A Level OCR CS

• Buses


  • Data Bus: Carries data between the processor and memory
  • Address Bus: Carries memory address being read from or written to
  • Control Bus: Sends control signals from the control unit
• Fetch-Decode-Execute Cycle


  1. Fetch: Retrieve instruction from memory into the CPU

  2. Decode: Interpret the instruction to determine what operation to perform

  3. Execute: Carry out the operation specified by the instruction
• Factors Affecting CPU Performance

  • Clock Speed
  • Number of Cores
  • Cache Memory
  • Pipelining
• Contemporary Processor Features


  • Simultaneous Multithreading
  • Out of Order Execution
  • Branch Prediction
  • Variable Clock Speed
  • Power Conservation
• Types of Processors


  • CISC (Complex Instruction Set Computing)
  • RISC (Reduced Instruction Set Computing)
• Graphics Processing Unit (GPU)


  • Designed for graphics processing but increasingly used in other fields
  • Specialised for parallel processing, often with SIMD architecture
• Multicore and Parallel Systems


  • Multiple cores in a single CPU or multiple CPUs can work on computations simultaneously for parallel processing
  • Extent of speedup depends on how much of the problem is parallelizable
• Von Neumann Architecture


  • Features a single control unit, arithmetic logic unit, and memory store for both instructions and data
  • Drawback: Reading an instruction prevents simultaneous reading and writing of data due to shared memory and data bus
• Harvard Architecture


  • Differs from Von Neumann by having separate memory units and buses for data and instructions
  • Advantage: Eliminates the restriction of a single memory store, allowing for simultaneous data and instruction operations
• Contemporary Processors are more complex than the traditional Von Neumann model
• Input, Output, and Storage Devices

  • Input devices
  • Output devices
  • Storage devices
• Types of Storage Devices

  • Magnetic storage
  • Optical storage
  • Flash storage
• RAM and ROM


  • RAM is volatile and used for programs and data in use
  • ROM is non-volatile and used for start-up instructions
• Virtual Storage


  • Virtual storage is external storage not directly connected to the computer
  • It can be accessed over a network or the internet
  • Virtual storage allows for automatic backup, sharing among multiple users, and accessibility from anywhere in the world
• Operating Systems


  • Windows
  • MacOS
  • Linux
  • iOS
  • Android
• Memory Management


  • Operating systems manage memory efficiently to avoid waste
  • Memory is divided into pages (paging) or segments (segmentation) for better utilisation
  • Virtual memory allows the use of secondary storage as an extension of physical memory, reducing costs
• Interrupts and Interrupt Service Routines (ISRs)

  • Interrupts signal the CPU when a device needs attention, with each having a priority
  • ISRs are executed by the operating system in response to interrupts, ensuring tasks are handled appropriately
• Scheduling Algorithms


  • Round robin
  • First come first served
  • Shortest job first
  • Shortest remaining time
  • Multilevel feedback queues
• Types of Operating Systems

  • Distributed
  • Embedded
  • Multitasking
  • Multi-user
  • Real-time
• BIOS and Device Drivers

  • BIOS (basic input/output system) helps boot the computer and is now often stored on flash memory
  • Device drivers allow the operating system to communicate with various hardware devices
• Virtual Machines


  • Virtual machines mimic physical computers and can run multiple operating systems simultaneously
  • Common uses include running one operating system within another and interpreting intermediate code for portability
• Applications Generation


  • Word processors
  • Spreadsheet packages
  • Presentation software
  • Desktop publishing software
  • Image editors
  • Web browsers
  • Database management systems
• Utilities


  • Anti-malware software
  • Disk defragmentation tools
  • Backup software
• Closed Source Software


  • Distributed only as executable machine code
  • Users don't have access to the source code
  • Developed by companies with resources and teams of programmers
• Open Source Software (OSS)

  • Source code is publicly available
  • Users can modify, recompile, and redistribute the software
  • Often developed by global teams of volunteers
• Advantages of Closed Source Software

  • Tends to be more polished
  • Developed by companies with resources
• Advantages of Open Source Software

  • Usually free
  • Users can adapt software to their needs
  • Developed by a global community
• Disadvantages of Closed Source Software

  • Vulnerable to exploitation but also benefits from community scrutiny
• Machine Code


  Instructions and data stored in binary, directly readable by the CPU
• Assembly Code


  A low-level language using mnemonics to represent instructions
• Assembler


  Converts assembly code into machine code
• Compiler


  • Converts high-level code into executable machine code
  • Produces an executable file that can be run immediately
  • Provides faster execution and security but slower compilation
• Interpreter


  • Converts high-level code line by line during runtime
  • Executes the program immediately but slower than compiled code
  • Useful for coding and debugging due to immediate feedback
• Stages of Compilation


  1. Lexical Analysis

  2. Syntax Analysis

  3. Code Generation

  4. Code Optimization
• Libraries


  • Pre-written code for common tasks, reusable by other programmers
  • Saves time and effort, covers complex tasks, and can be used across different programming languages
• Linkers


  Combine the code generated by the compiler with any necessary libraries to create a single executable file
• Loaders


  Take the executable file created by the linker, load it into memory, and prepare it for execution
• Waterfall Model


  • Linear and sequential approach to software development
  • Consists of distinct phases, such as requirements analysis, design, implementation, testing, deployment, and maintenance, with each phase flowing into the next
• Rapid Application Development (RAD)


  • Emphasises rapid prototyping and iterative development
  • Involves creating a working model of the software quickly and refining it based on feedback from users
• Spiral Model


  • Combines elements of both waterfall and iterative development
  • Involves iterative cycles, each consisting of planning, risk analysis, engineering, and evaluation