CompSci iGCSE theory

Why do programmers use hexadecimal numbers rather than binary numbers?

It is easier to say and memorise hexadecimal codes rather than binary codes as they are more simple

RAM

Random Access Memory
Volatile
Temporarily stores data (e.g. when apps are in use)

ROM

Read Only Memory
Non-Volatile

It stores firmware (BIOS,UEFI), which store instructions to boot-up the computer

What is an operating system?

Software which allows the user to control both hardware and software.
It hides the complexities of the hardware.

What is the user interface?

the part of the computer with which the user interacts with.

5 functions of the OS

Hardware management
File management
Memory management
Process management
Security management

Purpose of application software

Performs tasks related to end user

Define CLI

Command Line Interface
Users interact with the operating system using text commands

Define GUI

Graphical User Interface
Users interact with the OS with visual icons such as pointers, menus, windows

Advantages and Disadvantages of CLI

Advantages:
It takes up less system resources.
Useful for automated tasks
Commands are faster to type
Disadvantages:
Memorising the commands can be hard
Typing errors can occur
Less intuitive

Advantages and Disadvantages of GUI

Advantages:

Intuitive, user-friendly

Requires little prior knowledge of computers

Easier to understand and adaptable

Disadvantages:

Not good for repetitive tasks

Slower to find

Uses more system resources

Utility system software includes:

software which maintains and protects the OS(e.g. defragmenter)

What is cache

Cache is a device which is used between 2 devices that communicate at different speeds. It is commonly used between the CPU and RAM.
The CPU communicates at a much faster speed than RAM, so cache was made in order to increase the computer performance as CPU did not have to wait for RAM to fetch data.
It stores frequently used data.

What is the function of virtual memory

Used when RAM is full. The least frequently used data in RAM is swapped out into VM, and swapped back into RAM when there is space.
VM functions on an HDD

How many bits does ASCII use? Extended ASCII?

7 bits or 128 characters
Extended ASCII uses 8 bits

What is Unicode and why was it invented

Unicode is an international character code which includes all languages, made up of 2 or 4 byte (16 or 32 bit)
Unicode was invented as ASCII and extended ASCII had no standardisation because there were many different versions

What is the difference between logical right shift and arithmetic right shift?

In logical right shift, the empty spaces become zeros, but in arithmetic right shift, the empty spaces is one

Explain symmetric encryption

It uses the same pre-shared key to encode and decode data. The key is shared privately

Explain asymmetric encryption

A public key is shared publicly and it is used to encrypt data, but the message can only be decrypted by a private key, which is transferred to the recipient.

Compiler language

Translate high level language into machine code all at once
Executable format (.exe)
Independent, runs without needing translator software, easily distributable
If the code has errors or has a change, the code must be recompiled (creates more .exe files)
Needs to be compiled for specific machine architecture
Harder to debug
Fast execution speed
Memory heavy
The source code is not shown, which protects intellectual property

Creates two files to be maintained, one for editing and one for execution.

Interpreter language

Translates high level language into machine code one line at a time
Each line is executed after translation
If an error in the code is found, the program immediately stops
Dependent, needs translator software, not easily distributable
Cross platform
Easier to debug
Slow execution
No optimisation
Less RAM needed

Only one file is needed to be maintained

Needs to be translated everytime the program is run

Source code is made available

Why does the computer need to translate code before execution?

The code must be in binary or machine code in order for the processor to understand.

Computers use transistors, which has 2 states on or off. This can be represented by binary, which has 2 values, 1 or 0

4 ciphers and their types

Caesar cipher - shift cipher

Pigpen cipher - substitution cipher

Railfence cipher - transposition cipher

Vigenere cipher - Polyalphabetic substitution cipher

Bitmap graphics

Bitmap graphics contain the colour and position of millions of pixels (picture elements)
The colour is stored using hexadecimal code

What is colour depth

The number of bits used to encode the colour of each pixel
Standard colour depth is 24 bits (RGB)

High reso vs Low reso

Resolution is number of pixels per inch
High reso images have lots of small pixels
Low reso images have few big pixels

Vector graphics

Made up of simple shapes and lines
smaller than bitmap file
Not suitable for high quality pictures

Img reso formula

width*height

Img size formula

width x height x colour depth

Analogue sound

Sound which changes value constantly

What is sample rate and the unit

The number of times the audio amplitude is recorded
frequency
_hertz, standard is 44.1 kHz

Bit depth // Sample resolution

the number of bits available for each sample

Sampling

Measuring a soundwave at regular but different intervals

Audio file size formula

sample rate * bit depth * length

What is pseudocode

A standard way to show an algorithm without the rules of a programming language

What is an algorithm

A series of step-by-step instructions of a procedure

Lossy vs Lossless compression

Lossy compression : the original data is permanently deleted
Lossless compression : original data is not lost

NOT gates

Inverter
Outputs the opposite of the input

AND gates

Multiplication
Outputs the product of inputs

OR gates

Addition
Outputs the sum of inputs

NAND gates

NOT AND
Outputs the opposite of the product of inputs

NOR gates

NOT OR
Outputs the opposite of the sum of inputs

XOR Gate

Outputs zero when inputs are the same
Outputs one when inputs are different

Uses of logic gates

NAND gates are used in burglar alarms or buzzers
Circuits involving processing and computations
doorbells, street lights

Firmware

System software which is stored in ROM.
Initialises the hardware components and starts the OS when the computer is turned on.

Writing to magnetic storage (HDDs)

- Contains a stack of disks called platter

- Each disk has a magnetic coating on the surface.

- The data is recorded along tracks on the disks surface, separated by sectors

North = 1

South = 0

Reading from magnetic storage

The arm of the platter moves to be along the selected track
The disk spins so that the required sector of the track is under the arm's head
The magnetic coating sends a signal to the head, and the disk controller translates this into binary

Writing to optical data storage

A laser heats the surface of the disk, creating pits and land on the surface. The pits are lower than the land surrounding them, and they are unreflective.

Reading from optical data storage

The disk spins in the drive to ensure that all the data is read
A tracking mechanism moves the laser to the correct place
The laser shines onto the disk, and the light is reflected onto a sensor, which translates this into binary

Writing to Flash Drives

Control signals identify which bit is to be written to, and applies a high voltage
This pulls electrons into pools of the transistor, recording 1s and 0s
Electrons = 0
Empty pool = 1

Reading from Flash drives

Control signals identify which bit is to be read from, applying a small voltage.
The 1s and 0s from the pools are recorded

Why are SSDs not long term?

Writing to and reading from SSDs involve applying a voltage, which breaks down the transistor over time.

Why is camel case used?

There is easier readability
Consistency with programming
Variable names cannot include a space

High level programming language (HLL)

- Uses English command words, so it is easily understandable

- Has rich instruction set (sequencing, selection, iteration)

- Supports many different datatypes (Boolean, string, ...)

- Fewer lines of code can represent complex texts e.g. loops

- A line of code generates several executable instructions

- All HLLs need translator software

- Programmers do not need to know the specific machine architecture in order to understand it.

Low level programming language (LLL)

- Uses mnemonics/abbreviations or binary code, which is hard to understand

- Limited instruction set

- Cannot represent complex constructs

- No datatypes

- A line of code generates only one executable instruction.

- Programmers need to know specific details of machine architecture in order to understand it.

- Only assembler needs translator, otherwise LLL do not need translator software

Bit pattern

a collection of binary digits
e.g. ASCII code, hexadecimal

Memory address

A unique location in memory where instructions or data are stored

Buffer

most Input/Output devices have memory between the main system as they communicate at different speeds

Why does adding more RAM increase the computer's performance

More RAM means there is more space in RAM
This means VM needs to be accessed less, which increases the performance as the Swap In/Swap Out process takes time

ALU

Arithmetic Logic Unit
performs mathematical or logical operations in CPU

Register

storage inside the CPU
stores an instruction, an address, etc.

Control Unit

organises the actions of other CPU parts

Clock

an electronic device inside the CPU
used to sync the actions of other CPU units

CIR

Current Instruction Register
stores the current instruction being decoded/executed

ACC

Accumulator
used to hold the data during the calculations of the ALU

MDR

Memory Data Register

Stores the instruction that is currently being read from or written to.

MAR

Memory Address Register
stores the memory address of the memory currently being read/written

PC

Program Counter
stores the address of the next instruction

Fetch-Decode-Execute cycle

Fetch:

The address of the next instruction to be processed is copied onto MAR by PC

The control unit uses control bus to send read signal to read from memory

The data of the instruction’s memory address is copied onto data bus, and onto MDR

Decode:

The control unit analyses the data, and sends signals to other CPU components

Execute :

The instruction is executed by the ALU

The performance of the CPU

Clock Speed : The rate at which the instructions are processed by CPU. Faster clock speed, better performance

Number of processor cores: Parallel processing can be done, where the computer can multitask or perform the same task together

Size of cache : A larger cache means more frequently used data can be stored, so the CPU will not have to wait for RAM to access data, which improves bottleneck issue and speeds up processing

Cloud Storage

A form of secondary storage
Accessed via the Internet
Files in Cloud servers can be accessed anywhere on any device as long as there is internet

Virtualisation

Hiding the true physical nature of computer resources

Embedded device

a computer contained inside of another device that typically performs a single function.

Sequential computational model

follows instruction in an algorithm step by step

Parallel computational model

The tasks are shared equally between 2 or more processors in a computer
Needs an OS which can handle 2 or more processors
Needs software that is capable of splitting the tasks equally among the processors

Multi-Agent computational model

Separate algorithm are processed by different systems/agents to perform a function

Each agent acts independently, but they cooperate and communicate with one another

Network

an arrangement of computers and devices connected to share resources or data

Server

A powerful computer which provides the network with services such as storage or emails

Types of network connectivity

wired and wireless

Types of networks

PAN (personal area network) - data transmission across short distances

LAN (local area network) - a network across a small geographical area

WAN (wide area network) - network across a large geographical area

Network models

Client-server and peer-to-peer

Why are networks used?

To share access to the Internet

To share peripheral devices

To share file storage

To run software/security updates

Communication

How does a client-server model work?

The user uses its IP address to connect with the server’s IP. The server will authenticate the user, and the user will send a request to access its services. The server will provide its services.

How does a peer-to-peer model work?

There is no dedicated server ; each computer connected on a network can act as both a client or server. Simultaneously providing services to others while requesting services from others

Packet

In storage, data is in bits.
When data is sent over a network, it is a packet.
3 sections in packet :

- Header : Contains sender and receiver IP addresses, and the format of the packet (no. and order)

- Payload : contains the data

- Trailer : error checking e.g. checksum

Bus topology (Pros and Cons)

devices are connected along a single cable, with a terminator at the end to prevent data collisions.

Pros:

- Cheap to install

- easy to add more devices

Cons:

- when the cable is damaged, the entire network fails

- difficult to find a fault on the cable

- runs slower if there are more devices

- security risk

Ring topology (Pros and Cons)

cable connects the networked devices to each other in a loop/ring

Pros :

- adding extra devices won't affect performance

- easy to add devices

- no data collisions as the messages all travel in the same direction

Cons:

- whole network will fail when the cable fails or if a device fails

- it is a closed loop so the network must shut down for a while to add or remove a device

- hard to find fault

- expensive than bus

Star topology (Pros and Cons)

each network is connected to a central point ( a hub or switch)

Pros :

- when the cable is damaged, only the device connected to it fails, nott the entire network

- if a switch is used, the messages are sent to only the intended user, so it is more secure

- faults are easy to find

Cons:

- when the hub or switch fails, entire network fails

- expensive as u need to buy the switch/hub, and cables for each device

Mesh topology (Pros and Cons)

Devices in a mesh network will pass messages on to other devices within the network

Pros:

- not a lot of network traffic as messages can be easily rerouted

- high performance

Cons:

- difficult and expensive installation

- difficult to manage

Protocol definition

A set of rules which control the format of communications between devices on a network

What is eavesdropping

unauthorised sight of data being sent from one computer to another over a network (data interception)

Ways to prevent data interception

- Use a WEP or WPA encryption protocol and a firewall

- Use of wireless router by complex passwords

- Don't use public Wi-Fi connectivity

latency

The delay encountered by data travelling through network

Bandwidth

The amount of data which can be carried from 1 point to another on a network over a time

SMTP

Simple Mail Transfer Protocol
used when sending emails over the internet
controls the format of emails
commands email servers how to understand and respond to messages

POP(3)

Post Office Protocol (3)
used for retrieving email from an email server
you can download messages and then delete them from the server
there are storage limitations on the server, as well as privacy issues
more CPU time is used when a large number of emails are being downloaded

IMAP

Internet Message Access Protocol
allows emails to be accessed using multiple email clients
leaves messages on the server until deleted
changes are synced with the mail server in real time

Ethernet

A collection of family of protocols that is used for physical network connections in LAN

Wi-Fi

Wireless Fidelity

digital communications protocol which sets out how data is transmitted on wireless LANS

TCP

Transmission Control Protocol
provides a reliable connection between computers, checking whether the data is sent correctly
used when accessing web pages or email servers