A-Level Computer Science

A processor, also known as a central processing unit (CPU), is the primary component of a computer responsible for executing instructions, performing calculations, and managing data.

Clock speed refers to the number of cycles per second that a processor can execute instructions. It is measured in Hertz (Hz) or gigahertz (GHz).

Performs operations on data e.g. Addition, subtraction

Coordinating activities of the CPU

Quick, small stores of data within the CPU

Holds memory address for data or a instruction about to be used by the CPU

Holds actual data or instruction

Stores results of calculations in the ALU

Holds memory address of the instruction for each cycle

Fetch-The next instruction is retrieved by CPU from main memory

Decode-The instruction is broken down and decided so computer can understand

Execute-The CPU performs what the instructions told

A computer system built within a large device e.g. Camera, washer, car

Memory loses its data when power off

Memory retains its data when power is lost

Collection of wires that carry signals between various components of the computer system

Processes all of the data and instructions that make the system work - can be effected by clock speed, number of cores, cache size

A memory management procedure needed when the RAM is full so a temporary section is made which acts like a part of RAM

Solid state storage, non-volatile, more reliable/durable but can only be overwritten a limited number of times

CD, DVD, Blue-ray
+: cheap, easy to transport
-: slow, less storage than hard drives, stored data degrades over time, cannot be written over

Hard drives
+: fast access, stores large amounts of data, low cost
-: not very portable, easily be broken

USB, flash memory, SD
+: fast, small, light, easily potable, quiet
-: more expensive, storage capacity less, limited number erase/write cycles

Data is stored on multiple servers in a remote location
+: secure, can be accessed anywhere, no need to buy
-: needs internet, download and upload can be effected by internet connection, less control if data is held

Text commands where user has to type in command

maintains a computer

reorganises data on the hard drive to put fragmented files back together and moves files to collect all the free space

Full Backup - a copy is taken from from every file on the system
Incremental Backup - only files created or edited since last backup are copied

reduces file size so they take up less space on hard disk

scrambles data to stop others from accessing it

source code is made freely available and users can modify it
+: free, ,made for greater good, can be adapted
-: small, buggy, security holes, no warranties, no customer support

only the compiled code is released and the source code is kept a secret
+: warranties, well-tested, reliable, cheaper
-: expensive, software may not fit user needs

Personal Area Network

within the range of an individual person

Local Area Network (LAN)

a network that connects devices close to each other e.g. school, house

• Can be wired or wireless

• infrastructure owned by LAN owner

• Each device known as a node

Wide Area Network (WAN)

a network within a large geographical area e.g. internet

amount of data that can be transferred in a given time

Network Interface Card

allow a device to connect to a network

connect devices on a LAN

responsible for transmitting data between networks

a set of standards for connecting computers

client has connection to server, servers can backup and store centrally but can be expensive and difficult to maintain

no central server, each computer equal in responsibility, have to work as both server and a client.

+: better performance, rest of network not affected in one fails, simple to add more devices
-: wire needed for all devices, expensive

+: faster
-: expensive

assigned to all devices, unique to all devises and cannot be changed, permanent, identifies the actual device.

assigned either manually or automatic, the location of your device on the internet

Split data into packets to be sent across the network, each packet given a number order of data, each router reads packet header and decides which way to send it according to IP rules, packets then arrive and reassemble them in the right order.

A model for something explains it or shows how it can br produced

Different programming languages follow different paradigms

Some languages are better suited to solving particular computational tasks than others (e.g. querying a database, voice recognition, bitcoin mining)

Instructions given in sequence

Selection used to decide next instruction

Iteration used to repeat instruction(s)

Functions and procedures

BASIC,COBOL, PASCAL,FORTRAN,C

• Language model organised around objects and data (not actions and logic)

• Java, Python, C++,VB.net ,Ruby, Objective C

• Objects are components of an OOP programs that performs certain actions and knows

• A program is written using an OOP paradigm and uses a class called song to organise the songs that are streamed to customers

• The class song has these attributes:

  • title

  • number of listens

  • rating

• The constructor method will set the name attribute to the name that is passed in as a parameter. The constructor will also initially set the number of listens to 0 and the star rating to 0

Language based on and built around mathematical functions

Erlang, LISP, Haskell, Scala, ML

Lots of different languages

Constantly evolving

Set of instructions for the processor

Must be converted to binary

Machine code

Each instruction represented in binary

Processor has an instruction set that it understands

Different sets for different processors

Operation Code (Opcode)- the instruction to be performed (e.g. Add, Store, Subtract)

Operand-data to be used or its memory location

GOLDEN HANDCUFFS-brilliant but can’t go outside of it and edit it

Classes are the blueprints of an object that can be reused. If we take an example of a person, we could say that a person has a name, can do things like speaking and walking. This would be our blueprint. We can create multiple objects of this class by instantiating them, a bit like jelly coming out of a jelly mould. When an object is created, it has a method in the class called the constructor that sets up all the properties a newly created object should have.

Inheritance uses the ideas of parent and child (super- and sub-classes) to abstract certain behaviours and attributes for similar classes.

Encapsulation is a way of ensuring that methods and attributes have the right permissions set for accessing and altering data. This is sometimes called information hiding. These are usually termed as public and private and are usually used to stop data from being changed accidentally.

Polymorphism is a property of OOP that allows the programmer to make a program accept any data that they want into a method and it will be able to cope with it.

Problem expressed as set of facts and rules

Lossy compression- An algorithm that makes a file storage space smaller but the accuracy with which it represents data is reduced

Lossless compression-An algorithm that makes a file storage space smaller but accuracy with which it represents data is not reduced

Advantages to the user of a file being compressed

Downloads quicker. Saves user money by using less bandwidth

Some loss of detail is acceptable

Unlikely to be noticeable

Will make the file size smaller than lossless

A:

Reduced risk

Suitable for large projects

Good documentation control

Additional functionality can be added later

Software produced early in lifestyle

DA:

Can be very costly

Risk analysis requires high level of expertise

Success dependant on risk analysis

Not suitable for smaller projects

A:
End user involvement increases flexibility

Prototype provides tangible product early on

Shortened dev cycle

DA:
Inefficient for large scale projects

Results focused-solution may be inefficient

Changing user requirements

Type of agile software development

Focus on programming

Short iterations

User requirements drip-fed

Flexible process

A:

cost effective

robust solutions

meet user requirements

DA:
Needs constant end-user involvement

No quality assurance of coding

Often poorly documented

• Multiple identical systems in different geographical locations

• Transactions written to each systems

• Provides immediate backup in case of system failure in 1 location

• Serialisation technique

• Priority awarded to transactions based on dependencies and timestamps

• Prevents deadlock by blocking request until previous transaction is complete

• Only 1 transaction happens at a time

• Overlapping transactions may result in loss of data

• Implemented using timestamp ordering

• Timestamp issued at start of transaction

• Earlier timestamp given priority

• DB objects have ‘read’ timestamp AND ‘write’ timestamp

• Timestamps updated whenever object is read or written

• Only 1 user can access a record at a time

• Prevents data inconsistencies

• Can result in Deadlock

Deadlock Resolution

• DBMS is responsible for identifying and resolving deadlock (Timestamp ordering, serialisation, commitment ordering, redundancy)

• Set of rules that DBs use

• Ensure no inconsistent changes made

• Refer to transactions (actions that bring about a change to the data)

A- Atomic- A change is either completely performed or not at all

C-Consistent-A transaction must take a database from 1 consistent state to another

I-isolated- A transaction should not be visible to any other transaction or user until it is irrevocably committed

D-Durable- Once a change has been made, it must not be lost due to any subsequent failure

Every value of 1 attribute of a table must exist as a value of another attribute in another table

Changes to a DB might not make sense. E.g. Delete songs from your music library, playlist still refers to songs, playlist cannot locate the songs to play them

DBMS allows enforcement of referential integrity to prevent inconsistent changes

Data must be of a predefined type (e.g. string)

Each row in a table must have a unique identifier

• Ensures data is entered into DB is:

  • accurate

  • valid

  • consistent

• Validation rules to enforce

• May be compromised by malicious individuals, poor design, accident, etc

A Transaction refers to a change of state in a DB resulting from a single logical operation on data

Any relation DB requires basic functionality to be useful

All (except READ) result in transactions

Transactions must maintain DB integrity

• Only relevant/ valid data will be entered into DB

• Selection can occur during data capture (e.g. through form design)

• or during data entry (e.g. Boolean)

• Data retrieved from a DB using SQL statements

• Capture human-marked data from document forms such as surveys and tests

• e.g. read questionnaires

• Scans images of text

• Converts into editable and searchable data

• Used to digitise printed texts

• Scanners

  • Collect data by scanning barcodes with laser or LED lights

  • e.g supermarket checkouts

• Smart Card Readers

  • Collect data electronically from card with embedded microchip

  • ATMs

• Paper forms designed to elicit valid data from users

• Data must be manually entered

• Prone to errors in data entry

• e.g. passport applications

• Input boxes, radio buttons, check-buttons, select menus, text areas

• Data validated client-side (JavaScript)

• Validated data sent to server for further processing/ validation (PHP, ASP, Perl)

• Data can be automatically sent into DB user server-side scripting

• linked set of resources

• capable of sharing power and resources

  • printers

  • CD-ROM

  • DVD-Drives

  • databases

• Connected devices

• Communicate

• Theoretical arrangement of the components of a network

• Actual arrangement determined by physical factors

• Effects:

  • cost

  • performance

  • ease of installation

Both sides need an input and an output so it has to be a pair.

We use multiple twisted pairs to increase speed (more bandwidth)

Hub is sending packets to all devices

Switch takes packets of data to specific MAC addresses (has software to do this) (only sends to the 1 needed device). Useful for large networks as it reduces traffic, increasing the speed.

• 1 common linking communications cable (bus)

• Nodes connecting off the table

• Cheap

• Slows down when heavy traffic

• Lots of data collisions

• Breakage to bus will affect the whole network

• Limited distance coverage

• Shared link to server(s)

• Central node is the Hub

• Few data collisions

• Fast, robust and cost-effective

• Can set up independent segments

• Hub can be another node, or switch etc.

• Hub has seperate connections to each node

• 1 direction traffic

• fast performance

• 1 common linking table

• passes through NIC of each node

• Network must be able to cope if 1 node fails

• Good for high volumes of traffic

• SPF

• Decentralised design

• Wired or wireless

• No single point of failure (SPF)

• Each nodes connect to 2+ other nodes

• Nodes communicate directly with each other without need for internet connection

• Communications between workers

• Share data/files/ info

• Share peripherals, e.g. printers

• Easy to upgrade machines

• Logon from any connected machine

• Distributed processing

• Network Interface Card

• Used on a LAN

• Each node on the network needs a NIC in order to connect to the LAN

• Allows computers to communicate over a computer network by providing physical access to the network and a unique address for each individual node (MAC address)

• MAC- Media Access Control

• Unique number to identify individual network interfaces

• Used to communicate with physical layer of network

• Can be used to restrict network access

• Used on a LAN

• Used on a LAN

• Connects network segments

• No data collisions

• Forwards data packets between networks

  • Receive packet

  • Reads address info

  • Use routing table/ routing policy

  • Forward packet to next network

  • Repeat until packet reaches destination

• Wireless device that can be connected to a wired network

• Uses Wi-fi/ Bluetooth/ related standards

• Usually connects to router via wired network

• Relays data between wireless and wired devices

A:

• No building work needed

• Quick to add new workstations

• Flexible deployment of nodes

• Visitors can access network/ internet

DA:

• Slower performance

• Security issues

• Computers located in seperate locations

• 2 or more seperate LANs joined together

• Satellite/ fibre-optic/ telephone lines

• Infrastructure may be provided by others

• The Internet

• Storage Area Network

• Dedicated network used for large scale storage of data in data centres

• Common uses of a SAN includes email servers, databases, and high usage file servers

• Personal Area Network

• Used for data transmissions among devices (e.g. phones, tablets)

• Can be used for communication between personal devices or to connect to a higher level network and/or Internet

• It is not a place

• Data storage and servers moved off site

• 3rd party management maintenance, security, backups etc

A:

• Cheaper

• No in-house maintenance

• DA:
  Loss of control/ security

• All nodes have equal status

• Can share files and peripherals

• Prone to data collisions

• Overall speed of the network is reduced

• Each peer can be accessed by any other

• Can be implemented over the Internet… used to facilitate file sharing

• Most common network architecture

• Dedicated, high-spec machine

• Handles data storage and processing of shared files

• Handles services e.g. printing and internet access

• Clients request services from the Servers

• Data centres= multiple servers stacked together

• Virtualised servers= more efficient and less energy consumption