J277 (9-1) Computer Systems Revision

Binary, Denary & Hexadecimal

• Why Binary? Computers use binary because they are made of switches that can only be on or off (representing 1 or 0).
• Conversions: You need to be able to convert between all three number systems: Denary (base 10), Binary (base 2), and Hexadecimal (base 16).
• Binary Shifts: A left shift multiplies a number by 2 for each place moved. A right shift divides a number by 2 for each place moved.

File Sizes & Character Sets

• File Units: Be able to compare file sizes (e.g., 0.1 GB is 100 MB). Remember the order: Byte, Kilobyte (KB), Megabyte (MB), Gigabyte (GB), Terabyte (TB).
• Character Sets: A character set like ASCII stores a list of all available characters. Each character is given a unique binary code. Upper-case and lower-case letters have different codes.

LANs (Wired vs. Wireless)

• LAN (Local Area Network): Covers a small geographical area (e.g., one building). A key characteristic is that the organisation owns all the network hardware.
• Wired Benefits: More secure from eavesdropping, faster and more reliable connection (less interference).
• Wireless Benefits: Allows devices to be portable, easier to connect new devices, no need to run physical cables.

Network Protocols

Protocols are rules for communication. They are split into layers to make them manageable (each layer handles one job and passes data to the next).

• HTTP: Used for viewing standard webpages.
• HTTPS: A secure version of HTTP, used for sensitive data (like banking).
• FTP (File Transfer Protocol): Used for downloading/uploading files.
• IMAP (Internet Message Access Protocol): Used for checking and receiving emails.

Image Representation

• Metadata: Data *about* the data. For an image, this includes its width, height, and colour depth.
• Pixel Data: The image itself is stored as a sequence of binary values, one for each pixel, read from left-to-right, top-to-bottom.
• Colour Depth: The number of bits per pixel. A 4-bit colour depth can store $2^4$ = 16 different colours. Increasing it to 2 bytes (16 bits) would allow $2^{16}$ = 65,536 colours, but would also massively increase the file size.

Primary vs. Secondary Storage

• Primary Storage (RAM/ROM): Volatile (RAM) or Read-Only (ROM), holds data the CPU is *currently* using.
• Secondary Storage (HDD/SSD): Non-volatile, long-term storage. A computer needs both: secondary storage to hold the OS and files permanently, and primary storage (RAM) to load them into when they are in use.
• Virtual Memory (VM): A section of the secondary storage (e.g., HDD/SSD) used as temporary RAM when the RAM is full. Data is moved from RAM to VM, and then back to RAM when needed.

Network Security

• Spyware: Software that secretly records user actions. Prevented by Anti-malware.
• Brute-force attack: Tries every possible password. Can be slowed by Firewalls (blocking repeated attempts).
• Data Interception: Eavesdropping on data sent over a network. Prevented by Encryption.
• SQL Injection: Malicious code entered into a website form to steal data. Prevented by Penetration Testing (to find the vulnerability).

Software Models

• Client-Server: A model where a Client (e.g., your computer) requests services from a Server (e.g., a website's computer) which stores, processes, and provides the data.
• Proprietary Software: Source code is kept secret, and users buy a licence to use it. Benefit: The artist/programmer earns money and has full control.
• Open Source Software: Source code is publicly available. Benefit: Users can modify the code to fix bugs or add features.

Embedded Systems

• Definition: An embedded system is a computer built into a larger device to perform a specific, dedicated task (e.g., parking sensors, a microwave controller, engine management).
• It is *not* a general-purpose computer like a laptop.

Legal, Ethical & Privacy Issues

• Legal Issues: Involve breaking the law. For example, using facial recognition to store personal data (a face scan) without consent could breach the Data Protection Act (DPA).
• Ethical Issues: Morally questionable actions. For example, is it right to track people's movements in a shopping centre without telling them?
• Privacy Issues: Relate to an individual's right to be "left alone". Storing and tracking individuals' faces and locations is a major invasion of privacy.