Lesson 2. The System Bus

Lesson Objective

Have an understanding and knowledge of the basic internal components of the CPU.
Describe the purpose of a bus and how the address, control and data bus are used.
Understand the concept of addressable memory.

Lesson Notes

Basic Internal Components

This diagram shows the basic internal components of a computer system.

Key components are identified in the CPU and Computer System.

The system bus is not inside the CPU. It's on the motherboard and connects the CPU to other important parts of the computer system, such as the main memory (RAM) and sometimes other caches. In contrast, there are internal buses within the CPU that allow communication between its various parts. Think of the system bus as a highway connecting different cities, and the internal CPU buses as streets within a city.

The Processor

It is the brain of the computer. The processor is a device that carries out computation. The processor executes instructions and processes information.

What is it made of?

A thin slice of silicon approximately 2cm square
Contains millions of tiny transistors. The transistors control the flow of electrical pulses and are timed via the computer's clock.
Microscopic wires called BUSES connect groups of transistors.

Main Memory

RAM (Random Access memory)

Readable and writable.
Contents are volatile meaning when the power is lost the information is lost.

ROM (Read only memory)

Non Volatile contents are not lost when power is lost.
Cannot be edited once it has been set up. Used to store bootstrap program in PC.

Keyboard Input Controller

A keyboard input controller in the CPU is a hardware device that is integrated into the CPU itself. It is responsible for handling all keyboard input, from the moment a key is pressed until the corresponding character is displayed on the screen. Some specific tasks that a keyboard input controller in the CPU performs:

Detects when a key is pressed or released.
Provides support for special keys, such as the function keys and modifier keys.
Supports multiple keyboard layouts.

Disk I/O Controller

A disk I/O controller in a processor is a hardware component that manages the transfer of data between the processor and disk storage devices. It is responsible for translating commands from the processor into signals that the disk drive can understand, and vice versa. Some of the specific tasks that a disk I/O controller performs include:

Sending and receiving data between the processor and disk drive.
Managing the caching of data in memory.
Handling errors that occur during disk operations.

Video Display Unit (VDU) Controller

A VDU controller is a specialized integrated circuit that is responsible for controlling the display of text and graphics on a monitor. It is typically integrated into the processor or motherboard of a computer.

The VDU controller performs a number of tasks, including:

Generating the video signal that is sent to the monitor.
Generating and displaying text and graphics characters.
Handling cursor movement and other user interface functions.

Buses

Buses in a computer consist of a series of wires that transfer data signals between internal components. They typically consist of 8, 16, 32 or 64 lines.

The System Bus

The Three Box Model

Hardware refers to the physical components of a computer system. The core components of any computer system are:

The Processor
Main Memory
Input/Output Controllers (to communicate with I/O devices)

The processor controls the system bus and the system bus communicates between the all three boxes.

In order to perform operations on data the processor must get the data from main memory first.

An addressable memory system is used where each storage location in main memory has a unique address called a main memory address.

The Three Buses (System Bus)

The control bus is used to send control signals between each I/O controller and the processor, as well as between the processor and memory. Bidirectional.
The data bus sends data between CPU components. Bidirectional.
The address bus sends memory addresses from the processor to CPU components. One-way.

Image of Bus and Component Connections:

Words

Memory is divided up in equal units called words (smallest addressable unit - total amount of bits).

Word length is usually 8, 16, 32 or 64 bits.
Word length is also usually related to the Data Bus.
Each word has a separate memory address.

The width of the data bus is defined by the number of wires or lines it contains.

If the data bus is the same width as a computer word, data can be transferred to and from memory in a single operation.

If it is 16 lines and a word is 32 bits, it will require two memory access and data transfer operations.