memory and storage

• Primary storage


  • Random Access Memory (RAM)
  • Read only memory (ROM)
  • Registers
  • Cache
• RAM


  Holds the operating system, programs and data in use by the CPU when the computer is running, volatile (contents lost when the power is turned off), read and write, large in comparison to ROM
• ROM


  Holds the first instructions for when the computer is first turned on known as the bootstrap, programs may be stored in ROM in embedded systems, non-volatile (contents remain when power is turned off), read only, small in comparison to RAM
• Virtual memory


  Needed when there is not enough physical RAM to store the open programs, held on the hard disk, programs are transferred out to virtual memory from RAM when they are not currently being executed, programs are transferred back to RAM from virtual memory when they are needed
• Secondary storage types


  • Optical (CD-R/RW, DVD-R/RW, Blu-Ray)
  • Magnetic (Hard disk drive, tape)
  • Solid state (SSD, Memory sticks, Flash memory cards)
• Optical storage


  • Low capacity compared to other types, slow to access data, thin lightweight and portable
• Magnetic storage


  • High storage capacity, quick to access data, has moving parts which eventually fail, hard disks perform better if they are defragmented
• Solid state storage


  • Medium storage capacity, very quick to access data, no moving parts, very reliable, no noise, low power, no need to defragment, limited number of read/write cycles, expensive compared to other types of storage
• Adding binary numbers


  1. If there's an odd number of ones put a one

  2. If there's an even number of ones carry one

  3. If there's 1 one put one and don't carry anything
• Character set


  A defined list of characters recognised by the computer, each represented by a unique binary number
• Well-known character sets


  • ASCII - 7-bit, 128 characters
  • Extended ASCII - 8-bit, 256 characters
  • UNICODE - 24-bit, over 16 million characters
• Sound file size


  Total number of bits = (no. samples per second) x (no. bits per sample) x (length of sample in seconds)
• Bit depth


  Number of bits stored per sample, higher = higher quality and larger file size
• Sample rate

  Number of samples stored per second, higher = higher quality and larger file size
• Bitmap


  Images constructed from coloured squares called pixels
• Vector pictures


  Store the mathematics to draw coloured shapes
• Colour depth


  Number of bits for each pixel, determines number of possible colours (2^n)
• Metadata


  Additional data stored with the image to define width, height, colour depth, date, geolocation, file size, file type, compression type and author
• Greater colour depth and resolution

  Larger file size of the image
• Photographs are stored in 24-bit colour depth, over 16 million colours
• Compression


  Reducing the number of bits in a file, making the file size smaller and data transfer quicker
• Lossy compression


  • Some data is lost and cannot be recovered
  • Greatly reduces file size
  • Reduces quality
  • Suitable for images, sound and video
  • Cannot be used on text and executable files
• Lossless compression


  • No data is lost, just encoded differently
  • Can be turned back into original format
  • Can be used on all types of data
  • Usually less effective at reducing file size
  • Most suitable for documents and executable files
  • Reduced sound/image quality