The Central Processing Unit (CPU)Understanding computers

3rd Edition

• Draw a block diagram of the main components of a

computer: input, processor, output and storage

• Explain what RAM and ROM are used for

• Distinguish between main memory and permanent

storage devices

• Name the three stages in the Fetch Execute Cycle

• Define Hz, MHz and GHz and state how these relate

to the speed of the processor

Objectives

The Central Processing Unit (CPU)

Understanding Computers

What are these components?

The Central Processing Unit (CPU)

Understanding Computers

The components of a computer

Central

Processing

Unit

(CPU)

Input

OutputMain

Memory

Storage

The Central Processing Unit (CPU)

Understanding Computers

Fetch – Decode – Execute cycle

Decode

Execute

Fetch

The Central Processing Unit (CPU)

Understanding Computers

Fetch – Decode – Execute cycle

• The computer has a list of instructions in memory to

carry out:

• CPU Fetches top instruction from the list

• Instructions are passed to Decoder to interpret

• Decoder passes on the instruction

• Each instruction is Executed or carried out

• CPU Fetches top instruction from the list…

The Central Processing Unit (CPU)

Understanding Computers

Activity

• Pupil 1 – holds instructions in the order they are to

be processed

• Pupil 2 – fetches an instruction and gives to Pupil 3

• Pupil 3 – decodes instruction and tells Pupil 4 what

to do

• Pupil 4 – executes the instruction

The Central Processing Unit (CPU)

Understanding Computers

Processor speed

• One cycle per second = 1 Hertz (Hz) = 1 instruction

carried out each second

• 1 Kilohertz (kHz) = 1000 cycles per second

• 1 Megahertz (MHz) = 1,000,000 cycles per second

• 1 Gigahertz (GHz) = 1,000,000,000 (1 Billion) cycles

per second

• How fast is your computer’s processor?

The Central Processing Unit (CPU)

Understanding Computers

Worksheet 2

• Use the Internet to find answers to the questions on

the worksheet.

The Central Processing Unit (CPU)

Understanding Computers

RAM vs ROM

• RAM stands for Random Access Memory

• ROM stands for Read Only Memory

• Some data needs to be permanently held in

memory, even when the machine is switched off

• What does a computer do when you turn it on?

• Where are these instructions held?

The Central Processing Unit (CPU)

Understanding Computers

Starting up a computer

• When you switch on a computer, a small program

held in ROM called the bootstrap loader is

automatically executed

• The program performs some self-tests, and loads

the operating system into memory

The Central Processing Unit (CPU)

Understanding Computers

Plenary

• What is memory?

• What is the difference between a hard disk

and memory?

• What is a processor?

The Central Processing Unit (CPU)

Understanding Computers

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