CS

• Components of the CPU

  • Control Unit (CU)
  • Arithmetic Logic Unit (ALU)
  • Cache
  • Registers
• Control Unit (CU)


  Controls the flow of data in and out of the CPU<|>Manages and co-ordinates all of the Fetch-Decode-Execute cycle
• Arithmetic Logic Unit (ALU)


  Carries out the mathematical calculations<|>Logical operations and binary shifts
• Cache


  Stores frequently and recently used data and instructions for quick access<|>Faster read-write speed than RAM but not faster than the registers
• Registers


  Single piece of memory with a value<|>Temporarily holds addresses, instructions or data<|>Fastest read-write speed
• Stored Program Concept


  In the Von Neumann Architecture, both the programs and the data they use are stored in memory
• Purpose of the CPU

  To repeatedly execute instructions stored in memory<|>(To carry out the Fetch-Decode-Execute Cycle)
• Registers used by the CU to run Fetch-Decode-Execute Cycle

  • MAR - Holds address of data to be fetched/stored
  • MDR - Holds data/instructions fetched from memory
  • PC - Holds address of next instruction to be processed
  • ACC - Stores results of calculation done by ALU
• Fetch-Decode-Execute Cycle


  1. Fetch: Memory address copied from PC to MAR, CU fetches data/instructions from RAM, Data copied to MDR, PC incremented

  2. Decode: CU decodes the instruction in MDR

  3. Execute: Instruction carried out (e.g. calculation in ALU, loading from memory, writing to memory)
• Clock Speed


  Number of Fetch-Decode-Execute Cycles per second<|>Every action in CPU takes place on the tick of a clock<|>The higher the clock speed, the more Fetch-Decode-Execute Cycles done in a second<|>Measured in Hertz (Hz)
• Cache Size


  Cache is much faster than RAM<|>Data goes in and out much quicker<|>Recently and regularly used data is stored here<|>Makes CPU efficient as less time is wasted in waiting for data to be transferred
• Number of Cores


  A processor can have more than one core<|>Each core can process Fetch-Decode-Execute Cycles simultaneously<|>But only if the software is designed for multi-core processors
• Embedded Systems


  Used to control the function of electronic devices<|>Perform limited and very specific tasks<|>Their input is often controlled by a button/switch<|>It's a computer system embedded within a device<|>Designed for a specific purpose on circuit boards
• Primary Storage


  • RAM - Stores programs currently being used, copies data needed by programs, stores instructions/modules from open programs
  • ROM - Can store data without power, stores boot sequence instructions, can only be read, not written to
• Virtual Memory


  Used when there is not enough space in RAM for programs and data that is currently being worked on<|>It's a small section of secondary storage<|>It acts like RAM<|>When the program in virtual memory is required, it is swapped with something not in use in RAM<|>The process of swapping will take time as hard disk speeds are much slower
• Secondary Storage


  • Magnetic Storage - Uses different patterns of magnetisation in a tape/hard disk to store data
  • Optical Storage - Marks arranged in patterns, laser burns 'pits' into shiny surface
  • Solid State Storage - Sends a large electric current to force electrons through barrier and trapping them in position
• Characteristics of Storage Media

  • Capacity - The amount of memory the medium has / how much data it can store
  • Speed - How quickly the data can be accessed from it
  • Portability - Whether the medium can easily be moved from one computer to another
  • Durability - How strong the medium is & how many times it can be reused before it fails
  • Reliability - How consistently it records and plays back data over time / retains quality
  • Cost - Relative as there are many types of any one device and medium
• Suitability of Storage Media

  • CDs & DVDs - Useful for keeping data for later use
  • Memory sticks - More effective for more regular backup
  • HDDs - Mainly found on desktop computers
  • SSDs - Frequently used for operating system & software that need to be fast, require very little power, no moving parts so don't heat up much, very thin and reliable, perfect for small portable devices with built-in storage
• Binary


  All characters on the keyboard are represented by a binary number<|>These binary numbers are patterns of 0 and 1
• Character Set


  All the characters that can be represented on a computer by a binary number<|>Or a defined list of characters represented by a binary number on a computer
• ASCII


  Is an example of a character set<|>It consists of 128 characters, using 7 bits<|>ASCII codes are alphabetically ordered, so it's possible to figure out one code from another
• Extended ASCII (EASCII)


  It consists of 256 characters, using 8 bits<|>Additional characters in the set include symbols, other languages and mathematical characters
• Unicode


  It consists of 16, sometimes 24, sometimes 32 bits<|>This exponentially increases the number of combinations possible<|>It can represent more characters including ones from all other languages, symbols and emojis