1.2 memory and storage

• Primary storage


  Computer system needs this for any data that it needs to access quickly, including start-up instructions, operating system, running programs and associated data
• Types of primary storage

  • RAM
  • ROM
• RAM


  Holds operating system, running applications and associated data while computer is on and in use. Volatile - loses data when power is turned off. Can be read from and written to.
• ROM


  Stores the bootstrap loader/BIOS that helps 'boot' the computer. Non-volatile - does not require power to maintain contents. Read-only.
• Using virtual memory


  1. Allocate section of secondary storage to temporarily act like RAM

  2. Move data in RAM that is not currently required by CPU into secondary storage

  3. Move data back from secondary storage into RAM when required by CPU
• Using virtual memory


  Slows down computer performance
• Adding more RAM


  Reduces need for virtual memory, improving computer performance
• Secondary storage


  • Non-volatile, low cost, high capacity, reliable
• Magnetic storage (HDDs)


  Made of stack of magnetic disks (platters) that rotate, with moving read/write head. Reliable, cost effective, high capacity but not very portable.
• Solid-state storage (SSDs)


  Use flash memory, no moving parts. Faster access than HDDs, low power, robust, compact but lower capacity and higher cost.
• Optical storage


  Use properties of light to store data on rotating optical disks (CDs, DVDs, Blu-Ray). Easily damaged by mishandling and scratches.
• Bit


  Binary digit, stored state of a computer switch (on or off)
• Nibble


  4 bits
• Byte


  8 bits
• Storage units


  • Nibble
  • Byte
  • Kilobyte (KB)
  • Megabyte (MB)
  • Gigabyte (GB)
  • Terabyte (TB)
  • Petabyte (PB)
• Calculating required data capacity


  1. Add up estimated file sizes

  2. Add 10% for overheads
• Denary number range


  0 - 255
• Hexadecimal range


  00 - FF
• Binary number range


  00000000 - 11111111
• Left shift


  Same as multiplying by 2
• Right shift


  Same as dividing by 2
• Overflow error


  Carry bit/left over bit when adding binary numbers
• Most significant bit (MSB)


  Leftmost digit in binary number, represents 128 in decimal
• Least significant bit (LSB)


  Rightmost digit in binary number, represents 1 in decimal
• ASCII


  1. bit code representing English alphabet, numbers, some symbols and control characters (128 characters)
• Extended ASCII


  1. bit code including extra symbols, mathematical symbols and some non-English characters (256 characters)
• Unicode


  Originally 16-bit code representing many additional non-English characters and symbols (65,536 characters), now extended to use even more bits
• Pixels


  Stored in computer as binary codes, number of bits per pixel determines number of colours
• Image metadata


  • Colour depth
  • Resolution
• Higher colour depth


  Larger number of colours represented, better quality image but larger file size
• Higher resolution


  More pixels used, better quality image but larger file size
• Sound


  Analogue vibrations sampled at regular intervals and stored as digital binary values
• Sample rate


  Number of samples taken per second, measured in Hz
• Bit depth


  Number of bits used to store each sample
• Higher sample rate and bit depth


  Better approximation of original sound but larger file size
• Lossy compression


  Some data removed to make file smaller, original file cannot be restored
• Lossless compression


  No data removed, algorithms find patterns to reduce repeated data, original file can be restored