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GCSE Computer Science

The CPU (Central Processing Unit)
•
•
•
•
•

the main component in a computer for processing data and instructions
the computing equivalent of the human brain
a hardware device
made up of many subcomponents
ALU
input
device

registers

internal
memory

controller

output
device

control bus
address bus

data bus

RAM

The ALU (Arithmetic and Logic Unit)
•
•
•

processes and manipulates data
performs simple calculations (such as addition and subtraction) on the data that is
temporarily stored in the registers
performs comparisons on data, which allow programs to make use of choice (such as
an IF statement in a high-level language

Registers
•
•
•
•

storage locations found on the CPU
where data or control information is temporarily stored
usually much faster to access than internal memory, as they have to be accessed so
often
e
...
an accumulator, which is used by the ALU to store the results of its calculations

Internal memory
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•
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fast access temporary storage on the CPU
sometimes called level 1 cache memory
holds data moved from the registers when it is not being actively used, which can
either be written to RAM or called back into the registers for further processing
speeds up the processing of data
1

The controller
•

sends and receives signals that travel along a control bus from all parts of the
computer, ensuring that all processes take place at the right time and in the correct
order

The controller in operation
•
•
•

the controller: sends control signal to RAM
RAM:
looks at address bus to see location
RAM:
places required data on data bus

Buses
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•

allow data to be transferred to different parts of the computer
include the control bus, the address bus and the data bus

The control bus
•

used by the controller to send control signals to different parts of the computer

The address bus
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used to transfer the storage address of data

The data bus
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used to transfer the data

Input devices
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•

allow data (such as text, images, video or sound) to be entered into a computer
system
e
...
a graphics tablet, a mouse, a keyboard and a microphone

Output devices
•
•

allow data (such as printed document, on-screen data and sound) to be
communicated by the computer in a human-friendly form
e
...
speakers, a projector, a monitor and a printer
2

Secondary storage
•
•
•

also known as ‘backing storage’
where data is stored when it is not being actively used, usually for retrieval at a later
date
takes longer to access than memory

The most frequently used secondary storage media

Cores
•
•

the processing components within the CPU
In theory, a dual-core processor should be able to process instructions twice as fast
as a single-core CPU
...
g
...


Cache memory
•
•
•
•

fast access
very expensive (only small amounts are present in most computer systems)
improves the performance of the CPU
able to provide instructions and data to the CPU at a much faster rate than other
system memory (such as RAM)

The fetch-decode-execute cycle
1
...

2
...

3
...


4

RAM
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•
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•
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•

volatile
allows data to be changed
speed: 
used for the temporary storage of currently running programs and data
consists of a large number of store locations, each of which is identified by a unique
address
e
...
used to store the program and the data when working on a word-processed
document

ROM
•
•
•
•
•

permanent
does not allow data to be changed
speed: 
used for the permanent storage of data
e
...
used to store programs such as the BIOS – but it cannot be update

Flash memory
•
•
•
•
•

permanent
allows data to be changed
speed: 
used for the permanent storage of data
e
...
used to store programs such as the BIOS – and can be updated

Cache memory
•
•
•
•
•
•
•

volatile
allows data to be changed
speed: 
used for the temporary storage of frequently accessed data and instructions
consists of a small number of store locations that can be accessed very quickly by the
CPU
quicker than RAM
e
...
often stores the next set of instructions that need to be processed by the CPU

5

Hardware ports
•
•
•

serve as an interface between the computer and other computers or devices
a piece of equipment to which a plug or cable connects
e
...
HDMI, USB, audio in and audio out

Software ports
•

allow multiple software applications to use different software ports on the same
physical connection

Optical storage technology
•
•

uses laser beams, which are projected onto a CD/DVD/Blu-ray disk, and if light is
reflected back, data is read as a 1 and if light is not reflected back, data is read as a 0
can be used to read and write information to a disk

Magnetic technology
•

used in floppy disks, hard disks and tapes (magnetic mediums), on which data is
stored by writing data using a write-head and read by reading on a read-head

Solid state technology
•
•
•

used in storage media (such as USB flash memory sticks)
does not having any moving parts (such as read-head)
increasingly used to replace both magnetic and optical storage (especially in mobile
devices) where its low power consumption and high-speed access is advantageous

Storage in the cloud
•
•
•
•
•
•

a contemporary data storage facility
allows users to store their data on third-party servers and access it from many
computing devices
allows maintenance tasks, backup and data replication
requires Internet connection
may allow personal data that is held on a third-party server to be physically stored in
a country where adequate data protection legislation does not exist
leaves users solely reliant on the cloud storage provider when it comes to ensuring
that their data is stored safely and can be retrieved at a later date

6

GUI (graphical user interface)
•
•
+
+
+
+
+
−
−
−
−

a type of interface that allows users to interact with a computer system through
graphical icons
introduced to help users, especially beginners, use computer systems as CLIs
(Command Line Interfaces) were found to be difficult
intuitive
easy to navigate
uses windows, icons, menus and pointers
has no complicated commands
allows data between different software applications to be easily changed
requires a large amount of memory
relatively processor intensive
slower than a CLI
take up a large amount of hard disk space

Features of a GUI
•
•
•
•
•
•
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•

windows
icons
menus
pointers
assistants
customisation
shortcuts
task bar

Menu-driven interface
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•
+
+
+
+
+
+
−
−

allows people to interact with a computer system by presenting the user and
allowing them to work through a series of menus
e
...
the iPod Classic
does not require learning a lot of commands
intuitive
easy to navigate
ideal for beginners (logical)
does not require learning an expert language
requires little processing power
irritating (too many menu screens)
time-consuming
7

Voice-driven interface
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•
+
+
+
+
+
+
−
−
−
−

used to issue commands to a computer system and enter data into it
popular, as it is natural
much faster than keyboard input
does not require learning to type
has less danger of RSI
does not require large hardware (such as a keyboard)
useful for users with disabilities
allows multitasking
allows background noise to interfere
lacks privacy
may not recognise similar sounding words
may not understand users with speech impediments, sore throats and strong
accents

CLI (command line interface)
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•
•
+
+
+
−
−
−

entirely text-based
allows a user to communicate with a computer system by typing in specific
predefined commands
most widely used interface before GUIs were developed
quicker to input commands, as shortcut keys can be used
needs little memory and processing power
requires little storage space (no graphical images)
confusing for beginners
requires commands to be typed precisely
require learning a large number of commands

Touch sensitive interface
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•
•
+
+
+
+
−
−
−
−

extensively used in mobile computing devices
becoming more popular
interprets tapping and other actions
very intuitive
easy to use
does not require keyboard or mouse
requires little thinking (a form of direct manipulation)
easy to damage screen
difficult to select small items
may reduce image brightness
must be within arm’s reach of the user
8

HCI (human-computer interaction)
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the communication between people and computer systems

Human-computer interfaces
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allow a person and a computer system to communicate
provided by the operating system
identified by the style of communication

Data
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made up of raw facts and figures
can be represented in many different forms (such as text, numbers, images, sounds
and video clips)
can be processed to become information

Denary
•

Base 10

Binary
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Base 2
used in order for data to be processed by a computer system

Hexadecimal
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Base 16
widely used, as binary numbers can be quickly converted into hexadecimal numbers
which are more convenient for humans to use
used in MAC addresses

Characters
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can be letters, digits, spaces, punctuation marks and various other symbols
stored as binary numbers when stored on a computer system
can be represented differently by other computer systems, which is important for
computer systems to recognise otherwise data could not be exchanged between
computers
9

Character sets
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tables that map a character with a unique binary number
devised in order to allow for data exchange between computer systems

ASCII (American Standard Code for Information Interchange)
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a character set
7-bit
only able to represent 128 different characters, and computer systems need to be
able to store more

Unicode
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a standard character set that has combined and replaced many others
originally an extension to the ASCII character set
contains many of the characters used around the world

The AND logical operator
Input (A)
Input (B)
Output (A AND B)
0
0
0
0
1
0
1
0
0
1
1
1
• can be used for a control system that is required to turn on a sprinkler system in a
field when: the temperature rises above 25°C, and it has not rained in the last five
days

The OR logical operator
Input (A)
Input (B)
Output (A OR B)
0
0
0
0
1
1
1
0
1
1
1
1
• can be used for a control system that is required to close the windows on a
commercial greenhouse when: either the wind speed above 12 km per hour, and it is
raining

10

The XOR logical operator
Input (A)
0
0
1
1

Input (B)
0
1
0
1

Output (A XOR B)
0
1
1
0

The NOT logical operator
Input (A)
0
1

Output (NOT A)
1
0

Data types
Data
type
Integer
Real
Boolean

Description

Examples

Whole numbers, positive or
negative
Numbers, including fractions or
decimal points
True or false

42, -11, 0
12
...
50, 28
...
g
...
g
...
g
...
g
...
g
...

44 kHz on most audio CDs, which produces a large amount of data and may need to
be compressed

Pixels
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thousands of small-coloured dots
make up an image

14

The OS (the Operating System)
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•
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software that manages a computer system
loaded by the bootstrap loader
manages resources

The OS in managing peripherals (such as input and output devices)
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•

communicates with and receives data input from an input device
communicates with and sends output to an output device

The OS in managing printing using spooling
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•

Data is stored on hard disk/in a queue/in memory
...


Spooling
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allows user to carry on working/log off when waiting for a job to print

The OS in managing secondary storage
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•
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ensures that data is stored and can be retrieved correctly from any disk drive
creates and maintains filing systems (such as FAT or NTFS)
organises files in a hierarchical directory structure

The OS in managing memory (RAM)
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•

ensures that programs/data do not corrupt each other
ensures that all programs and data including itself is stored in correct memory
locations

The OS in managing processes
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•

ensures that different processes can utilise the CPU and do not interfere with each
other or crash
ensures that all tasks on a multitasking OS appear to run simultaneously

15

The OS in managing security
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allows creation and deletion of user accounts
allows users to logon and change passwords

The OS in managing file compression
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reduces the amount of data, making the file smaller, saving disk space

Disk defragmentation
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when fragmented files are split up and stored on different parts of the disk
when file parts are physically rearranged (reorganised, moved and reordered on disk
into the order required for access)
will slow down disk access speed

File compression
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when the amount of data is reduced and the file is made smaller, saving disk space

The OS in providing a user interface
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allows copying/deleting/moving/sorting/searching of file or folders
allows access to system settings such as hardware
allows users to have more than one window open
allows customisation of interface, e
...
change desktop background/layout
allows user to switch between tasks (programs/windows)
provides a graphical user interface
provides user with errors/help messages
provides a command line interface

Libraries
• collections of commonly used private functions and subprograms which can be
called from within your program at any time, but only when the appropriate library
has been linked
• used by most computer languages, but custom libraries can be created
+ store related private functions and subprograms in the same location
+ save time
+ reliable and don’t require further testing
+ easier to maintain as programs contain less code
16

Private functions
• e
...
standard mathematical operations (such as square root and random number
generators)
Subprograms
• e
...
input/output routines (such as saving data to disk)
IDEs (Integrated Development Environments)
• also known as software development environments
• provide programmers with various tools that are needed to create computer
programs
The features of an IDE
• the editor
• the compiler
• the interpreter
• the linker
• the loader
• the debugger
• the trace
• the break point
• the variable watch
• the memory inspector
• error diagnostics

The editor
• allows a programmer to enter, format and edit source code
The compiler
• converts source code into executable machine code, allowing the program to be run
at any time
The interpreter
• converts each line of source code into machine code and executes it as each line of
code is run
• performs the conversion process each time the program needs to be run
The linker
• allows previously compiled code from software libraries to be linked together
17

The loader
• loads previously compiled code into memory
The debugger
• helps locate, identify and rectify errors in a program
The trace
• displays the order in which the lines of a program are executed and, possibly, the
values of variables as the program is being run
The break point
• interrupts a program on a specific line of code, allowing the programmer to compare
the values of variables against expected values
• allows the program code to be executed one line at a time (single-stepping)
The variable watch
• displays the current value of any variable, which can be ‘watched’ as the program
code is single-stepped to see the effects of the code on the variable
• may be set, which will interrupt the program flow if the watched variable reaches a
specified value
The memory inspector
• displays the contents of a section of memory
Error diagnostics
• used when a program fails to compile or run
• displays error messages to help the programmer diagnose what has gone wrong
File transfer
• the ability to transfer data from one location to another
• done by copying a file from one folder (directory) to another or from one storage
medium to another
• can be used to organise files better using subfolders, and to back up work onto a
secondary storage device (such as a flash memory stick)
Formatting
• the process of preparing a disk for use
• involves setting out a new file system on disk and erasing all data in readiness for
new data to be stored

18

Compressing
• the process of making a file size smaller
• allows more data to be stored on the disk
• allows files to be transferred more quickly
• has two types: software-based compressing and hardware-based compressing
• has two types: lossy and lossless
Software-based compressing
• often included as a facility of an operating system, so is readily available on most
computer systems
• slows down the process of reading and writing to disk
Hardware-based (disk-based) compressing
• requires specialist hardware, which can be expensive
• does not affect the speed of access too much
• always lossless
Lossy compression
• reduces the amount of data that needs to be stored
• results in the deterioration of the quality of an image
• used to compress multimedia data (such as sound and video), especially in
applications that stream media over the Internet
• e
...


100 kB

5 kB

Compression ratio
Original file size

•

Compression ratio =

•

e
...
Compression ratio =

Compressed file size
100
5

= 20 (or 20:1)

Lossless compression
• uses an algorithm that compresses data into a form that may be decompressed at a
later time without any loss of data, returning the file to its exact original form
• preferred to lossy compression when the loss of any detail could have a detrimental
effect

19

System maintenance tools
• included in operating systems
• allow users to maintain the upkeep of their computer system
System restore (roll back)
• the process of replacing lost or corrupt data by replacing it with an earlier backup
Disk defragmentation
• the process where files are physically re-arranged on disk that they are no longer
fragmented and the parts of each file are stored together
• improves the speed of accessing data from disk
Fragmented files
• files that are stored on computer systems that are split and stored on different parts
of the disk
• take longer for the disk heads to move between parts of the files, slowing down the
process of loading them
The control panel
• used by many operating systems to give the user control of software and hardware
features
• enables the user to change settings such as sound, device settings and display
settings all from one convenient location
Virus protection software
• also known as anti-virus software
• a program that can be loaded into memory when the computer is running
• monitors activity on a computer system for the signs of virus infection
• needs to be updated regularly to combat viruses
Viruses
• computer programs that are able to copy themselves onto other programs, often
with the intention of maliciously damaging data
• can have catastrophic effects on computer systems
• need to be prevented
• each have their own unique ‘signatures’ that are known to virus protection software
and stored in a database, which are scanned for in the data
• many thousands, and new viruses are created daily

20

Firewall
• a software or hardware security system
• controls the incoming and outgoing network traffic
• monitors where data has come from and where it is going to
• determines if the communication is allowed
• checks a list of predefined rules (such as if the port is listed as a common port for
browsing web pages)
• analyses packets of data to determine whether they should be allowed through or
not
• if advanced, analyses the contents of each packet of data to ensure that it matches
what is expected to be sent to that port
Software applications
• used in the day-to-day use of computer systems
Word processing
• used for storing, manipulating and formatting text
• can be used to type a letter
Spread sheets
• used for organising and analysing numerical data in tabular form
• can be used to model a household budget
Presentation
• used for displaying information in the form of a slide show
• can be used to present new ideas to management
Database
• stores a comprehensive collection of related data
• can be used by a gas company to store customers’ records
Drawing
• used for producing and editing digital images
• can be used to create a company logo

21

Packets
• collections of data that are transmitted over packet-switched networks
• include source address, destination address, that enables the data to be
reassembled into its original form, other tracking information, data and a checksum
that checks that the data has not been corrupted
Protocol
• an agreed-upon format that allows two devices to communicate
The possible rules of a protocol
• handshaking (where two devices establish their readiness to communicate)
• how the sending device will indicate that it has finished sending a message
• how the receiving device will indicate that it has received a message
• the type of error checking to be used
• agreement on the data compression method to be used
IP (Internet Protocol)
• combines with TCP to allow communication between computer systems on a
network
• sets out the format of packets and an addressing system
TCP (Transmission Control Protocol)
• combines with IP to allow communication between computer systems on a network
• allows packets to be sent and received between computer systems
HTTP (Hypertext Transfer Protocol)
• used to transfer multimedia web pages over the Internet
FTP (File Transfer Protocol)
• used when copying a file from one location to another via a network or the Internet
• typically used for the transfer of large files
• allows broken communications to resume transferring a file rather than having to
restart
A protocol stack
• when a programmer takes the rules of a protocol and implements them when
creating a program
• could lead to minor communication problems

22

Networks
• consist of a number of computer systems connected together
+ share hardware and software
+ share data/files
+ easier for internal communication/can send emails
+ central backup
+ easier to monitor network activity
+ centrally controlled security
+ can access data from any computer
− expensive (employing a network manager, the initial cost of servers, communication
devices)
− cause security problems (sending files could cause a virus)
− cause data to be accessed by hackers more easily
− affect all workstations if the server is down
LAN (Local Area Network)
• where the computer systems are located close to each other (such as a school)
• many could be linked using a WAN
WAN (Wireless Area Network)
• where the computer systems are located distant from each other (such as the
Internet)
Network topologies
• the theoretical layouts of computer systems on a network
Bus network
• has computer systems (the nodes of the network) each connected to the bus (a
single cable on which data can be sent), and terminators on each end to ensure that
the network functions correctly
+ makes it easy to implement and add more computer systems to the network
+ quick to set up (well-suited for temporary networks)
+ cost-effective (as there is less cabling)
− has difficulties in troubleshooting the bus
− has limited cable length and number of stations (so the performance degrades with
added computers)
− goes down if there is a problem with the main cable or connection
− low security (all data transmissions can be seen by all computers)
− requires proper termination
− increases probability of data collisions
23

Ring network
• has computer systems connected in a ring or loop along which packets are sent
+ allows data to be quickly transferred without a bottleneck (there are consistent date
transfer speeds)
+ allows the transmission of data to be relatively simple (as packets travel in one
direction only)
+ allows adding nodes with very little impact on bandwidth
+ prevents network collisions
− breaks if any of the computer systems fail (as data cannot be transferred efficiently)
− has difficulties in trouble-shooting the ring
− forced to be temporarily shut down to add another node (as they are all linked
together)
Star network
• has computer systems, each of which are connected to a central node (the hub)
+ has good performance
+ easy to set up
+ allows adding nodes without taking the network down
+ barely affected by any non-centralised failure
+ has minimal security collisions
+ has better security
− expensive to install (as it requires more cabling)
− requires extra hardware (such as a hub)
A NIC (Network Interface Card)
• required to connect a computer system to a network
Linking a stand-alone computer to a network
• requires a NIC (Network Interface Card)
• two ways: to allow a cable to connect your computer system to the network
(provided by a physical hardware port), and to connect a computer system using a
wireless connection (Wi-Fi)
Typical network speeds
• a copper cable: 100 Mbps – 1 Gbps
• a fibre-optic connection: 1 Gbps – 10 Gbps
• a Wi-Fi connection: 54 Mbps – 108 Mbps (can be severely affected by distance and
atmospheric conditions (such as heavy rain))

24

Network security
• of paramount importance to any network as the loss of data (particularly personal
and confidential data) can have many serious consequences
User access levels
• a method used to allow certain users read and/or write access to data on a
computer system
• e
...
An administrator will have read and write access to all data, while a secretary
will not have access to confidential data (such as employees’ salaries)
Suitable passwords
• should be complex, with a combination of uppercase and lowercase alphanumeric
characters and non-alphanumeric characters
Brute force attack
• when a hacker has access to programs that have the ability to try multiple guesses in
quick succession
• slower with complex passwords
The number of attempts taken to brute force a password
• attempts= number of characters password length
• e
...
‘Computer1’: attempts = (26 + 26 + 10)9 = 62 = 13,537,086,546,263,552
Encryption
• the conversion of data using an algorithm into a form called cyphertext that cannot
be easily understood by people without the decryption key
• involves performing the XOR logical operator on the original data and a key (a secure
binary number known only to the sender and recipient)
• e
...

Original Data
10101010
Key
11110000 XOR
Cyphertext
01011010
• has other techniques (such as SHA256 and Blowfish)
Network policies for acceptable use
• documents (each several pages long) written and agreed by a committee to outline
the rules that users are required to follow while using a computer network following
publication
• typically include a list of unacceptable types of website that should not be visited,
activities that are not allowed on the network (such as gambling), and installation of
unauthorised software
25

Network policies for disaster recovery
• for recovery after fire, flood, lightning, terrorist attacks, hardware failure (such as
power supply unit failing), software failure (such as virus damage), accidental
damage and malicious damage (such as hacking)
• (before the disaster) risk analysis, preventative measures and staff training
• (during the disaster) staff response (implementing contingency plans)
• (after the disaster) recovery measures, purchasing replacement hardware,
reinstalling software, restoring data from backups
Network policies for backup
• set out how often and to what medium backups are made
• uses medium (historically magnetic tape) that is generally different to the active
storage medium
• typically require that three different backups be kept at any given time, with one of
these being stored offsite (the oldest being the grandfather, the second oldest being
the father and the most recent being the son, with the grandfather being
overwritten each time a new backup is added)
Backup
• the process of copying data so that it can be preserved and restored if the original
data is lost
• should be made regularly (as the older the backed-up data becomes, the less likely it
is to match any current data stored on a computer system
Archiving
• the process of storing data which is no longer in current or frequent use
• holds data for security, legal or historical reasons
• frees up resources on the main computer system and allows faster access to data
that is in use
The Internet
• a WAN (wide area network)
The www (World Wide Web)
• a service provided over the Internet

26

Moderns
• abbreviation of ‘modulator/demodulator’
• devices that allows digital computer systems to communicate over analogue systems
(most commonly the telephone network)
• when in use, make the telephone line unavailable to make telephone calls
• have a typical data transfer speed of 56
...
HTML
• one of the main programming languages used when creating webpages

...
PNG
• a format used for storing compressed images
• uses lossless compression
• favoured for excellent quality
• generated using a non-copyright algorithm

...
MP3
• the format used to store compressed audio
• uses lossy compression
• favoured for its small file sizes that allow quick download speeds while maintaining
reasonable good quality
27

Packet switching
• the process of delivering packets from one computer system to another using a
designated device (such as a switch and a router)
• used in the Internet
Routing
• the method of selecting paths along which packets are sent on a computer network
• involves constructing a routing table (which are essential to maintain to ensure that
packets are delivered as quickly as possible) in the memory of specialist computer
systems (such as routers, switches, bridges, firewalls and gateways) which stores a
number of paths along which it is best to send packets to reach a specific destination
• mostly use only one network path at a time, but there are some multipath routing
techniques
MAC (media access control) addresses
• also known as a physical address and a hardware address
• a unique hexadecimal number given to any communication device (such as a NIC)
• e
...
74:E1:B6:8E:18:27
• stored in a communication device’s ROM
• uses hexadecimal notation to allow over 281 trillion different combinations of MAC
address
• stored in the routing tables in computer systems on its network (as the addresses
are permanent and do not change like IP addresses)
IP addresses
• allocated to computer systems on networks (usually by a DHCP server, although you
may assign your own IP address if you do not wish to rely on the services of a DHCP
server)
• e
...
195
...
213
...
g
...

The IP address is typed into the address bar of the web browser
...

The computer system sends a request to its DNS server for the IP
address that is mapped to the domain name of the website
...

The DNS server returns the IP address
...

The user’s computer system communicates with the computer system
where the website is stored
...
co

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