1. Central Processing Unit
The CPU
Central Processing Unit

2. Activity 1 5 minutes to discuss and feedback on the following:
What is this?
What is it for?
What do you notice?
What questions do you have?
What do you think? (hypothesis)

3. The CPU – Central Processing Unit
Learning Objectives:
i)State the purpose of the CPU
ii) Describe the function of the CPU as fetching and executing instructions stored in memory
What is the Purpose of the CPU?
The CPU is often described as the 'brain of the computer'.
The purpose of the CPU is to process data. It is where all the searching, sorting, calculating and decision making takes place in the computer.
Whatever you are doing on your computer (writing an essay or sending an ), its the CPU that has dealt with all of the data and processing required to make these tasks possible.

4. The CPU – Central Processing Unit
Learning Objectives:
i)State the purpose of the CPU
ii) Describe the function of the CPU as fetching and executing instructions stored in memory
The Fetch-Decode-Execute Cycle A standard process describes the steps needed for processing to take place. It is called the Fetch - Decode - Execute cycle or sometimes simply called the Fetch-Execute Cycle. First of all, both the data itself AND the program that is using the data are loaded into main memory (RAM) by the operating system. The CPU is now ready to do some work.

5. The CPU – Central Processing Unit
Learning Objectives:
i)State the purpose of the CPU
ii) Describe the function of the CPU as fetching and executing instructions stored in memory
The Fetch Stage
The first step the CPU carries out is to fetch some data and instructions (a program) from main memory (RAM) and then store them in its own temporary memory areas.
These memory areas are called 'registers'.
This is called the 'fetch' part of the cycle.
Can I Have Some Data?
CPU
RAM / Memory
Yep! Here it is!

6. The CPU – Central Processing Unit
Learning Objectives:
i)State the purpose of the CPU
ii) Describe the function of the CPU as fetching and executing instructions stored in memory
The Fetch Stage - Continued
For this to happen, the CPU makes use of a vital hardware path called the 'address bus'.
The CPU places the address of the next item to be fetched on to the ‘address bus’.
Data from this address then moves from main memory into the CPU by travelling along another hardware path called the 'data bus'.
CPU
RAM / Memory
Address
Bus
CPU
RAM / Memory
Data
Bus

7. The CPU – Central Processing Unit
Learning Objectives:
i)State the purpose of the CPU
ii) Describe the function of the CPU as fetching and executing instructions stored in memory
The Decode Stage
The next step is for the CPU to make sense of the instruction it has just fetched.
This process is called 'decode'.
The CPU decodes the instruction and prepares various areas within the chip in readiness of the next step.
Making Sense of Instructions

8. The CPU – Central Processing Unit
Learning Objectives:
i)State the purpose of the CPU
ii) Describe the function of the CPU as fetching and executing instructions stored in memory
The Execute Stage
This is the part of the cycle when data processing actually takes place.
The instruction is carried out on the data (executed).
The result of this processing is stored in yet another register.
Once the execute stage is complete, the CPU sets itself up to begin another cycle once more.

9. The CPU – Central Processing Unit
Learning Objectives:
i)State the purpose of the CPU
ii) Describe the function of the CPU as fetching and executing instructions stored in memory
Summary

10. Activity 2
10 minutes: Compete the first activity sheet: “The stages of the Fetch – Decode – Execute Cycle”

11. The CPU – Central Processing Unit
Learning Objectives:
i)State the purpose of the CPU
ii) Describe the function of the CPU as fetching and executing instructions stored in memory
Starter – 5 mins
Open your work from last lesson and discuss with your neighbour.
Are you able to explain the Fetch-Decode-Execute Cycle to the person next to you?
Look at the lesson presentation again & see if you can improve your work.

12. The CPU – Central Processing Unit
Learning Objectives:
i)State the purpose of the CPU
ii) Describe the function of the CPU as fetching and executing instructions stored in memory
RECAP

13. The CPU – Central Processing Unit
Learning Objectives:
i)State the purpose of the CPU
ii) Describe the function of the CPU as fetching and executing instructions stored in memory
What makes up a CPU?
In order to appreciate how the Fetch-Decode-Execute cycle works you need to have a basic understanding of the three main parts of a CPU. These are the:
Control Unit
Immediate Access Store (IAS) or “Cache”
Arithmetic and Logic Unit (ALU)
Arithmetic and Logic Unit (ALU)
Control
Unit
Immediate Access Store (Cache)
Inputs and Outputs
Main Memory
(RAM)

14. The CPU – Central Processing Unit
Learning Objectives:
i)State the purpose of the CPU
ii) Describe the function of the CPU as fetching and executing instructions stored in memory
The Control Unit
The Control Unit has three main jobs:
It controls and monitors the hardware attached to the system to make sure that the commands given to it by the application software are used. For example, if you send something to print, the control unit will keep a check that the instructions are sent to the printer correctly.
It controls the input and output of data so that the signals go to the right place at the right time
It controls the flow of data within the CPU (the Fetch-Execute cycle described above).

15. The CPU – Central Processing Unit
Learning Objectives:
i)State the purpose of the CPU
ii) Describe the function of the CPU as fetching and executing instructions stored in memory
Immediate Access Store (Cache)
This holds the data and programs needed at that instant by the Control Unit.
The CPU reads data and programs kept on the backing storage (RAM/Hard Drive) and stores them temporarily in the IAS (Cache).
The CPU needs to do this because Backing Storage (e.g. the hard disk) is much too slow to be able to run applications directly.
For example, let's imagine that a modern CPU was slowed down to carry out one instruction per second. In this scenario, if the hard disk slowed down to an equivalent rate it would take three months to supply the data needed by the CPU!
So data and programs are first of all loaded into main memory, then the CPU fetches JUST the part it immediately needs and loads that into its own internal registers as part of the Fetch-Execute cycle.

16. The CPU – Central Processing Unit
Learning Objectives:
i)State the purpose of the CPU
ii) Describe the function of the CPU as fetching and executing instructions stored in memory
Arithmetic and Logic Unit (ALU)
This is where the computer processes data either by manipulating it or acting upon it.
It consists of two parts:
Arithmetic part, which does exactly what you think it should: it performs any calculations needed on the data, e.g = 5
Logic part - this deals with logic and comparisons. For example, it works out if one value is greater, less than or equal to another.

17. Summary of the CPU
An input device (e.g. keyboard) sends data to the CPU. The Control Unit receives this data.
The Control Unit sends this data into main memory to be used later.
When the time is right, the data will be transferred from main memory into cache (IAS)
The data will then be sent to the ALU for processing
The control unit will send the processed data back (for example to an output device such as a screen or monitor).

18. Activity 3
15 minutes: Compete the second activity sheet: “The Journey of Data through the CPU”

19. The CPU – Central Processing Unit
Learning Objectives:
i)State the purpose of the CPU
ii) Describe the function of the CPU as fetching and executing instructions stored in memory
RECAP
What is the Purpose of the CPU?
The CPU is often described as the 'brain of the computer'.
The purpose of the CPU is to process data. It is where all the searching, sorting, calculating and decision making takes place in the computer.
Can YOU describe the function of the CPU?
What is the cycle that takes place?