Computer Science 1.1 -

• Secondary Storage


  Memory areas that the CPU cannot access as quickly as primary storage, including USB drives, CDs, hard drives, etc.
• Primary storage


  Memory areas that the CPU can access very quickly, including CPU registers, cache, ROM and RAM
• Two Main Tiers of Storage

  • Primary storage
  • Secondary storage
• Secondary storage


  • Non-volatile - it's where all data (operating systems, applications and user files) are stored when not in use
  • Includes magnetic hard disk drives, solid state drives, CDs and SD cards
  • Read write speeds are much slower compared to primary storage
• Hard Disk Drives (HDDs)


  • Traditional internal storage in PCs and laptops
  • Made up of a stack of magnetised metal disks that spin thousands of times a second
  • Data is stored magnetically in small areas on the disk's circular tracks
  • A moving arm can access these areas and read or write data
• Optical Discs


  • CDs can hold around 700 MB of data, DVDs can hold 4.7 GB and Blu-rays can hold around 25 GB
  • Come in three forms: read-only, write-once, rewritable
  • Use is declining due to increased internet speeds and streaming/download services
• Magnetic Tapes


  • Have much greater storage capacity than HDDs and an extremely low cost per GB
  • Often used by large organisations to store huge amounts of data in archive libraries
  • Come in plastic cassettes and require a special tape-drive for reading/writing
  • Read/written sequentially, so slow when finding specific data but fast once in the correct place
• Solid State Drives (SSDs)


  • Storage devices with no moving parts, used for the same purpose as HDDs
  • Most use a type of flash memory
  • Have significantly faster read/write times than HDDs
  • Can give much quicker times for booting up and opening programs and files
• Other types of flash storage


  • USB pen drives
  • Memory cards (e.g. SD cards)
• Advantages of HDDs


  • Cheaper
  • Higher capacity
  • Longer read/write life than SSDs
• Advantages of SSDs


  • Faster
  • Don't need defragmenting
  • More shock-proof than HDDs
  • Silent operation
• Optical discs are cheap and robust secondary storage
• Magnetic tapes are used for archiving by large organisations
• SSDs are fast and reliable secondary storage
• HDDs are high-capacity, reliable storage
• There are advantages to using both HDDs and SSDs
• Secondary storage is non-volatile - it's where all data (operating systems, applications and user files) are stored when not in use
• Primary storage refers to the memory areas that the CPU can access very quickly
• Secondary storage includes magnetic hard disk drives, solid state drives, CDs and SD cards
• Read/write speeds are much slower on secondary storage compared to primary storage
• On-splatile memory


  Memory that can only be read, not written to
• ROM

  • Comes on a small, factory-made chip built into the motherboard
  • Contains all the instructions a computer needs to properly boot up
• BIOS


  The instructions that tell the CPU to perform self checks and setup the computer
• What happens when the computer is powered on

  1. CPU reads the instructions from ROM

  2. Performs self checks and sets up the computer

  3. Copies the operating system into RAM
• Although the CPU can only read ROM, it is possible to update (flash) the BIOS on a ROM chip
• RAM


  Where the computer puts everything it's working on
• A 2 TB hard drive does not mean the computer has 2 TB of memory
• Clock speed


  The speed at which the CPU operates
• CPU


  • The higher the clock speed, the more instructions it can process per second
  • Some CPUs have multiple cores which can process multiple instructions simultaneously
  • But higher clock speeds can also lead to increased power consumption and heat generation, potentially causing damage
• Cache


  Very fast memory in the CPU that stores regularly used data so the CPU can access it quickly
• Levels of cache memory

  • L1 (quickest but lowest capacity)
  • L2 (slower than L1 but higher capacity)
  • L3 (slower than L2 but highest capacity)
• Registers


  Tiny bits of data needed by the CPU that are super quick to read/write to
• Key CPU registers


  • Program counter
  • Memory address register (MAR)
  • Memory data register (MDR)
  • Accumulator
• Fetch-Execute Cycle


  1. Fetch instruction from memory

  2. Decode instruction

  3. Execute instruction
• The Von Neumann architecture describes how most computers work, with the CPU executing a program stored in memory
• Embedded systems are computers built into other devices like dishwashers and TVs, usually dedicated to a single task
• Embedded systems are often used as control systems to monitor and control machinery
• Embedded systems are usually easier to design, cheaper to produce, and more efficient at their task than a general purpose computer
• A computer system consists of hardware and software that work together to process data and complete tasks
• Hardware is the physical components of a computer, while software is the programs and applications that run on the hardware