Memory and storage

• RAM


  Random access memory
• ROM


  Read-only memory, purpose is to store the computer's boot program
• Volatile


  Refers to memory that loses its contents when power is turned off
• CPU


  Central processing unit, the 'brain' of the computer
• Storage tiers


  • Primary
  • Secondary
  • Tertiary
• Optical storage is in decline


  Reason: unknown
• Cache memory


  Used next to the CPU to provide very fast access to data, acts as a buffer between RAM and CPU
• When data is requested

  It first checks cache, then RAM, then hard disk
• Virtual memory


  Part of the hard drive used as an extension to RAM when there is not enough RAM to hold all the data and programs
• When a computer runs out of RAM

  It uses virtual memory
• Accessing hard disk


  Significantly slower than accessing RAM
• Relying heavily on virtual memory


  Causes significant performance drop, called 'disk thrashing'
• Flash memory


  Type of ROM that can be re-written, used to store boot programs and operating systems
• Flash memory devices range from a few GB to 256 GB
• Access times for flash memory

  Faster than hard disks but slower than RAM
• Memory types from slowest to fastest

  • Hard Drive
  • Flash Memory
  • Virtual Memory
  • ROM
  • RAM
  • Cache
• Virtual memory is using part of the hard disk as if it is memory
• Virtual memory is needed when the amount of physical RAM is not large enough to hold all the open programs and data in use
• The problem with using virtual memory is that instructions can only be executed from the cache and RAM, so they must be transferred back from virtual memory before they can be used, making the computer slower
• Example of virtual memory in use

  • Operating system and presentation app open, then graphics app opened but doesn't fit in RAM so moved to virtual memory
  • Presentation app moved to virtual memory to make space in RAM for graphics app
  • Graphics app moved to virtual memory when switching back to presentation app
  • Doubling RAM size reduces need for virtual memory, improving performance
• A computer's operating system uses 6GB of virtual memory
• Virtual memory


  Moves the least recently used instructions and data from main memory to virtual memory to allow other data to be loaded into main memory
• Advantage of virtual memory

  Allows for continuous operations even when the computer's main memory is full
• Disadvantage of virtual memory

  Computer's performance is compromised as it is much slower to access data
• Types of secondary storage

  • Hard Drive
  • Solid State Drive
  • Optical
  • Magnetic Tape
  • Flash (USB Stick)
• Read/Write


  Reading data from a storage device, writing/saving data to a storage device. This is normally used when referring to the speed of which a device can read/write information.
• Hard Disk Drive


  • Generally reliable and long lasting
  • Dropping them will cause damage
• Optical storage


  • DVD
  • CD
  • Blue-ray
• SSD


  Solid State Drive
• HDD


  Hard Disk Drive
• The CPU in the computer fetches data and instructions from memory to process
• Primary memory


  The only memory the CPU can access directly
• Types of primary memory

  • RAM (Random Access Memory)
  • ROM (Read Only Memory)
• RAM


  • Memory that can be accessed and changed by the computer at any time
  • Stores the programs and data that the computer is currently working on
  • Needs power to store data and is volatile
  • Fast processing
• What RAM may contain

  • Operating system
  • Music player software
  • Music file
  • Web browser software
  • Website
  • Word processor software
  • Word-processed file
  • Anti-virus software
• RAM


  Random access
• ROM


  • More complex form of electronic circuit that does not require power to maintain the contents and is non-volatile
  • Information on a ROM chip is often programmed at manufacture
  • Contains the computer's BIOS (Boot Input/Output System)
  • Medium speed
• ROM


  Used to store the boot program, a sequence of instructions to start the computer and load the operating system
• ROM cannot be overwritten by the computer, hence Read Only Memory
• Operating system


  Programs and data in use by the CPU