1. Computer Systems Nat 4/5 Computing Science Computer Structure:
Lesson 5:
Computer Architecture

2. Lesson Aims By the end of this lesson: Pupils at will be able to:
Label a computer system in terms of a 5 block diagram
Describe the phrase ‘word length’
Describe the purpose of a processor
List and describe the parts of a processor:
Control Unit, ALU, registers
Describe what a bus is and does

3. Computer architecture
Computer architecture = the structure of a computer system
Hardware = Physical parts of a computer system
Device = One item of hardware
Computer system has:-
a processor
Input device(s)
Output device(s)

4. Computer architecture
Nat 4/5
Computer architecture
Storage Devices
Output
Devices
Input Devices
Processor
Memory
(RAM & ROM)

5. Typical desktop system
Nat 4/5
Typical desktop system
Processor - Carries out instructions
Input Devices - Used to enter data e.g. keyboard and mouse
Output Devices - Generate output e.g. monitor, printer.
Memory - Stores programs and data - RAM and ROM
Storage devices
Store permanent copies of files.e.g. hard disk drive, DVD-Rewriter

6. Nat 4/5
The Processor
Program / software = a set of instructions that tell the computer what to do
The processor is the ‘brain’ of the computer – but can’t think just carries out instructions
1 chip
Small piece of silicon

7. Stored program Instructions are stored in memory
Processor fetches one instruction at a time from memory and carries it out
Different programs allow the processor to carry out different tasks

8. Arithmetic and Logic Unit (ALU)
Parts of a processor
Processor
Control Unit
Arithmetic and Logic Unit (ALU)
Registers

9. Control Unit Controls other parts of the processor
Nat 5
Control Unit
Controls other parts of the processor
Makes sure that instructions are carried out in the correct order

10. Arithmetic and Logic Unit (ALU)
Carries out calculations
Makes decisions (logical operations)
e.g. deciding one a number is the same or less than another number is a logical operation

11. Registers Storage locations inside the processor Hold:-
data being processed
instructions being executed
memory addresses being accessed
Small amount of storage space compared to RAM – does not hold a whole program - just the instructions being executed

12. Memory Memory contains many storage locations
Each storage location has a unique address
2 types of memory chips as in a computer:-
RAM and ROM

13. RAM – Random Access Memory
Stores instructions and data that the computer is currently using
Holds data while the computer is switched on - contents are lost when the computer is switched off
User’s data stored in RAM

14. Read Only Memory
Stores programs and data permanently - contents are not lost when the computer is switched off
Programs are stored in ROM chips when the computer is manufactured
Stores part of the Operating System

15. Buses Bus = a set of wires that connects parts of the computer
e.g. memory and input/output devices
Every computer has these buses:-
Address bus
Data bus
Control bus

16. Buses Control Unit RAM Registers
Nat 5
Buses
Control Bus
Processor
Main Memory
Control Unit
RAM
Data Bus
ALU
ROM
Registers
Address Bus
A bus is a collection of wires which connect parts of the computer

17. Nat 5
Data Bus
Carries data between the processor, memory and other parts of the computer e.g. input and output devices
Bi-directional – data can travel in either direction on the data bus
64 wires in the data bus
16 wires in the data bus

18. Address Bus Address of a storage location = a binary number
Address bus carries this information from the processor to memory to identify the location being accessed
Uni-directional – data only travels from the processor to memory

19. Control Bus Consists of separate wires – each with it’s own function
Clock line - regularly pulses with electricity to control the speed of the processor e.g. 3.6 GHz
Read line – used when the processor is about to read from memory
Write line – used when the processor is about to write data to memory

20. Summary The Processor has 3 main parts:-
Nat 5
Summary
The Processor has 3 main parts:-
Control Unit
Controls the timing and order of execution of instructions
Arithmetic and Logic Unit (ALU)
Performs the arithmetical and logical functions
Registers
Store information
Buses connect parts of the computer:-
Address bus – carries address of location being accessed
Data bus – carries data
Control bus – separate wires with their own job to ensure things happen at the correct time

21. Questions What is computer architecture? What is hardware?
What is a device?
What is software another name for?
Name five parts of a computer system.
Which part of the processor:-
carries out sums and makes decisions?
controls when instructions are fetched and executed
stores data, addresses or instructions

22. More questions Name 2 types of memory chip.
What are computer buses (include what a bus does).
Name 3 buses.