1. CPU Lesson 2

2. Von Neumann Architecture
Control unit coordinates all of the activities taking place inside of the CPU. Summaries as follows:
It controls the execution of instructions in the correct sequence
It decodes instructions
It regulates and controls processor timing using regular pulses from the system clock
It sends and receives control signals to and from other devices within the computer
Arithmetic and ALU carry out the following instructions:
Logical Operations: AND,OR, NOT
Shift Operations: Bits in the computer world can be shifted left and right by a certain number of places
Arithmetic Operations: Addition, subtraction, multiplication and division

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

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
Program Counter
Main Address Register – hold address (location in memory) of the instruction or piece of data to be fetched or stored
Main Data Register - contains the data to be stored in the computer storage (e.g. RAM), or the data after a fetch from the computer storage
Current Instruction Register – holds the current instruction to be executed, having been fetched from memory.
Arithmetic Logic Unit – carries out calculations on the data. +, *, -, /
Accumulator - accumulator is a register in which intermediate arithmetic and logic results are stored.
Registers are very fast memory locations within the CPU used in the execution of instructions
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
Registers - A special, high-speed storage area within the CPU. All data must be represented in a register before it can be processed. For example, if two numbers are to be multiplied, both numbers must be in registers, and the result is also placed in a register. (The register can contain the address of a memory location where data is stored rather than the actual data itself.)

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
Electrically Erasable Programmable Read-Only Memory

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. 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
A system bus is a single computer bus that connects the major components of a computer system, combining the functions of a data bus to carry information, an address bus to determine where it should be sent, and a control bus to determine its operation.

12. 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
An address bus is a computer bus architecture used to transfer data between devices that are identified by the hardware address of the physical memory (the physical address), which is stored in the form of binary numbers to enable the data bus to access memory storage.

13. 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
A data bus is a system within a computer or device, consisting of a connector or set of wires, that provides transportation for data.

14. 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.

15. 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)

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

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

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

19. CPU Fetch-execute cycle
The basic operation of a computer is called the ‘fetch-execute’ cycle. The computer fetches the instruction from its memory and then executes it. This is done repeatedly from when the computer is booted up to when it is shut down

20. 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
A program counter is a register in a computer processor that contains the address (location) of the instruction being executed at the current time. As each instruction gets fetched, the program counter increases its stored value by 1. After each instruction is fetched, the program counter points to the next instruction in the sequence. When the computer restarts or is reset, the program counter normally reverts to 0.

21. 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.
Current Instruction Register - This holds the current instruction to be executed, having been fetched from memory.

22. 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

23. 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
Current Instruction Register - This holds the current instruction to be executed, having been fetched from memory.
CIR
MDR
Data Bus

24. 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)
Op-Code short for Operational code

25. 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

26. 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

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

28. 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