CPU: Fetch, Decode, Execute Cycle

Systems Architecture

• The Central Processing Unit (CPU) is the primary component for processing instructions and executing programs.
• The CPU relies on the fetch-decode-execute cycle to carry out software and hardware operation tasks.
• In the fetch stage, the CPU retrieves an instruction from memory, using the Memory Address Register (MAR) and Memory Data Register (MDR).
• The Program Counter (PC) tracks the next instruction's memory address and updates after each fetch.
• During the decode stage, the Control Unit (CU) interprets the instruction and determines the necessary operation.
• In the execute stage, the instruction is carried out, potentially involving the Arithmetic Logic Unit (ALU), data movement between registers, or interaction with input/output devices.
• The CPU cycle repeats millions to billions of times per second, depending on its clock speed.
• The Control Unit (CU) coordinates activities within the CPU to execute instructions correctly and move data efficiently.
• The Arithmetic Logic Unit (ALU) performs mathematical calculations and logical operations.
• Registers are small, internal CPU storage locations that temporarily hold data for quick access.
• The Accumulator (ACC) stores intermediate results of calculations, reducing the need to access slower memory.
• The Memory Address Register (MAR) holds the memory location address being accessed.
• The Memory Data Register (MDR) stores data transferred between the CPU and memory.
• The Von Neumann architecture stores program instructions and data in the same memory unit.
• The Von Neumann architecture creates a bottleneck, since the processor can only fetch one instruction or data item at a time from memory
• The Harvard architecture stores instructions and data separately, allowing simultaneous access and improving speed.
• Harvard architecture is used in embedded systems and specialized computing devices.
• Clock speed, measured in GHz, affects CPU performance, with higher speeds generally meaning faster processing.
• Excessive clock speed increases heat generation and power consumption.
• Multi-core processors execute multiple tasks simultaneously, improving multitasking and parallel processing.
• Cache memory, located close to the CPU, stores frequently accessed data for faster retrieval, improving system performance.

Memory and Storage

• Memory stores both instructions and data needed for processing.
• Primary storage includes Random Access Memory (RAM) and Read-Only Memory (ROM).
• RAM is volatile memory, losing its contents when the computer is powered off.
• RAM stores data and programs actively in use by the CPU.
• More RAM allows a computer to run more applications simultaneously without slowing down.
• ROM is non-volatile, retaining data even when powered off.
• ROM stores essential firmware like the Basic Input Output System (BIOS).
• When RAM is full, virtual memory uses a portion of the hard drive for temporary storage.
• Accessing data from virtual memory is slower than RAM, reducing system performance.
• Secondary storage devices such as Magnetic storage, solid-state storage, and optical storage save files and applications permanently.
• Magnetic storage (HDDs) use spinning disks and a read/write head to store data.
• HDDs offer large storage capacities at a lower cost, but are susceptible to damage and slower than SSDs due to moving parts.
• Solid-state storage (SSDs) and flash memory use electronic circuits to store data.
• SSDs are faster, more reliable, consume less power, and are more durable since they have no moving parts.
• Optical storage (CDs, DVDs, Blu-ray discs) use laser technology to read and write data.
• Optical storage is less common due to limited capacity and slower speeds which is unlike modern storage alternatives.
• Cloud storage stores data remotely on internet-connected servers.
• Cloud storage provides remote access, automatic backups, and collaboration features.
• Cloud storage relies on an internet connection and has potential security risks and subscription costs.
• Lossy compression reduces file size by removing unnecessary data, sacrificing some quality.
• Lossy compression is used in JPEGs, MP3s, and MP4s.
• Lossless compression reduces file size without any data loss, preserving the original quality.
• Lossless compression is used in PNG images and ZIP files.

Networks, Security, and Cyber Threats

• Computer networks allow devices to communicate and share resources.
• Local Area Networks (LANs) cover small areas and are typically controlled by a single organization.
• Wired LANs use Ethernet cables for fast, secure communication.
• Wireless LANs (Wi-Fi) offer convenience but are vulnerable to interference and security threats.
• Wide Area Networks (WANs) span large areas and connect multiple LANs.
• WANs use external infrastructure and are susceptible to latency and security risks.
• Routers connect different networks and direct data between them.
• Switches efficiently route data between devices within a LAN.
• Network Interface Cards (NICs) allow devices to send and receive data over wired or wireless connections.
• Network security protects data from cyber threats which can include malware, Phishing attacks, Denial of Service and SQL injection.
• Malware (viruses, worms, trojans) infects systems, corrupting files or stealing information.
• Phishing attacks deceive users into providing personal details.
• Denial of Service (DoS) attacks overload networks, rendering them unavailable.
• SQL injection exploits database vulnerabilities to manipulate or steal information.
• Security measures can include Firewalls, Anti-malware software, and Encryption
• Firewalls block unauthorized access between a computer and the internet.
• Anti-malware software detects and removes malicious programs.
• Encryption converts data into an unreadable format, protecting it from unauthorized access.
• Strong password policies and multi-factor authentication reduce the risk of unauthorized access.

Legislation and Ethical Considerations in Computer Science

• Laws and regulations govern the use of computers, data, and digital communication.
• These laws ensure privacy, security, intellectual property rights, and responsible technology use.
• The Data Protection Act (DPA) 2018 is the UK’s implementation of the General Data Protection Regulation (GDPR).
• The Data Protection Act (DPA) 2018 governs how personal data is collected, stored, and processed.
• The Data Protection Act (DPA) 2018 gives individuals more control over their personal information.

Key principles of the Data Protection Act (DPA)

• Lawfulness, fairness, and transparency: Data must be collected legally and with consent.
• Purpose limitation: Data should be collected for a specific, legitimate purpose.
• Data minimization: Organizations should only collect necessary data.
• Accuracy: Data must be kept up to date and corrected if inaccurate.
• Storage limitation: Data should not be kept for longer than necessary.
• Integrity and confidentiality (security): Data must be protected from breaches and unauthorized access.
• Accountability: Organizations must take responsibility for following the law.
• Failure to comply with GDPR and the DPA 2018 leads to heavy fines and legal action.
• The Computer Misuse Act 1990 protects computer systems from cybercrime, including hacking and malware.

Offences under the Computer Misuse Act

• Gaining unauthorized access to computer material is illegal.
• Gaining unauthorized access with intent to commit a crime is illegal.
• Causing damage by modifying computer material is illegal.
• Supplying or creating tools for hacking or cybercrime is illegal.
• Punishments for breaking this law range from fines to long-term prison sentences.
• The Copyright, Designs and Patents Act 1988 protects intellectual property.
• The Copyright, Designs and Patents Act 1988 makes it illegal to copy, distribute, or modify someone else’s work without permission.

How the Act applies to computer science

• Software piracy is illegal.
• Downloading copyrighted media illegally is a violation of copyright laws.
• Plagiarism is an offence under this law.
• Companies use Digital Rights Management (DRM) to prevent illegal copying of digital content.
• Some software developers use open-source licenses, allowing others to freely use and modify their code.
• The Freedom of Information Act (FOIA) 2000 gives the public the right to request access to information held by public sector organizations.

How the Act works

• Citizens can request data from public bodies.
• Organizations must respond within 20 working days and provide the requested information unless it falls under exemptions.
• Private companies are not covered by the act.
• Computer science raises ethical, environmental, and cultural concerns.

Ethical Issues

• Ethical concerns focus on responsible technology use and the impact of digital advancements on society.
• Privacy concerns: Should companies be allowed to track and sell user information?
• AI and automation: Should artificial intelligence replace human jobs?
• Cybersecurity vs. government surveillance: At what point does surveillance become an invasion of privacy?
• Digital divide: Not everyone has access to high-speed internet and digital devices, which limit opportunities in education and employment

Environmental Issues

• Computers and technology contribute to environmental issues, including e-waste and energy consumption:
• E-waste (electronic waste) causes environmental pollution.
• Energy consumption contributes to carbon emissions.
• Companies are increasingly looking for renewable energy sources to power their data centers.

Cultural Issues

• Technology affects different cultures in various ways, sometimes leading to social challenges:
• Censorship and internet restrictions are present in some countries.
• Automation and AI are replacing traditional jobs, creating both opportunities and economic challenges.
• Cultural homogenization can lead to a loss of local traditions and languages.

Learn about the Central Processing Unit (CPU) and its role in processing instructions. Explore the fetch-decode-execute cycle, including the roles of the Memory Address Register (MAR), Memory Data Register (MDR), Program Counter (PC), and Control Unit (CU). Understand how the CPU repeats this cycle to carry out software and hardware operations.