Computer science paper 1

• Secondary Storage

  Non-volatile storage where programs and data are kept for later use
• Two Types of Internal Storage


  • Hard Disk Drives (HDDs)
  • Solid State Drives (SSDs)
• Hard Disk Drives (HDDs)

  • Moving parts
  • Store data magnetically on metal disks
  • Can be noisy
• Solid State Drives (SSDs)

  • No moving parts
  • Use flash memory for faster read/write times
  • Usually quiet/silent
• Flash drives & memory cards are solid state storage used to expand the capacity of small devices
• Optical discs can be read-only, write-once or rewritable
• Magnetic tape is used by organisations to store huge amounts of data
• External HDDs & SSDs are portable versions of internal storage, often used for backups
• Magnetic tape has the highest average capacity
• Magnetic tape has the lowest average cost per GB
• Flash drives & memory cards have the highest portability
• Optical discs are easily scratched and have limited rewrites
• Magnetic tape is suitable for long-term storage
• SSDs have a limited number of rewrites
• HDDs can be damaged by impacts, heat and magnets
• Five Functions of an Operating System (OS)

  • Peripheral management and drivers
  • Providing user interface
  • Memory management and multitasking
  • File and disk management
  • User management
• Peripheral management and drivers

  • Communicates with internal hardware and peripherals using drivers
  • Chooses correct drivers for connected hardware on startup
  • Installs and updates drivers automatically
• Providing user interface


  • Graphical User Interfaces (GUIs) have windows, icons, menus and pointers
  • Command-Line Interfaces are text-based and use fewer system resources
• Memory management and multitasking

  • Moves application data to main memory when in use and removes it when no longer needed
  • Allocates memory addresses so applications don't overwrite or interfere with each other
  • Divides CPU processing time between tasks to complete them efficiently
• File and disk management


  • Organises files into a hierarchical structure of folders
  • Deals with naming, saving, moving, editing and deleting files and folders
  • Splits the hard disk into sectors and decides where files are written to
  • Maintains the hard disk with utility software
• User management


  • Controls which users, and how many users, can access the computer system
  • Grants users access to specific data and resources
  • Uses anti-theft measures like password or pin protection
• Defragmentation software


  Reorganises a hard disk by putting related data back together, speeding up reading/writing files
• SSDs don't need to be defragmented as they can access fragmented files quickly
• Defragmentation process


  1. Gaps appear as files are moved, deleted, or change size

  2. OS splits new files into blocks to fill the gaps

  3. Defragmentation software puts the fragmented files back together and groups free space
• Compression software


  Reduces size of files by permanently or temporarily removing data from them
• Compressed files take up less disk space and are quicker to upload and download, but need to be extracted before use
• Encryption software


  Scrambles (encrypts) data to stop third-parties from accessing it, requiring a special 'key' to decrypt
• Strong encryption makes it almost impossible to find the key through brute force methods
• Lossy compression


  Permanently removes data from the file
• Lossless compression


  Temporarily removes data to store the file, and restores it to the original state when opened
• Lossy compression allows for a bigger reduction in file size, but loses data and reduces quality, while lossless compression has a smaller reduction in size but no loss of quality
• Character


  Uppercase and lowercase letters, digits 0-9, and symbols like ?, + and &
• Character set


  Collections of characters that a computer recognises from binary representation, used to convert characters to binary code and vice versa
• ASCII


  A 7-bit character set that can represent 128 different characters, with an extra bit added to make each character 1 byte
• Unicode


  A character set that covers all major languages, including ones that use different alphabets, using multiple bytes per character
• The first 128 characters in Unicode are the same as ASCII
• Calculating text file size

  File size (in bits) = number of bits per character x number of characters
• Bitmap image


  An image made up of lots of tiny dots called pixels, with the colour of each pixel stored using a binary code
• Image resolution


  The number of pixels in a bitmap image, given as width x height
• Colour depth


  The number of bits used to represent each pixel, determining the number of colours that can be used