J277 (9-1) Computer Systems Revision

Number Bases (Denary, Binary, Hex)

• Binary (Base 2): Uses 0s and 1s. This is the language of computer hardware (On/Off). An 8-bit number is a 'byte'.
• Denary (Base 10): The human counting system (0-9).
• Hexadecimal (Base 16): Uses 0-9 and A-F. It's a shorthand for binary. One Hex digit represents a 4-bit 'nibble'. (e.g., B = 11, which is 1011 in binary).

Images & Sound

• Bitmap Images: Stored as a grid of pixels. File size is determined by: Resolution (Width x Height) x Colour Depth.
• Colour Depth: The number of bits used to store the colour of one pixel. 8-bits can store 256 colours ($2^8$).
• Digital Sound: An analogue wave is converted by sampling.
  • Sample Rate: Number of times per second the wave's amplitude is measured (in Hz).
  • Bit Depth: Number of bits used to store each measurement.

Binary Addition & Shifts

You add one column at a time, from right to left. Remember the rules:

• 0 + 0 = 0
• 0 + 1 = 1
• 1 + 0 = 1
• 1 + 1 = 0, carry 1
• 1 + 1 + 1 = 1, carry 1

Binary Shifts: Shifting all bits left multiplies by 2 for each place. Shifting right divides by 2 for each place.

Secondary Storage

This is non-volatile (keeps data with no power) and long-term. There are three types:

• Optical: Uses a laser. (e.g., CD, DVD). Cheap, but slow and easily scratched.
• Magnetic: Uses read/write heads on platters. (e.g., HDD). High capacity and cheap, but has moving parts (less durable).
• Solid-State (SSD): Uses flash memory (no moving parts). (e.g., SSD, USB Stick). Very fast and durable, but more expensive.

Operating System (OS) Functions

The OS is the main software that manages all the hardware and software. Its key jobs are:

• Memory Management: Manages RAM, moves data to/from virtual memory, and allocates memory to programs.
• Peripheral Management: Manages all input/output devices (like printers, keyboards) using drivers.
• User Management: Handles logins, permissions, and access rights.
• File Management: Organises files and folders, handling creation, deletion, and renaming.

Utility Software

Software designed to maintain the system.

• Encryption: Scrambles data to make it unreadable without a key.
• Defragmentation: Reorganises files on a (magnetic) hard drive to be stored in contiguous blocks, speeding up access.
• Compression: Reduces file size for easier storage/transfer.

LAN vs WAN

• LAN (Local Area Network): Covers a small geographical area (e.g., one building or campus). The organisation *owns* the hardware.
• WAN (Wide Area Network): Covers a large geographical area (e.g., countries). Infrastructure is *rented* from providers. The Internet is the biggest WAN.

Topologies & Protocols

• Star Topology: All devices connect to a central hub/switch. Reliable (if one cable fails, only one node goes down), but if the hub fails, the network fails.
• Mesh Topology: Every node connects to every other node (Full) or some nodes (Partial). Very resilient (no single point of failure), but expensive and complex to cable.
• IPv4 Address: A unique 32-bit address, written as 4 denary numbers (0-255). e.g., `192.168.1.100`.
• DNS (Domain Name System): The "phonebook" of the internet. It translates human-readable URLs (e.g., `google.com`) into computer-readable IP addresses.

CPU Registers

Registers are tiny, super-fast memory locations inside the CPU, each with a specific job:

• Program Counter (PC): Holds the memory address of the next instruction to be fetched.
• Memory Address Register (MAR): Holds the memory address of the instruction or data currently being fetched or stored.
• Memory Data Register (MDR): Holds the actual data or instruction that has just been fetched from, or is about to be written to, memory.

Legal & Ethical Issues

• Data Protection Act (DPA): Law governing how organisations must store and process personal data securely and fairly.
• Ethical Issues: Problems that aren't illegal but are morally questionable, like planned obsolescence (designing products to fail) or bias in AI.
• Environmental Issues: The problem of e-waste and the energy consumption of data centres.