1.2 Memory and Storage

• Primary Storage (Memory)


  The "main memory" of a computer system, used when the computer system is actually being used, temporary and must be high performance
• A computer system uses primary storage to hold data and programs that are currently being used and processed
• Most primary storage is volatile, which means that its contents are lost when the power is turned off
• Random Access Memory (RAM)

  A temporary store of instructions and data in use by the CPU for currently executing programs

  
• Programs and data are loaded from the hard disk/solid state storage to RAM for processing

  
• RAM is volatile, which means that the contents are lost when the power is turned off
• RAM is read and write capable, which means that you can store new data on it and also read data from it
• Read Only Memory (ROM)


  A form of memory in a computer system that holds the instructions that are executed when the computer is first turned on, also known as the 'bootstrap loader'
• ROM is non volatile, which means that the contents remains when the power is turned off
• ROM is read only, which means that it can only be read from and the contents are not changed
• The main purpose of ROM is to hold the boot-up program, but it does also have other uses
• Differences between RAM and ROM

  • RAM is volatile, ROM is non-volatile
  • RAM is read and write capable, ROM is read only
• Virtual Memory


  A special technique used by modern operating systems to overcome the limitation of running short of RAM

  
• Virtual memory acts as RAM when RAM is full

  
• When you add more RAM

  The computer is less reliant on virtual memory, and the computer's performance will increase (it will be faster)
• Virtual memory has a slower access speed because the hard drive or other primary storage device has a slower access speed than RAM, and all data that is in Virtual Memory needs to be loaded into RAM first before it can be processed by the CPU
• Advantages of virtual memory include allowing more programs to run even when RAM is full, and allowing very large programs to run by loading only one section of the program at a time
• Secondary Storage


  An important component of computer systems used to store programs and data when the computer is not actively using it
• Secondary storage is needed to store programs and data when the computer is not turned on, or when it is running a different program
• RAM is volatile, meaning it loses its contents without a power source, so a secondary storage device is needed to store data and program files when the power supply is turned off
• Hard Drive


  • Metal coated disks which spin at 7200 revolutions per minute, with a magnet used to move the disk head across the disk
• Accessing data on a secondary storage device is considerably slower than accessing data that is stored in primary storage (RAM)
• Optical Storage


  Uses a laser beam to melt a silver coated plastic disk, with a binary 1 represented by a small hole and a binary 0 represented by no hole
• The main optical storage formats are CDs, DVDs and BluRay which all work in a similar way but have different properties which enable more data to be stored

  
• Disadvantages of optical storage include the disks can be easily scratched and degrade over time, they have a limited capacity, and there are sometimes compatibility issues between different formats
• Magnetic Storage


  Stores data using magnetic signals stored on magnetic surfaces, such as magnetic cassette tape or solid disk drives
• Tape drives are still used occasionally as a backup device
• Disadvantages of magnetic storage include a limited life, they can be easily damaged, they are physically large, and they have complex moving parts
• Solid State Storage


  Uses a special form of RAM style chip to store the data, which can retain its data even when the power is turned off, and can operate at extremely high speeds

  
• Solid state devices include solid state drives, memory cards, USB pen drives, and mobile phone SIM cards

  
• Currently, solid state devices are more expensive than magnetic and optical devices
• Characteristics of Storage Devices

  • Capacity
  • Speed
  • Portability
  • Durability
  • Reliability
  • Cost
• When choosing a storage device for a specific application, you need to consider the six characteristics of capacity, speed, portability, durability, reliability and cost
• Bit


  One single on or off signal
• Nibble


  Four bits
• Byte


  Eight bits, or two nibbles

  
• Kilobyte


  1024 bytes, or two to the power 10
• Larger Data Units


  • Kilobyte
  • Megabyte
  • Gigabyte
  • Terabyte
  • Petabyte
  • Exabyte
  • Zettabyte
  • Yottabyte
• You must be able to convert between the larger data units
• Bit


  One single on or off