UNIT 3: HARDWARE

COMPUTER SCIENCE O-LEVEL

[DATE]
[Company name]
[Company address]
 The CPU & Microprocessor
What is the role of the central processing unit (CPU)
in a computer?
 A computer system consists of hardware and software
 The main hardware components of a computer system are input devices,
the central processing unit, primary memory, secondary
storage and output devices
 Examples of input devices - keyboard, mouse, game controller, sensors,
microphone and webcam
 Examples of output devices - monitor, phone screen, speakers, printer, and
motors
 Data and commands are inputted by the user using an input device, the
central processing unit (CPU) processes data by executing
instructions and the results are outputted to an output device

A diagram showing the input, process, output sequence followed by

computer systems

Step Example

Input  A keyboard is used to input a number

Process  If the instruction being executed is ADD, the

inputted value is added to an existing value

Output  The result of the calculation is outputted to the user

via the monitor
What is a microprocessor?
 A microprocessor is a type of integrated circuit on a single chip
 An integrated circuit is a small electronic circuit made up of transistors,
capacitors, resistors and other electronic components
 The integrated circuit contains a central processor designed to
perform arithmetic and logic operations, which include adding, subtracting,
transferring numbers from one memory location to another, and comparing
two numbers
 The single chip also contains input/output interfaces, and memory
 Microprocessors are a compact way of processing data and can be used in a
wide range of electronic devices, including general-purpose computer
system and Embedded system

Von Neumann Architecture

The components in a CPU, in a computer that has a Von Neumann
architecture

 John Von Neumann developed the concept of the stored program

computer in the 1940s

 The Von Neumann computer architecture which most modern day computers
use, is based upon this concept
o The key feature of the stored program concept, and Von Neumann
architecture, is data and instructions are stored in the same
memory ( RAM ) as binary
o Another feature of Von Neumann architecture is a central processing
unit (CPU) fetches instructions from memory
and executes them one at a time (serially)
o The CPU then stores the results back into memory
Components of the Central Processing Unit

A diagram showing the main components of the CPU

 The main purpose of the CPU is to execute instructions and process data
 The CPU has two main components - the Control Unit (CU) and
the Arithmetic Logic Unit ALU)
o The Control Unit controls the flow of data around the CPU
o The Control Unit also sends control signals to the different
components instructing them what to do e.g. read, write, add, subtract
o The Control Unit decodes instructions (into
an opcode and operand )
o The Control Unit controls the timings of operations (the clock speed)
o The Arithmetic Logic Unit(ALU) performs the calculations required to
execute the instructions, these include ADD and SUBTRACT
o The ALU also carries out logical operations such as COMPARE
o The ALU has a built-in register where it stores interim results of
calculations
o After calculations, the ALU sends data to the MDR
 The CPU also contains a number of registers which are small memory
locations within the CPU, which temporarily store data needed to execute
an instruction
 Special purpose registers have specific roles to play in the execution of an
instruction
Special purpose register Definition

Program Counter (PC)  stores the address of the next instruction to be

fetched from memory

Memory  stores the address of the instruction or data to be

Address Registers (MAR) fetched from or written to memory

Memory Data Register  stores the data that has been fetched from memory
(MDR) or being written to memory
 Data from MDR is sent to ALU to be executed

Current Instruction  stores the instruction the CPU is currently

Register (CIR) decoding or executing

Accumulator  Temporarily stores the results of the calculations

performed by the arithmetic and logic unit

Buses
 Components within the CPU and wider computer system are connected by
buses. These are wires down which electronic signals and data travel. The
different buses are collectively called the system bus
 The system bus is made up of three different buses - the data
bus, the control bus and the address bus
o The data bus transmits data from the CPU to memory or input/output
controllers. It is bidirectional which means data can travel in both
directions
o The address bus transfers addresses from the CPU to memory. It
is unidirectional which mean addresses only go from the CPU to
memory
o The control bus transfers control signals from the control unit to
other components in the computer system such as memory or
input/output controllers. The control bus is bidirectional
A diagram showing how the different buses connect the components in a
computer system

The Fetch, Decode, Execute cycle

 The Central Processing Unit (CPU) executes instructions by performing the
Fetch Decode Execute cycle
o The CPU fetches an instruction from memory
o The instruction is then decoded ( by the Control Unit) into
an opcode and an operand
o The instruction is executed and the whole cycle is repeated with the
next instruction in the process
 A diagram depicting the Fetch Decode Execute cycle

Step Detailed Explanation

Fetch  The memory address of the instruction to be fetched is stored in

the Memory Address Register and is sent down the address bus.
 The data/instruction at the memory address is transferred back to
the CPU, via the data bus, where it is stored in the Memory Data
Register
 The instruction is copied into the Current Instruction
Register and the Program Counter increments

 The instruction in the Current Instruction Register is decoded, by

Decode the Control unit , into an opcode and an operand

Execute

 The instruction is executed by the Arithmetic Logic Unit and the

opcode is performed upon the operand.
 The result is stored in the accumulator or written to a memory
location within memory

CPU Performance
 Central Processing Units can have multiple cores
o A dual-core processor has two cores
o A quad-core processor has four cores
o Each core runs separate fetch, decode,
execute cycles, independently from one another and at the same
time (simultaneously) meaning parallel processing can take place
o Multiple cores enables multitasking (running more than one program
at the same time)
o Some programs cannot be split between cores
o The more cores a computer has the more instructions that can be
executed per second resulting in better performance
 Each core has a clock speed
o The clock speed is how many instructions the core can execute each
second
o The clock speed is measured in Hertz
o Modern cores can execute billions of instructions per second
o A gigahertz (GHz) is a billion instructions per second
o A megahertz (MHz) is a million instructions per second
o A CPU core with a clock speed of 3.4GHz can execute 3.4 billion
instructions per second
 Cache is a small amount of memory situated within or close to the CPU with
very fast read/write speeds
 o It is used for storing frequently used instructions/data, recently used
instructions, and instructions that are to be fetched and executed next
in a process.
o The impact of increasing the amount of cache is that more data can be
stored there and accessed faster than if it was in RAM …which
improves the performance of the CPU.
 Double the number of cores does not necessarily mean double the number of
instructions executed a second. The cores might have different clock speeds
and cache sizes

Instruction Sets
 An instruction set is a list of all the commands that can be processed by a
CPU
 Each command has a unique binary code
 The table below shows an example instruction set. Each instruction has
a mnemonic that indicates what the instruction does alongside a
corresponding binary code
 After an instruction is decoded into an opcode and an operand, the CPU finds
the opcode in the processor’s instruction set. It then knows what operation to
perform when executing the instruction
 Instruction lists are machine-specific
 This means a program created using one computer’s instruction set would not
run on a computer containing a processor made by a different manufacturer
 For example, a computer program created using Intel’s instruction set would
not run on a device containing an ARM processor

Embedded Systems
 An embedded system is a computer systems with a either one function or
limited specific functions built within a larger mechanical device
 Its purpose is to control the device and allow a user to interact with it
 It runs on firmware and does not have additional peripherals
 An embedded system is different to a general purpose computer system like a
laptop or desktop computer which can be used to perform many different
tasks
 The vast majority of microprocessors manufactured are for use as embedded
systems
 Some embedded systems are microcontrollers meaning they are part of an
integrated circuit with built in memory
 Type of embedded system Description

Integrated circuit containing a CPU and memory (RAM or

Microcontrollers
ROM) built in to the same chip
Integrated circuit containing only a CPU on the chip
Microprocessor
RAM , ROM , peripherals need to be added

 The microprocessor used in an embedded system is often custom designed

 Embedded systems often have firmware which is software built into the
system which cannot be reprogrammed by the user. The software may be
able to be updated e.g. GPS software in a car navigation system
 Embedded systems usually have some form of analog or digital input

Diagram of an embedded system

Embedded system Possible input devices Possible Output

Screen showing the time

Buttons to set the
Digital clock
time/mode/alarm
Alarm
Pedestrian button

Traffic Lights Timer Lights

Movement sensor
Lighting system Movement sensor Lights
Security system Keypad to enter alarm code Alarm

Camera
 Movement sensor
Actuator controlling movement
Vending machine Keypad to make choice
of choice
Keypad to set temperature
Central Heating
Heat
system
Temperature sensor

 There are many advantages of embedded systems

o low power consumption
o small physical size
o low cost to manufacturer
o they can be controlled remotely
o can operate in real time and respond to inputs very quickly

Input Devices
 Every computer system receives commands and data, from the real world,
via input devices
 Input devices converts inputs into digital data which can be processed
 For conventional computer systems, such as desktop computers and laptops,
the most common input devices are the mouse and the keyboard
 There are a wide range of input devices used in more specialist computer
systems such as scanners and sensors

Barcode Scanner
Diagram of a barcode being scanned by a handheld barcode scanner

 Barcodes are a series of black and white bars which represent a code
 A barcode reader shines a red laser at the barcode to read the data it
represents
 The light from the white lines is reflected back
 Black lines absorb the light so less is reflected
 The different levels of reflection are converted into a binary value which can
be processed by a microprocessor
 A barcode reader can be handheld or built into a larger machine such as a
self-service checkout at a supermarket
 Barcodes can be used in many ways such as identifying a product being
sold or tracking a package through the delivery process
 Benefits include faster checkouts, automated stock control, less chance
of error due to manual entry of data, and more detailed tracking
information

Digital Camera
 A digital camera works by capturing light and converting it into a digital
image
 Light enters the camera through the lens, it reaches an image sensor where it
is split into millions of pixels (small squares). Each pixel measures light
intensity which is converted into binary and represents a colour.
 Digital cameras are integrated into smartphones , used in security
systems and by professional photographers to create high quality digital
images
 An advantage of digital cameras is they show a preview of the image
 They also instantly create an image which can then be
easily duplicated and transmitted via bluetooth or WiFi
 Software can be used to edit digital photos, for example applying a filter or
retouching a photo

Keyboard
 A keyboard is the most common device used for text-based data input
 They are connected either by a USB cable or wirelessly to the computer
system
 They are built into laptops.
 Smartphones and tablets have virtual keyboards
 Each key on a keyboard has a peg underneath it which makes contact with a
conducting membrane. This is then converted into an electrical signal to
transmit a unique character code

Microphone
  A computer microphone works by converting sound waves into electrical
signals that can be processed by the computer.
 They can capture any real world sound and convert it into digital
data which can be stored, duplicated or modified
 The microphone has a diaphragm that vibrates in response to sound waves.
These vibrations are then converted into electrical signals by a coil of wire
attached to the back of the diaphragm. Changes in the signal are recorded by
a microprocessor using a analogue to digital converter
 Microphones are used to record music, telephone calls, communicate
online and dictation

Optical Mouse
 A mouse use a laser to detect and track movement, this is then processed
by a microprocessor which interprets the movement and replicates it
when moving a virtual cursor on-screen
 Items can be selected or moved using the left mouse button, whilst the right
button usually displays additional menus
 They can be wired or wireless
 They are used to control the cursor in a Graphical User Interface (GUIs)
 They are simple to use and provide the user with an intuitive way to navigate
the computer
 They are reliable due to no moving parts

QR code scanner
 https://pixabay.com/vectors/qr-code-scanner-bar-code-156717/

Diagram of a QR code being scanned by a mobile device

 A QR code is a visual representation of data using black and white

squares
 QR codes can represent over 7000 digits whereas a barcode represents up
to 30 digits
 QR codes are scanned by a camera (often on a mobile device)
 A piece of software convert the squares into binary data
 QR codes often link to a website where more information can be found
 They can also be used to advertise products, share contact details,
provide promotional codes, train tickets, and event tickets

Touchscreen
 A touchscreen can be classed as both an input device and an output device

Types of touch screen

Type How it works Benefits/Drawbacks Uses

Resistive screen Resistive screens consist of + Cheap to produce.
two conductive layers. The
top layer is flexible. When + Resistant to surface
Used in cash machines,
the screen is touched the two contaminants
information kiosks,
layer connect, completing a
medical equipment
circuit + Can be activated with
nearly every object (stylus,
finger, gloved hand)
Infra-red LEDs shine infrared light + Excellent image quality,
screens across a screen forming a
matrix. When the screen is + high precision
touched the beams are
interrupted + durable
Tablets, laptops,
smartphones
+ allows for multiple
touches at the same time

- Requires a bare finger or

stylus for activation
Capacitive Made up of a protective +Excellent image quality
screens layer, a transparent
Large scale commercial
conductive layer and a glass +Unlimited touch-life
displays
substrate. Touching the
screen changes the +Scale well
Information kiosks
electrostatic field of the
conductive layer - Sensitive
Medical equipment
to interference from light,
water, snow
2D and 3D scanners
 A two-dimensional (or flat-bed) scanner shines a strip of light over a
document. Reflected light is measured for each pixel. This data is converted
into binary data so the document can be digitally recreated
 A three- dimensional laser scanner shines a light over an object.
The geometry and dimensions are recorded so the object can be
recreated digitally
 The digital 3D model can be modified using specialist software
 A 3D printer could then print out the model
 2D scanners can be used for creating digital versions of documents or
photographs
 Reading passports at airports
 3D scanners can be used to create 3D models for use with computer-aided
design (CAD) software
 Creating replicas with 3D printers
 Typical uses of 3D scanners and printers are dentistry, product development,
medical
 A cheap and quick way of creating a digital representation of both 2D and
3D objects so they can be manipulated, stored, transmitted or copied

Summary of input device uses and benefits

Device Example Uses Benefits

Barcode scanner
Faster checkout, less errors, detailed
Checkouts, parcel delivery
stock/tracking information

QR code scanner
Can hold more information than a
barcode
Advertising products, linking to
websites, e-tickets
Can be scanned using a camera on a
mobile device (smartphone)
2D and 3D scanners Converts 2D and 3D objects into
digital data which is easier and
Scan documents, photos and objects
quicker to transmit/duplicate or
manipulate
Digital camera
Smartphones, professional
Instant preview and image
photography, security systems
Keyboard Entering text Universal, easy to use
Mouse Navigating a GUI
Intuitive
Making selections and entering
commands
Microphone Recording real-world sound

Musical recordings

Telephone calls

Dictation
Touchscreen Entering text and commands on
mobile devices (tablet, smartphone)
Can be durable
Cash machines
Good image quality
Interactive advertisements
Intuitive/easy to use
Information screens
 Output Devices
 An output device shows the results of the processing in a way humans can
understand
 In a general purpose computer system, the main output device is
the monitor/screen
 Other output devices include projectors, printers, speakers and actuators

Actuators
 Actuators are used in conjunction with a motor to translate energy (electrical,
air, hydraulic) into real-world movement of a physical object.
 They come in a variety of sizes and have been made specifically for a
particular function.
 Examples of use include turning a wheel, opening or closing a door,
controlling a conveyer belt, operating machinery, moving robotic arms,
vibrating a machine, starting or stopping a pump, opening or closing a
valve
 They are often used with sensors. The input of the sensor is checked
against stored values. If the input is within a certain range an actuator is
used to provide movement of a physical object

Light projectors
 Used to project computer outputs onto a large screen
 Used to give presentations in both business and education settings
 There are two types - Digital light processing (DLP) projectors and Liquid crystal
display (LCD) projectors.
 DLP systems use millions of micro-mirrors arranged in a grid on a microprocessor
within the projector. Light is shone through colour filters and the mirrors. The
position of the mirrors can be altered to change the intensity of the light
 LCD projectors use three mirror filters to separate an image into red, green and blue
wavelengths. The three images are then combined to produce the full colour image
which is passed through the lens on to the wall/screen

Device Advantages Disadvantages

Digital light
Higher contrast ratios. Image tends to suffer from
projector (DLP)
“shadows” when showing a moving
Smooth video image
 DLP do not have grey components
Higher reliability
in the image

The colour definition is not as good

Smaller and lighter
as LCD projectors

Better suited to dusty atmospheres than

LCD projectors
LCD projector
Contrast ratios are not as good as
Give a sharper image than DLP projectors
DLPs
Better colour saturation and intensity than
Have a limited life
DLP projectors
Use less power and generate less heat LCD panels degrade over time.
Quieter running that DLP
Laser and Inkjet printers
 Both printers create hard copies of a digital document
 Hard copies are needed when you do not have access to an electronic device

Inkjet printers:

 Have a print head which moves across the page

 Spray liquid ink droplets from ink cartridges through very fine nozzles onto the
paper (these are in the print head)
 The droplets can be piezoelectric or thermal bubble technology
 Inkjet printers produce high quality hard copies of digital images or documents

Laser printers:

 Are very fast when making multiple copies of a document

 Are useful for high volume print jobs for example producing leaflets
 Have a low running cost per page.
 Are often used in business and education setting
 Have large toner (dry ink) cartridges and large paper trays
 Use positive and negative charges to make the toner particles stick to piece of paper,
heat is then used to bond the particles to the paper

3D printers
 3D printers create 3D models of a digital model by depositing layers of material,
such as plastic or resin, on top of one another to slowly build up a 3D object
 They allow for precision and can be used in medicine to create prosthetics and blood
vessels
 3D printers can be used to create rapid prototypes
 Models can be transmitted digitally and then models printed out all across the world
Liquid Crystal Display screens
 LCD screens are made up of millions of tiny liquid crystals.
 The display is made of pixels arranged in a matrix
 The display requires backlighting
 They are used for TVs, monitors, tablets and phones
 They have low power consumption and run at a cool temperature
 They do not suffer image burn or flicker issues
 They provide bright images and colours
 They are cheaper to produce than LED screens

Light emitting diode (LED) screen

 An LED screen is made up of tiny LEDs
 They do not need backlighting which makes them thinner and lighter to
hang on the wall
 LED screens are replacing LCD displays in TVs, monitors, laptops and mobile
devices
 They have better image quality and a longer life span
 They can be used to create very large screens that can be used to show
visual content at events such as football matches and music festivals
 They consume very little power making them suitable for being switched on
for many hours in the day

Organic light emitting diode (OLED) screen

 Much thinner and lighter than traditional LCD screens
 Use organic light emitting diodes (OLEDS).
 They use organic carbon compound to create semiconductors
 No form of back lighting is required
 You can have very thin, flexible screens

Speakers
 Speakers are used to take digital sounds or recordings and output them as
sound waves which can be heard by humans
 The digital data is changed into a electric current using a digital to analogue
converter (DAC)
 It is then passed through an amplifier to create a current large enough to
drive a loudspeaker
 The loudspeaker converts the current into a sound wave
 Typical uses include listening to music, listening to video sound, telephone
calls and alarms

Sensors
 Sensors are input devices
 They measure a physical property of their environment such as light levels,
temperature or movement
 Sensors can be used for monitoring and controlling systems.
o A monitoring system tracks the state of a system, it gathers data and may
issue warning messages
o A control system will control the system based upon the input from sensors
o For example, if the water temperature in a fish tank fell below the acceptable
level, a control system would start up a heater. The system will then
continue to check the water temperature readings and when they are within the
acceptable range it will switch off the heater
 This process of the outputs affecting the following set of input values
received from the sensors is called a feedback loop

Sensor type
What it measures Typical use

Acoustic Sound levels To detect changes in sound

levels of industrial machinery

To monitor noise pollution

In security system to detect

suspicious sounds
Accelerometer Acceleration rate, tilt, vibration Detecting sudden changes in
vehicle movement and deploy
safety features if needed

In mobile phones to detect

orientation of the device
Flow Rate of gas, liquid or powder Detect changes in the flow
flow through pipes in water system
Gas Presence of a gas e.g. carbon Detect levels of gas in
monoxide confined spaces

Detect gas levels when fixing

gas leaks
Humidity Levels of water vapour Monitor humidity in
greenhouses
Infra-red Detecting motion or a heat Security systems detecting
source intruders who break the beam

Measures heat radiation of

objects - used by emergency
services to detect people
Level Liquid levels Detects levels of petrol in a
 car tank

Detect levels of water in a

water tank

Detect a drop in water levels

due to a leak
Light Light levels Automatically switching on
lights when it gets dark (street
lights, headlights)
Magnetic field Presence and strength Anti-lock braking system

Monitoring rotating machinery

such as turbines
Moisture Presence and levels of Monitoring moisture in soil
moisture
Monitoring damp in buildings
pH Acidity or alkaline Monitoring soil to ensure
optimum growing conditions

Monitor pHlevels in chemical

processes
Pressure Gas, liquid or physical Monitoring tyre pressure
pressure
Monitoring pressure in pipes
during manufacturing process
Proximity Distance Monitoring position of objects
in robotics

Used in safety systems to

prevent objects colliding
Temperature Temperature in Celsius, Used to maintain temperature
Fahrenheit or Kelvin in swimming pools

Used to control temperature in

chemical processes

Primary Storage
 Primary storage is directly accessible by the CPU , it includes RAM, Cache,
and ROM
 The purpose of Random Access Memory (RAM) is to store data,
instructions and software (including parts of the OS) currently in use
o RAM is also called main memory
o RAM has faster read/write speeds than secondary storage
 o RAM is volatile which means it is temporary memory and all data will
be lost when the computer is switched off
o The larger the main memory, the more data the computer access
quickly, which can improve the computer's performance
o A standard desktop computer has between 4GB and 16GB of RAM
 The parts of the operating system currently in use will be stored in RAM
 The purpose of cache is to store frequently/recently used instructions and
data
o Cache has faster read/write speeds than RAM
o There are different levels of cache
 Level 1 cache is situated within the CPU
 Level 2 and 3 cache are situated on the motherboard
o If you increase the amount of cache you can store more frequently
used data and increase the performance of your computer
 If you have too much cache it will decrease the data access
speeds because the data requested will take longer to find
o Cache is volatile and all data will be lost when the computer
is switched off
 A diagram showing the parts of a computer system which are classed as
primary storage

 The role of Read-only memory (ROM) is to store the computer’s boot-up

sequence
o The boot-up sequence is a set of instructions the computer executes
when then computer is initially switched on
o This sequence will run a series of checks to ensure all of the
system hardware is working
o It will then load the operating system which will move from secondary
storage to RAM
 ROM is non-volatile which means data will be stored when the computer is
switched off
 ROM is read-only which means data cannot be written to ROM

The difference between ROM and RAM

RAM ROM

Volatile Non-volatile
Can read and write data to RAM Read-only

Secondary Storage
 Secondary storage is not directly accessed by the CPU
 It is used for permanent storage of data
 Secondary storage is non-volatile. This means data is stored even when
power is lost to the device
 Secondary storage is needed as primary storage (such as RAM ) is volatile
and will not retain data when the computer is turned off
 Unlike ROM, secondary storage can be read from and written to. ROM is
read only
 Common examples of secondary storage include hard disk
drives (HDDs), solid-state drives (SSDs), external hard drives, USB flash
drives, CDs, DVDs, and Blu-ray discs. These can be used to store data to
transfer it to another computer
 Data access speeds for secondary storage devices are typically slower than
primary storage (such as RAM)
 Secondary storage devices often have large capacity (can store large
amounts of data)
 Software such as the operating system is saved permanently on secondary
storage. The instructions and data that makes up the software then moves to
RAM when it is in use
 Secondary storage devices can be internal (inside the computer) and external
(outside the computer)
 Diagram showing different internal and external secondary storage devices

Comparison of primary and secondary storage

Primary storage Secondary storage

Volatile / temporary Non-volatile /permanent

RAM and cache can be read from and written to

Secondary storage can be read from and written to
ROM is read-only

Can be directly accessed by the CPU Cannot be directly accessed by the CPU

Smaller capacity Larger capacity

Data access speeds faster than secondary storage Data access speeds slower than primary storage

Stores data and instructions Stores data and instructions

Types of secondary storage

  There are three types of secondary storage - magnetic, solid
state and optical

Magnetic
 Magnetic hard disk drives have been the main type of internal secondary
storage in personal computers for many years however solid state drives are
increasingly popular due to their durability, low power consumption and high
read/write speeds
 A magnetic hard disk is made up of several metal discs coated with a
magnetic material
o These are called platters
o Iron particles on each platter are magnetised to represent a 0 or 1

A diagram showing the structure of a magnetic hard disk

 Each platter is divided by concentric circles creating several tracks and

wedge shaped sectors. Where they intersect is a track sector
 A diagram showing how each platter is divided by tracks and sectors

 The hard drive spins the metal disk(s) at a high speed (typically around
5400-7200 RPM) using a motor
 A read/write arm, controlled by an actuator, moves the head over the
surface of the disc to the location of the data
 The data is read/written using electromagnets

Solid State
 Solid State secondary storage is made of transistors that are arranged in a
grid layout
 It uses NAND and NOR gates in electrical circuits to persistently control the
flow of electrons
 Current flowing is 1, no current is 0 - which is how electrical flow can
represent binary values
 A NAND gate will take in two inputs and produce an output (1/electrical
current) as long as both inputs are not 1

The truth table truth table for a NAND gate

InputA InputB Output

0 0 1

0 1 1
 1 0 1
1 1 0

 A NOR gate will produce an output (1) only when both inputs are 0

The truth table for a NOR gate

InputA InputB Output

0 0 1
0 1 0
1 0 0
1 1 0
Optical
 Optical devices include CDs, DVDs and Blu-rays
o Blu-rays have the largest capacity
o CDs have the lowest capacity
o CD-R are read-only (you cannot save data on to them)
o CD-RW can be written to and read from
o DVD-RW can be written to and read from
 All optical devices work by shining a laser at the disk and processing the
reflection
 An arm moves the laser across the surface of the disk
 In CD-Rs a laser burns the data, permanently on to the disk, by
creating pits and lands
 The laser is also used to read the data from the pits and lands
 When the laser light hits the point where the pit changes into a land or vice
versa the light scatters and is not reflected back as well. This is captured by a
sensor and can be interpreted as a change in the binary value

A diagram showing how data is burnt on to optical discs to create lands and
pits which can then be used
 In CD-RW the chemical composition of the disk is changed to represent 0s
and 1s. This change can be overwritten

Virtual Memory
 The purpose of RAM is to store current data and program instructions in use
 The amount of physical RAM available is limited. The average modern
desktop computer has between 4GB and 16GB of RAM. Some programs
are larger than this
 Virtual memory is part of secondary storage which acts as RAM when RAM
is full
 It prevents the computer system crashing when RAM is full
 Data that needs to be accessed by the CPU directly is switched by the
operating system from virtual memory into RAM and an inactive page from
RAM replaces it in virtual memory. This is called page switching

A diagram showing page switching between RAM and virtual memory

 The table describes the steps that are taken when a program is opened on a
computer

Step Description

1  The memory manager software built into the Operating

System checks to see if there is sufficient space in RAM to
move the new program’s instructions into RAM, so that they
 can be directly accessed by the CPU

2  If RAM is full, the memory manager will move out the most
inactive pages of data and put them in virtual memory until
needed

3  The new program instructions are moved into RAM

Cloud Storage
 Cloud storage is the concept of storing data on remote servers, that can be accessed
via the internet, instead of on a local storage device (such as an SSD or HDD)
 The physical servers, where the data is stored, are owned and managed by a hosting
company

o Cloud storage warehouses often have thousands of servers containing

harddrives
o The hosting company provides services such backing up the data, managing
access to the data as well as general security
 The data can be accessed at any time, on any device, as long as there is an internet
connection
 The data is accessible using a web browser or dedicated software that connects to the
cloud storage service
 There is potentially limitless storage, although it does need to be paid for
 Online storage means it is easy to share data and collaborate with others
 If the internet connection is slow/lost the access to your data is affected
 A diagram showing different devices connecting to remote servers

Advantages of cloud storage

 Cloud storage offers several advantages over storing data on a local secondary storage
device

Benefit Explanation

Accessibility
 Data can be accessed from anywhere
 Data can be accessed by anyone with the relevant
permissions, making it quick to share files and
collaborate with others
 Data can be accessed on any device with an internet
connection

Scalability  Cloud storage providers allow customers to increase or

decrease their storage capacity as needed
 Reliability  Cloud storage providers often use multiple servers to
store and backup data, reducing the risk of data loss due
to hardware failure

Security
 Cloud storage providers offer advanced security
features, such as data encryption and multi-factor
authentication multi-factor authentication, to protect
user data from unauthorised access
 There is no need to hire specialist staff as IT services
being provided by the cloud storage provider

 You also do not need to purchase local secondary storage devices

 One cloud storage centre is more environmentally friendly than millions of
individual servers
 There are some drawbacks to using cloud storage
o If the user has a slow or unreliable internet connection they would have
problems accessing their files
o If you are storing a lot of data it can be expensive
 You have to pay the cloud storage provider
 You may need to commit to an annual subscription
 You may have to pay for high download and upload data transfer limits
o Data being sent over the internet has the potential to be intercepted
o Security is managed by the cloud storage provider which means the user does
not have complete control over it but is still legally responsible for ensuring
any personal and sensitive data is secure

Network Interface Card

 A Network Interface Card (NIC) or Network Interface Controller is an internal
physical component which enables computers and other devices
to connect to a network
 They are also known as network adapters
 NICs have a built in ethernet port and can be connected to a network via
an Ethernet cable
 You can have Wireless Network Interface Cards (WNIC) which allow a device
to wirelessly connect to a network
 The primary function of a NIC is to send and receive data packets between
the computer or device and the network
 The NIC converts data to be sent into signals that can be sent across the
transmission medium
o If this is through a wired network, the signals will be voltages through
an ethernet wire or pulses of light through a fibre
o If this is through a wireless network, the signals will be radio waves
  NICs are likely to be integrated into the motherboard but can be added as
an expansion card

MAC Addresses
 A Media Access Control/MAC address is used to identify a device on a
network
 It is a unique code which consists of six pairs of hexadecimal codes
separated by a colon
 Each pair of hex digits is a Byte of data. So in total a MAC address is 6
bytes or 48 bits long
 An example MAC address is b4:71:ac:f3:21:a2
 The MAC address is assigned to the NIC by its manufacturer
 The first three pairs are the manufacturer ID number and the last three
pairs is the serial number of the NIC and identifies the device on the network

An example MAC address - the first part identifying the manufacturer and the
second part identify the device

 A MAC address cannot be changed. It is static

A table listing the differences between an IP address and a MAC address

MAC Address IP Address

Permanently assigned to a Can be used to identify devices on a network
NIC and cannot be changed but are dynamic and can be changed
MAC addresses are written in
IP addresses are written in denary
hexadecimal
MAC addresses are 6 bytes
IP addresses are 4 bytes long
long
MAC addresses are used to MAC addresses are used to identify a device
identify a device on a LAN on a WAN

IP Address
 An IP (Internet Protocol) address is a unique numerical identifier assigned
to devices connected to the internet
o IP addresses are used to identify and locate devices on the internet
o This allows data to be sent and received between devices
 An IPv4 address is four numbers separated by a full stop
o Each number is a byte and range between 0-255
o There are over 4 billion unique addresses using the IPv4 system
o For example, an IPv4 address might look like this: 192.164.3.1

 Routers and gateways have a unique public address

 Public addresses are assigned by your internet service provider and is the
IP address for the network
 Public addresses are often static addresses meaning they do not change

A diagram showing how Private and Public IP addresses are used to identify
devices on the Internet and within a LAN

 A router can assign private IP addresses to devices connected within its

network
 These are often dynamic IP addresses meaning they can change
  Dynamic addresses are assigned from a list of available addresses at the
time they are required

 The IP4 system is running out of possible addresses due to the enormous
increase in networked devices
 A new system called IPv6 has been developed. It provides enough unique
addresses for every networked device on the planet
o IPv6 uses a 128 bit address
o Digits are organised into eight groups
 Each group is made up of four hexadecimal digits
 For example bb43:ff3f:0000:549b:e43e:db84:1f1f:0b9e

A comparison of IPv4 and IPv6 IP addresses

IPv4 IPv6

32-bit address 128 bit address

Four groups of denary Eight groups of four
digits separated by a full hexadecimal digits separated by
stop a colon
Over 4.3 billion unique Almost infinite amount of unique
addresses addresses
Router
 A router is an important piece of network hardware. It connects one network
to another network
o This means it can be used to connect a local area network (LAN ) to
the internet, which is a wide area network(WAN )

 An important role of the router is to analyse data packet and direct them on
to their destination
A diagram showing multiple networks connected by routers, represented by
the blue circular objects

 A router being used to connect a LAN to a WAN will have a public IP

address , which has been assigned to it by an Internet Service Provider
 It is this public IP address which other routers use to identify and direct
packets to the network
 An important role of the router is to analyse data packets and direct them
on to their destination
o The IP address of both the sender and intended recipient is stored
in the header of the data packet
 The header contains information about the packet
 The payload is the actual data being sent
 If the data packet is coming into the LAN, the router will send the data packet
to the specific device, within its LAN, that the packet is meant for
 If the packet is being sent from a device within the LAN, it will read the header
of the packet to determine the intended destination IP address
o It will then forward the packet on to the destination
 It might have to travel through several routers before it gets to
its destination
 Each pass from router to router is called a hop
 The network access device or ‘home hub’ used in your home network will
have a router built into it

A summary of the steps a router takes when it receives a data packet

Step Description

1  a router receives incoming data

packets from one network and analyses the
 packet header to determine the destination
IP address

2  It then looks up the IP address in a routing

table (routing table of known networks) to
determine the next network where the packet
should be sent

3  The router then forwards the packet to the

appropriate network or device

 This process is repeated by every router the data packet passes through, until
it reaches its destination
 In addition to routing data between networks, routers can also perform other
functions such as…
o assigning IP addresses to devices within the LAN
o filtering incoming traffic based on certain criteria, such as IP
address, port number, or protocol type