and how its components — the ALU, CU, and registers — work together to process instructions.

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What Is a CPU?

The CPU (Central Processing Unit) is the main processing component of a computer. It is often described as the "brain" of the computer because it carries out the instructions that make up a program. Every input you give, every calculation that is performed, and every output you receive passes through or is coordinated by the CPU.

Modern CPUs are fabricated on a single silicon chip and contain billions of transistors — tiny electronic switches that represent binary 1s and 0s.

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Von Neumann Architecture

Most modern computers follow the Von Neumann architecture, proposed by mathematician John von Neumann in 1945. The key features of this architecture are:

• Programs and data are stored together in main memory (RAM).
• Instructions are fetched from memory one at a time and executed in sequence.
• There is a single set of buses (data, address, and control) connecting the CPU to memory.

Feature	Description
Stored program concept	Instructions and data share the same memory
Sequential execution	Instructions are processed one after another
Single memory space	Both data and programs live in RAM
Buses	Data bus, address bus, and control bus connect components

Exam Tip: You may be asked to label a Von Neumann architecture diagram. Make sure you can identify the CPU, main memory, and the three buses.

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Components of the CPU

The CPU contains three main components that you must know for GCSE:

1. The Arithmetic Logic Unit (ALU)

The ALU performs all arithmetic and logical operations within the CPU:

• Arithmetic operations: addition, subtraction, multiplication, division.
• Logical operations: comparisons such as equal to, greater than, less than, AND, OR, NOT.

Every calculation your computer makes — from adding up a spreadsheet to rendering a game frame — passes through the ALU.

2. The Control Unit (CU)

The Control Unit manages and coordinates the activities of the CPU:

• It fetches instructions from main memory.
• It decodes instructions to determine what operation is needed.
• It sends control signals to other components (ALU, memory, I/O devices) to tell them what to do.
• It controls the timing of operations using the system clock.

Think of the CU as a traffic controller — it does not process data itself but tells every other component when and how to act.

3. Registers

Registers are tiny, ultra-fast storage locations inside the CPU. They hold small amounts of data that are currently being used or processed. Because they are built directly into the CPU chip, registers are the fastest form of memory in a computer.

Key registers you should know:

Register	Full Name	Purpose
PC	Program Counter	Holds the memory address of the next instruction to be fetched
MAR	Memory Address Register	Holds the address of the memory location about to be read from or written to
MDR	Memory Data Register	Holds the data that has been fetched from memory or the data about to be written to memory
ACC	Accumulator	Stores the results of calculations performed by the ALU

Exam Tip: A very common exam question is: "What is the purpose of the program counter?" The answer: it holds the memory address of the next instruction to be fetched and executed.

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The System Bus

The CPU communicates with the rest of the computer via the system bus, which consists of three separate buses:

Bus	Function	Direction
Data bus	Carries data between the CPU, memory, and I/O devices	Bidirectional (data can flow both ways)
Address bus	Carries the memory address that the CPU wants to access	Unidirectional (CPU to memory only)
Control bus	Carries control signals (e.g., read, write, clock)	Bidirectional

How the Buses Work Together

1. The CPU places the address of the required data on the address bus.
2. A control signal is sent along the control bus to tell memory whether it is a read or write operation.
3. The data is transferred along the data bus between the CPU and memory.

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Putting It All Together

Below is a simplified text diagram of the Von Neumann architecture:

+---------------------------------------------+
|                     CPU                      |
|  +-------+   +-----------+   +----------+   |
|  |  CU   |   |    ALU    |   | Registers|   |
|  +-------+   +-----------+   |  PC      |   |
|                              |  MAR     |   |
|                              |  MDR     |   |
|                              |  ACC     |   |
|                              +----------+   |
+---------------------------------------------+
       |              |              |
  Control Bus     Data Bus     Address Bus
       |              |              |
+---------------------------------------------+
|              Main Memory (RAM)               |
+---------------------------------------------+

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Summary

• The CPU is the main processing unit, executing program instructions.
• The ALU handles arithmetic and logic.
• The CU fetches, decodes, and coordinates execution.
• Registers (PC, MAR, MDR, ACC) provide ultra-fast temporary storage inside the CPU.
• The system bus (data, address, control) connects the CPU to main memory and I/O.
• Most modern computers use the Von Neumann architecture where programs and data share the same memory.

Key Vocabulary: CPU, ALU, CU, register, program counter, MAR, MDR, accumulator, Von Neumann, data bus, address bus, control bus.