Computer Science

What i don’t know

Problem Solving
1. Sorting & searching algorithms
  1. Bubble sort - Compares adjacent items of an unsorted list and switches the items place so the the list end up sorted in either ascending or descending order
  2. Merge sort - Divides the list in half, continuously until list size =1 (recursion). Then, compared adjacently to sort list
  3. Binary search - A search algorithm that finds the position of a target value within a sorted array by repeatedly dividing the search interval in half and eliminating the half unfit to hold the target value.
  4. Linear search - Goes through the list item by item until match is found or the entire list has been searched, making it a simple but inefficient method for unsorted lists.
  Linear search vs Binary search
  - Linear search is less efficient than binary search, particularly for large datasets, as the target item = x could be at the very end of the list.
  - Binary search needs the list to be sorted first unlike linear search.
Programming
1. Develop code
  1. Type coercion - Changing data type into another.
2. String
  1. String traversal - Loop through each character in a string.
  2. Concatenation - Joining 2 or more items together
3. Data Structures
  1. Array
    1. Record - Elements stored are different data types but related like name, age, F/M which is string, integer, Boolean. Each element is “Field”.
    2. Multidimensional arrays - Each item at an index is a diff array. Has columns & rows
4. Input/output
  1. Validation
    1. Range check - Ensures data entered within a specific range
    2. Presence check - Ensure value has been entered
    3. Look-up check - Checks if entered value is of a predefined set of values
    4. length check - Length of value entered fits a specific range
5. Subprograms
  1. Function - Always returns a value to the main program
  2. Procedure - Doesn’t return value to the main program
  3. Built-in function - Provided in programming software’s to save time and perform common tasks
6. Testing & Evaluation
  1. Errors
    1. Run-time error - Occur during program execution. The hardest to spot
    2. Logical error - The code works however, the desired output isn’t being executed. Error in logic
    3. Syntax error - Problem in user code when grammar rules of programming are broken.
    4. Integrated Development Environment (IDE) - A package that helps programmers develop a code.
  2. IDE (Integrated development environment - helps programmers develop a program code through syntax highlighting etc.
Data
1. Binary
  1. Binary shift - When binary number moves to the left or right by a specific amount based on multiplication or division.
  2. Logic shift - same as binary shift blank spaces replaced with zero
  3. Arithmetic shift -
    - Used with signed numbers in 2’s complementary form.
    - Left shift is identical to Logic Shift but, left most bit doesn’t move or change (MSB).
    - Right shift all bits moved right and blank spaces replaced with copies of MSB.
2. Data representation
  1. ASCII (American Standard Code for Information Interchange)
    1. Originally 7-bits so 128 characters can be stored (upper case, lower case, symbols, punctuation etc.)
    2. Extended ASCII 8-bits 256 codes.
    3. Many different versions of extended ASCII available
  2. Unicode
    - Standardized
    - Solution to Many diff ext. ASCII.
    - represented using 2 or 4 bytes.
    - Can represent code in all known languages.
  3. Colour depth - Number of bits used to represent colour of each pixel. 1-bit is 2¹ or 2 etc.
3. Data & storage compression
  1. File sizes

Computers
1. Machine & computational models
  1. Sequential computers - Follow instructions in an algorithm step by step in order from start to finish
  2. Parallel computers - Task distributed among 2 or more processors.
  3. Multi-agent computer -
    - Separate tasks processed by different systems to perform particular function.
    - Each agent is independent.
2. Hardware
  1. RAM -
    - Volatile (looses data when powered off)
    - Measured in gigabytes
  2. ROM -
    - Non-Volatile
    - Stores permanent data called firmware
  3. CPU saving data
    - CPU tells bus to tell memory where to save data & what data to save
  4. CPU reading data
    - CPU must specify memory location using memory address (a unique number that identifies a storage location)
  5. Cache memory - Small memory used to communicate between 2 devices with different speeds, usually CPU & RAM.
  6. Virtual memory - When computers RAM is full instead of closing programs the “memory manager” of the operating system swaps out unused programs to the hard disk in a pretend memory and swap in the needed program to free area.
    1. Disadvantages -
      - Read/write speed of hard disk much slower than RAM
      - Significant drop in performance
      - Disk thrashing occurs when hard disk and RAM continuously swap data.
  7. Fetch-Decode-execute
    - Accumulator - holds results of calculations performed by ALU.
    - Registers - memory locations, relatively small number of them.
    - Control unit - Controls steps of the cycle synchronised by an electric clock
    - Bus;
      - Address bus - Carries memory address between CPU & memory to identify memory location
      - Data bus - Carries value to be read/written to memory
      - Control bus - carries signals that determine whether to read/write data & when to do so
    - Bus width - number of wires that make up a bus
    - PC sends current instruction to MAR and holds next instruction to be executed. That address is sent along address bus and waits for signal from control bus. That signal sent from control unit to main memory and that data fetched is sent to MDR via data bus. Copy of instruction sent to CIR which is then executed by CPU.
  8. Factors affecting processor performance
    - Clock speed
    - Number pf processors
    - Size of cache
  9. Secondary storage -
    1. Magnetic (Hard disk)-
      - North & South poles represent 1 & 0.
      - Do not require power.
      - Consist of stack of disks called “platters” with magnetic coating.
        Data recorded along circular tracks
        arm moves across to be above right track
        Surface like magnet cause tiny current
        Translate to 1&0
    2. Optical (DVD, CD)-
      - Made of several layers
      - Data written along single track moves from center out in spiral.
      - 2 lasers, one to read, one to write.
      - When writing -
        laser heats surface creating non-reflective “pits”, lowering the surface compared to “lands”.
        Takes time.
      - When reading -
        laser reflects differently detected by light sensor
        Spiral track on outside is longer so rotation speed decrease to ensure data detected in constant speed
    3. Solid state/flash (flash drive) -
      - Uses chips called “NAND”, special transistors that can trap electrons in pools.
      - Full pool is 0
      - Empty pool is 1
      - Faster since alot of tiny NANDS can be stored in small area.
        Control signal (CS) identify which bit is to be read out.
        If empty pool transistor turns on and 1 read out.
        If not transistor off and 0 read out.
        When written, higher voltage applied pulling e^-n into pools.
        Erasing also needs higher voltage.
  10. Programming languages
    1. Instruction set - A list of all possible commands the CPU knows
    2. Machine code - Binary code represent each instruction in IS.
    3. Translator - Converts source code to machine code
    4. Source code - high-level text programmers write
    5. Assembler - Translate mnemonic(each instruction in instruction set) of assembly language to machine code to carry out for micro processor.
    6. Assembly language - Low-level language
    7. Emulator - Hardware or software that allows computer system to act like another.
  11. Translators
    1. Interpreter -
      - Translates line by line
      - No object code file
      - Find errors when they happen
      - Slower than compiler coz of translating source code will running.
      - Can only be run if the coding program is also available on device
    2. Compiler -
      - Translates whole code at once
      - Saves high-level language as machine code or object code file. Coz need to be executed/run
      - Cannot produce object code unless whole program correct
      - Report errors initially
      - Hard to debug
      - Easier to prevent code from being altered.
      - Can be run cross platform

Communication & the internet

Networks
1. Client server network - network with at least one server to provide services to client computers. The process;
  - A client will make a connection to the server using servers IP address.
  - Once connection made, client will make server request to the server.
  - If request is valid, the server will send requested data to the client using clients IP address.
2. Peer-to-peer network - A network that doesn’t have any dedicated servers.
  - Each computer can act as both a client & a server.
  - Each computer can also request services
Network security
1. Importance
  1. Confidentiality - ensure only intended audience is able to access data in network - encryption
  2. Correctness - network useless unless its correct - data must not be allowed to be changed
  3. Availability - network useless if data cannot be accessed when needed - complex systems will likely fail here
2. Authentication & validation
  1. Authentication - Verify identity of subject
  2. Validation - Check if input is correct or acceptable
3. Ways to secure network
  1. Access control - controls if a user will gain access to particular files - along with whether they gain read & write access or read only access
  2. Firewall - Monitors & controls data moving from one network to another

Protocols

A protocol is a set of rules that dictate how data is transmitted between computers.
A protocol may include;
- How each computer should act
- How to acknowledge that data is received
- Routing info
- How to assemble data at destination
- How speed of sender and receiver can be synchronised
Email protocols
Network protocol
1. Ethernet - Family of protocols used in wired Local Area Networks (LANs), covering physical aspects like cable types and connectors, as well as logical aspects like error checking and data transmission speed.
2. Wifi - technology that lets you connect to the internet wirelessly. It uses a set of rules created by the Wi-Fi Alliance to make sure all Wi-Fi devices can talk to each other.
3. Transmission Control Protocol/Internet Protocol (TCP/IP) - set of protocols governing how data is transmitted over networks.
  1. IP - Sends data to right address
  2. TCP - Works with data packets

Protocols of network layers

Layer	Description
Application layer	Allows applications like web browsers and email clients to communicate over a network.
Transport layer	Manages host-to-host communication, dividing data into packets and ensuring safe arrival. Manages data flow so data doesn’t overload receiver.
Internet layer	Routes data packets using IP address
Network layer	Transmits data through local networks like Ethernet.

Protocols in application layer

Protocol	Description
File Transfer Protocol (FTP)	Rules for how files are transmitted between computers.
Hypertext Transfer Protocol (HTTP)	Used for sending requests and responses between a web browser and a web server.
Secure HTTP (HTTPS)	Enables secure communication between a client and server through encryption.
Simple Mail Transfer Protocol (SMTP)	Protocol for sending email messages from a client to a server.
Post Office Protocol (POP)	Used by a client to retrieve emails from a mail server; emails are typically downloaded.
Internet Message Access Protocol (IMAP)	Allows emails to be read and managed on the message server without requiring download, suitable for users with multiple devices.

Mobile networks

First digital communication
Enabled texting

Increased data transmission speed
Gave wireless access to internet
Enabled video calls, downloading & streaming

Higher speeds
Made mobile games popular
Downloading is faster
Larger capacity so bigger number of users can be supported at the same time

Smarter, faster, more efficient
lower latency(delay in data travelling)

The internet and the world wide web
1. How the internet works
  1. Domain names - used to identify one or more IP addresses
  2. Internet service provider (ISP) - Allows access to internet
  3. Domain name server (DNS) - translates domain names to IP addresses.
  4. If one DNS doesn’t know the corresponding IP address it will send it to another DNS and so on.
  5. When a internet connected computer wants to send data to another computer.
    1. The data is split into packets
    2. Each packet has a header that contains the sender’s address, destination address, current packet number and total number of packets
    3. Each packet is sent to your ISP(Internet service provider)
    4. ISP will hv a router. The router will inspect the packet header and decide where to send the packets
    5. Once the packet reaches the destination the packets need to be rearranged correctly as they might have arrived in the wrong order using TCP.
    6. This is done using the packet header information.
2. Accessing the WWW
  - The WWW is accessed using web browsers
  - Web browser - Allows user to view websites using the internet
  - The web browser converts the data received from web server to human-readable format.
3. How the WWW works
  1. The user enters URL of the page they want to visit.
  2. The computer uses DNS to find the IP address of the required web server.
  3. Web browser connects to the web server using the IP address.
  4. Web page transferred from one computer to another using HTTP or HTTPS
  5. data is sent in HTML format

The bigger picture
1. Computer & the legal impact of technology
  1. Intellectual property (IP) - Unique product of human mind
  2. Copyrights & patents
    1. Patent - Protects the idea or design of an invention
  3. Licensing - States that even if a user has purchased it, they don’t own.
  4. creative commons - organization that provides license for any creative piece of intellectual property
  5. open source license - available freely in the internet; the code can edited by anyone
  6. Proprietary software - closed source and thus, users are not allowed to modify it.
2. Current & emerging trends
  1. DNA computing
    - Instead of traditional computer chips, DNA computing uses molecules of DNA to process information.
    - It works by using chemical reactions inside DNA to solve complex problems.
    - Very powerful because it can do many calculations at the same time (parallel processing).
    - Scientists hope this could be useful for medicine & solving big problems faster than normal computers.
  2. Quantum computing
    - They use quantum bit (qubits) instead of normal computer bits.
    - Qubits can exist multiple states at once (superposition).
    - Can process info much faster.
    - Qubits are connected in a way that changing one instantly affects the others (entanglement).
  3. Nanotechnology
    - Building tiny machines & circuits at the atomic and molecular level.
    - This means making smaller, faster processors and memory devices.
    - Working on nano sized transistors that can make computers incredibly powerful.
    - Creates better battery life, display and medical treatments.