1. Hook – CPU Knowledge Check
Use the BBC Bitesize link to complete the ten MCQ test
Click here to try the test
LP – to be able to describe how a CPU works (Unit 1)

2. Key Questions to consider
What is inside a CPU?
How does a CPU know what to do with instructions and data, and what order to do them in?
How does data and instructions get to the CPU?
What is the Fetch-Execute cycle?
To explain in detail how registers and buses are used within the fetch-execute cycle
To be able to describe the difference between the MAR/MDR and ALU/CU
To be able to label a CPU diagram and state the purpose of the fetch-execute cycle
LP – to be able to describe how a CPU works (Unit 1)

3. 1 – Key Concepts about the CPU
Program Counter (PC) and the Memory Address Unit (MAR)
The PC holds the ADDRESS of the instructions needed for each cycle. Once an instruction has been fetched, it will hold the address for the next one that is needed. The PC is also known as the CU
The MAR is a small area of memory that holds addresses of memory. It uses the ADDRESS BUS to swap collect instructions from the main memory
Current Instruction Register (CIR) and Memory Data Register (MDR)
The CIR holds the instruction that has just been fetched from main memory and is about to be executed. It will be split into an Op code (the operation to be performed) and the Operand (the data in which the operation will be performed on)
The MDR collects data from the main memory by using the DATA BUS, and then sends it onto the CIR.
Arithmetic Logic Unit (ALU) and the Accumulator
The ALU performs arithmetic (Add/Subtract) and logical (AND/OR) operations. The ACCULULATOR holds the result of a calculation or an operation. For example, when a value is inputted into the computer, it will be held here. Any calculation applied to it is saved here too.
The accumulator is an example of a another REGSITER (Like the MAR and MDR)
Travelling from the main memory to the CPU
Data and instructions from the main memory is taken to one of two registers, the MAR or the MDR.
The addresses of data/instructions are carried from the MAR to the Main memory via the ADDRESS BUS
The data/instructions themselves are carried from the Main memory to the MDR via the DATA BUS
1 – Key Concepts about the CPU
LP – to be able to describe how a CPU works (Unit 1)

4. LP – to be able to describe how a CPU works (Unit 1)
Cache Memory and Main Memory
Cache is memory stored within the CPU, other than the registers, it is the fastest memory for the CPU to use for regularly used data. When the CPU requests data it takes it from here first, then the RAM (Main memory). However, the cache is very small and can’t store much data compared to the RAM and secondary storage. Because of this it is very expensive. Cache comes in three sizes (L1, L2 and L3)
Secondary Storage
Used for long term storage of data and instructions
Examples include –
Hard Disk Drive (HDD) Solid State Drive (SSD)
Flash Memory (USB sticks and SD cards)
Optical Discs (DVD-R and Blu-ray)
RAM (Random Access Memory)
Holds Data and Instructions that are currently in use by the processor
Located on the Motherboard
Directly Accessible by the processor
All data/instructions are lost once power is turned off
ROM (Read Only Memory)
Instructions are permanently etched onto a ROM Chip
When power is turned off – instructions still remain on the ROM chip.
Bootstrap Loader is held in ROM
Gives the instructions to start up the Operating System
Memory – Key Concepts
LP – to be able to describe how a CPU works (Unit 1)

5. Activity 1 Use the key concepts slides to help you complete each task
Answer the exam style questions using the CPU key concepts
Label the Key terms to the Internal CPU diagram (10 Key Terms)
Use the slides to evidence work for each success criteria
Match the CPU terms to the meanings (10 Key Terms)
LP – to be able to describe how a CPU works (Unit 1)

6. Activity 1 - Bronze
Self Assess - Score
/10
CIR
Accumulator
ALU
Processor
Main Memory
Address Bus
PC (Program Counter)
MAR
Data Bus
MDR
Processes data and executes instructions
It is a very small area of memory that is used to hold a memory address. The memory address is provided by the Program Counter.
Once data is retrieved from Main Memory (RAM) it travels along the Data Bus to here. Data/Instructions are held here temporarily. Any instructions will then be passed to the CIR.
This holds the instruction that has just been fetched from main memory and is about to be executed. The Current Instruction is split into an Op Code (the operation to be performed) and the Operand (the data in which to perform the operation on).
All Data and Instructions currently in use are held here. When the power is turned off, all data/instructions are lost. (Volatile Memory)
Carries addresses from the Processor (MAR) to Main Memory
Carries Data/Instructions from Main Memory back to the Processor (MDR)
Used to hold the result of a calculation or an operation. When a value is inputted into the computer it will be held here. If a value is added – then the result is temporarily held here. This is also another Register, like the MAR and MDR
Holds the address of the next instruction that will be fetched.
Performs arithmetic (Add / Subtract) and logical (AND, OR) operations
LP – to be able to describe how a CPU works (Unit 1)

7. Activity 2 - Silver
CIR
Accumulator
ALU
Processor
Main Memory
Address Bus
PC (Program Counter)
MAR
Data Bus
MDR
0001
0010
0011
0100
0101
0110
0111
1000
1001
1010
1011
1100
1101
INP
Add 5
STO 8
ADD 8
Self Assess - Score
/10
LP – to be able to describe how a CPU works (Unit 1)

8. Activity 1 - Gold LP – to be able to describe how a CPU works (Unit 1)
Self Assess - Score /8
Exam Style Question
Answer
What is the difference between the address bus and the data bus?
What does the accumulator do?
What does the CIR split an instruction into?
What are the names of the THREE registers within the CPU?
LP – to be able to describe how a CPU works (Unit 1)

9. Diagrams of a Computer System
1945: John Von Neumann
Processor
Main Memory
I/O
Diagrams of a Computer System
LP – to be able to describe how a CPU works (Unit 1)

10. Activity 2 Use the key concepts slides to help you complete each task
Identify the key differences between memory and storage (Cache, RAM, ROM and secondary storage)
Use the revision book to describe the key functions of the FDE (Fetch-Decode-Execute) cycle
Use the slides to evidence work for each success criteria
Categorise the components of a computer system as internal or external
LP – to be able to describe how a CPU works (Unit 1)

11. External Components (Peripherals)
Activity 2 - Bronze
Internal Components
External Components (Peripherals)
The processor
Main memory (RAM, ROM)
I/O controllers
Buses
Keyboard, mouse, printer, disk drives
I/O Devices / I/O Ports
Secondary Storage
LP – to be able to describe how a CPU works (Unit 1)

12. Activity 2 Silver LP – to be able to describe how a CPU works (Unit 1)
Fetch
Decode
Execute
Activity 2 Silver
LP – to be able to describe how a CPU works (Unit 1)

13. Activity 2 - Gold LP – to be able to describe how a CPU works (Unit 1)
Memory Type
Key points
RAM
ROM
Virtual Memory
Secondary Storage
LP – to be able to describe how a CPU works (Unit 1)

14. Plenary / Extensions
Produce FIVE Questions + Mark Scheme related to CPU components, the FDE cycle and memory types (1-4)
Use the revision book to create a diagram of a CPU which describes the CU, ALU, Registers, Buses, Inputs and Output Devices (I/O) and Main Memory (4–7)
Create a chart that shows the fastest to slowest memory types from Cache, Main Memory and types of secondary storage (7–9)
LP – to be able to describe how a CPU works (Unit 1)

15. Plenary LP – to be able to describe how a CPU works (Unit 1)
Exam question
Answer
LP – to be able to describe how a CPU works (Unit 1)

16. Extension 1
LP – to be able to describe how a CPU works (Unit 1)

17. Extension 2
LP – to be able to describe how a CPU works (Unit 1)