1. Unit 1.1 System Architecture Lesson 2

2. Big Picture How are instructions executed by the processor?
What is the purpose of the processor?
Where are instructions that are currently in use stored?
What else is stored in main memory?
How are instructions executed by the processor?

3. Learning Objectives To be able to label an internal diagram of the CPU
To be able to describe the roles of the MAR and the MDR in the fetch part of the fetch-execute cycle
To be able to describe the purpose of the accumulator
To be able to explain the purposes of the ALU, CU and the cache
The describe the importance of the Program Counter in the Fetch-Execute cycle

4. Engagement - Activity 1 On the Diagram Given…
Use research to try and identify the terms on the sheet
Write the definitions/explanations of what they do in the boxes
Then try and label the diagram accurately

5. Activity 1 - Answers Processor Main Memory 0001 INP Address Bus 0010
0011
0100
0101
0110
0111
1000
1001
1010
1011
1100
1101
INP
Add 5
STO 8
ADD 8 PC
MAR
CIR
MDR
Data Bus
ALU
Accumulator

6. Key Words Von Neumann Architecture ALU (Arithmetic Logic Unit)
MAR (Memory Address Register)
MDR (Memory Data Register)
Program Counter
Accumulator
ALU (Arithmetic Logic Unit)
CU (Control Unit)
Cache
Fetch/Execute
Busses

7. Computer Systems Internal Components The processor
Main memory (RAM, ROM, EEPROM)
I/O controllers
Buses
External components (peripherals)
Keyboard, mouse, printer, disk drives
I/O Devices / I/O Ports
Secondary Storage

8. The Processor The “brains” of the computer
Carries out instructions (Executes Instructions)
Processes Data
Retrieves Data/Instructions from Main Memory (Fetch)
Stores Data into Main Memory once Executed

9. Main Memory 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

10. Main Memory 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

11. Main Memory
EEPROM – Electronically Erasable Programmable Read Only Memory
E.g. Sim cards / Flash Memory
Instructions/Data can be erased electronically and replaced

12. I/O Controllers
These are used to allow an interface between a hardware device external to the motherboard (e.g. a Keyboard) and the Processor itself.
I/O controllers are used for:
Keyboard, Mouse, Disk Drive, VDU

13. System Busses
A bus is a set of parallel wires connecting two or more independent components of a computer system in order to pass signals between them.
The System is split into three separate busses:
The data bus
The address bus
The control bus

14. Address Bus
Carries addresses from the Processor to main memory or other I/O devices
It is one direction (Uni-Directional)
The processor generates an address
All data/Instructions are returned on the Data Bus

15. Data Bus
Carries Data/Instructions from Main Memory to the Processor (or from other secondary storage devices) to the processor.
Bi-Directional (two way)
Data can be read/written

16. Control Bus Control signals are sent along the control bus
E.g. Memory Read, Memory Write
This instructs which was data will be travelling to/from memory.

17. Peripherals
Peripherals are any devices that are not directly connected to the CPU
E.g. mouse, keyboard, printer, hard disk drive, cd-rom drive
These devices are known as I/O devices (Input/Output Devices)

18. I/O Ports
I/O Ports allow communication from an I/O device and the motherboard (hence CPU)

19. Secondary Storage Used for long term storage of data and instructions
Hard Disk Drive
Solid State Drive
Flash Memory
DVD-R
Blu-ray

20. A Computer System
Processor
Main Memory
I/O
1945: John Von Neumann

21. Processor Control bus (read/write Signals) Processor Main Memory
Address bus
System Clock Hz
Data bus

22. What Happens in the Fetch – Decode – Execute cycle?
Fetch part of the cycle
Program counter is incremented for each instruction of the program being executed
The contents of the Program Counter are put into the MAR (Memory Address Register)
The address is transferred along the Address Bus to Main Memory (this address indicates which part of memory to fetch the data/instructions from

23. What Happens in the Fetch – Decode – Execute cycle?
Fetch part of the cycle
The data/instruction that has been addressed is transferred back to the processor along the data bus
This is held in the Memory Data Register
The instruction would then be transferred to the Current Instruction Register.

24. Processor - Registers PC – Program Counter
Holds the location of the next instruction/data address in Main Memory
MAR – Memory Address Register
The contents of the PC are copied here and then transferred along the Address Bus
Main Memory PC
MAR
Address Bus

25. Processor - Registers MDR – Memory Data Register
Once Data/Instructions are brought from the Memory Address in Main Memory – they are placed in the MDR.
CIR – Current Instruction Register
The instructions stored in the MDR are copied here
Main Memory
CIR
MDR
Data Bus

26. What Happens in the Fetch – Decode – Execute cycle?
Decode / Execute Part
The instruction to be decoded is held in the Current Instruction Register
The instruction is split into an Op-Code and an Operand
The instruction is carried out by the ALU (Arithmetic Logic Unit)

27. Executing Instructions
ALU (Arithmetic Logic Unit)
Performs arithmetic and logical operations including +, -, AND, OR
Accumulator
Results of calculations are placed into the Accumulator
Accumulator
ALU

28. Activity 2
Produce a Labelled Diagram of the Internal Components of a Computer System
See Lesson 2 Activity 2
In Pairs Describe in words how the Fetch-Execute Cycle Works

29. Activity 3 – FDE Cycle

30. Control Unit
Sends Control Signals between the different internal components
Memory Read
Memory Write
Hard Disk Drive Read
I/O Write

31. Plenary Questioning Produce Question + Mark Scheme
Exit Pass – Describe the differences between the registers on the processor
Exit Pass – Describe the differences between the ALU and the CU

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