The Role of the Motherboard and Buses

This lesson covers the motherboard and the system bus (address bus, data bus, and control bus), as required by OCR J277 Section 1.1.1. Understanding how components communicate is essential for explaining how a computer system functions as a whole.

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The Motherboard

The motherboard is the main circuit board inside a computer. It is the physical foundation that connects all the components of a computer system together.

What Does the Motherboard Do?

Function	Description
Physical connections	Provides slots and sockets for the CPU, RAM, storage, and expansion cards
Communication pathways	Contains the buses (wires/traces) that allow components to send data to each other
Power distribution	Distributes electrical power from the power supply to all components
BIOS/UEFI chip	Contains the firmware needed to start the computer (boot process)

Key Connectors on a Motherboard

• CPU socket — where the processor is installed
• RAM slots (DIMM) — where memory modules are inserted
• PCIe slots — for expansion cards (graphics card, network card, sound card)
• SATA connectors — for connecting storage devices (HDDs, SSDs)
• USB headers — for connecting USB ports
• Power connectors — receive power from the PSU (power supply unit)

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The System Bus

The system bus is a collection of wires (also called traces on the motherboard) that carry signals between the CPU, memory, and other components. It consists of three separate buses:

1. The Address Bus

Property	Detail
Purpose	Carries the memory address that the CPU wants to access
Direction	Unidirectional — data flows only from the CPU to memory/devices
Width	Determines the maximum amount of memory the CPU can address

How address bus width affects memory:

The width of the address bus (measured in bits) determines how many unique memory addresses the CPU can access:

• A 16-bit address bus can address 2^16 = 65,536 memory locations (64 KB)
• A 32-bit address bus can address 2^32 = 4,294,967,296 memory locations (4 GB)
• A 64-bit address bus can address 2^64 memory locations (over 16 exabytes)

OCR Exam Tip: The formula for calculating addressable memory is: Number of addresses = 2^n, where n is the width of the address bus in bits.

2. The Data Bus

Property	Detail
Purpose	Carries data between the CPU, memory, and I/O devices
Direction	Bidirectional — data can flow in both directions
Width	Determines how much data can be transferred in one operation

How data bus width affects performance: