Last Minute Revision Notes for OCR GCSE Computer Science

Central Processing Unit (CPU): The main processor in a computer that carries out instructions of a computer program using data.
- Distinction between Instructions and Data: Instructions are the commands executed by the CPU, and data is the information processed by those commands.
Components of CPU:
- Arithmetic Logic Unit (ALU): Performs arithmetic and logic operations required for executing instructions.
- Control Unit (CU): Coordinates operations within the CPU, managing the fetch-execute cycle and decoding instructions.
- Cache: A small, fast type of volatile memory that stores frequently accessed data and instructions to improve speed. It's checked before the CPU accesses RAM.
- Registers: Ultra-fast, small storage locations within the CPU for temporary data storage. Key registers include:
- Memory Address Register (MAR): Holds the address of a memory location that is being accessed.
- Memory Data Register (MDR): Contains data that is either being fetched from or written to memory.
- Program Counter (PC): Holds the address of the next instruction to be executed.
- Accumulator: Stores the results of calculations performed by the ALU.

Fetch-Execute Cycle: A process consisting of three main steps:
- Fetch: The instruction is retrieved from memory using the address in the Program Counter.
- Decode: The Control Unit interprets the instruction to understand what action is needed.
- Execute: The ALU performs the required operation, often fetching additional data if necessary.
Clock Speed: Indicates how many cycles per second the CPU can perform, measured in gigahertz (GHz). Higher clock speeds lead to faster processing.
Number of Cores: Refers to the number of processing units within the CPU. More cores allow simultaneous processing of multiple instructions, enhancing performance.
Embedded Systems: Computers designed for specific tasks within another device (e.g., household appliances, cars).

Primary Storage: RAM (volatile) and ROM (non-volatile). Cache and registers are not typically categorized in primary storage.
- RAM: Temporary storage that loses data when powered off. Virtual memory can sometimes be used when RAM is full.
- ROM: Contains the startup program to load the operating system and is non-volatile, retaining information when powered off.
Secondary Storage: Long-term storage of data and programs, primarily three types:
- Magnetic Storage: Example is Hard Disk Drives (HDD), utilizing magnetic polarity to store data.
- Optical Storage: Example is DVDs, using light reflections to represent data.
- Solid State Storage: Example is Solid State Drives (SSDs), which offer faster data access without moving parts.
Binary Units: Know conversions from bits to kilobytes, megabytes, etc. Basics include:
- 1 Byte = 8 bits
- 1 Kilobyte (KB) = 1,000 Bytes

Character Sets: ASCII (8-bit, 256 characters) vs. Unicode (16+ bits, thousands of characters).
Images: Composed of pixels, resolution is width x height. Color depth indicates bits per pixel.
Sound: Sample rate (measured in Hz) and bit depth are crucial for digital sound quality.

Types of Networks:
- LAN (Local Area Network): Covers a small area, like a building.
- WAN (Wide Area Network): Covers a large geographic area with infrastructure owned by multiple entities.
Network Topologies:
- Star Topology: Central node manages all connections (easy to add devices but reliant on central device).
- Mesh Topology: Every device connects directly, high performance but complex.
Network Hardware:
- Devices like Switches and Routers manage data packets across networks using MAC addresses and IP addresses.
Protocols: Set of rules for data communication, examples include TCP (splits packets, error handling), POP, IMAP, SMTP (email protocols). The Internet uses IP addresses for routing.

SQL Injection: Attack through inserting SQL code into user input forms to manipulate databases.
Brute Force Attacks: Attempting all password combinations. Mitigation: use of complex passwords.
Denial of Service (DoS) Attacks: Flooding servers to disrupt service.
Social Engineering: Manipulating people to gain confidential info, e.g., phishing.

Firewalls: Monitor and control incoming and outgoing network traffic based on security rules.
Antimalware: Detects and removes malicious software.
User Access Levels: Controls who can access sensitive information.
Encryption: Secures data by converting it into a format that can only be read by someone with the decryption key.

Role of OS: Manages hardware and software resources, provides an interface for users, and oversees tasks like multitasking and peripheral communication.
Utility Software: Supports system maintenance tasks (e.g., defragmentation). Not to be confused with application software targeted at users.
Data Protection and Legal Aspects:
- Data Protection Act: Obligates organizations to protect personal data and provides rights to individuals regarding their personal data.
- Computer Misuse Act: Makes hacking and unauthorized computer access illegal.
- Copyright, Designs and Patents Act: Protects intellectual property, ensuring creators retain rights to their works.

Privacy Issues: Balancing data usage with individual rights.
Environmental Issues: Considerations on recycling and resource extraction for technology.
Exam Tips: For long-form answers (8-mark questions), focus on balanced arguments, context-relevant points, and logical flow in writing. Avoid lengthy introductions and conclusions, emphasizing clarity and detail in your responses.