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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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