Computing Paper 1

• • Memory address copied from Program Counter to MAR
  • Instruction fetched from address in the MAR and copied to MDR
  • Program counter increases increment
  • Decode - Fetched instruction is decoded by the control unit
  • Decoded instruction is executed
• MAR - Holds memory address about to be used by CPU
• MDR - Holds data being transferred between CPU and RAM
• Program Counter - Register that holds the memory address of instruction for each cycle
• Accumulator - Register that stores results of ALU calculation
• Arithmetic Logic Unit carries out the mathematical and logical operations
• Control Unit executes instructions and controls the flow of data in the CPU
• Cache is very fast memory that stores regularly used data so CPU can access it quickly
• Registers are tiny, super fast memory to temporarily hold data needed by the CPU
• Primary storage is needed to temporarily store the data the computer needs - RAM and ROM
• RAM is volatile, and is used to store data currently in use by CPU. It is read and write memory
• ROM is non-volatile, and is used to store BIOS which contains startup instruction for CPU. It is read only memory
• Secondary Storage is permanent storage for programs and data when not being used by CPU. Hard drives, SSD's, USB sticks etc.
• Virtual memory is secondary storage that is used to temporarily store data when the RAM is full
• Magnetic Storage (Secondary)

  • Reliable
  • High Capacity
  • Cheap
  • Longer Life
  • Slow
  • Not portable
• Examples of Magnetic Storage are Hard Disk and Magnetic Tape
• Solid State Storage: SSD, USB, Memory Card

  • Fast
  • Quiet
  • Don't need defragmenting
  • Expensive
• Optical Storage: CDs

  • Portable
  • Waterproof
  • Shockproof
  • Slow
  • Low Capacity
• Units of Data Storage:

  
• Overflow is where the result of a calculation requires more bits than expected - (When calculation should be 8 bits and you get 9 bit result)
• ASCII uses 8 bits, which only gives 256 possible characters
• Unicode uses 16 bits, that allow for 65,536 possible characters
• File size (in bits) = number of characters x bits per character
• Total number of colours = 2^n

  n = colour depth of image
• file size (in bits) = length x width x colour depth
• Metadata is data stored within an image

  • Length
  • Width
  • Colour depth
  • File format
• • Sampling is where analogue signals (waves) are converted to digital
  • Works by taking amplitude of sound wave at regular intervals (depending on sample rate) and plotting it digitally
  • This means that a digital representation of the sound wave has been created
• Improve sound file by:

  • Increasing sample rate: More samples taken per second meaning more accurate to original flow
  • Increasing bit depth: More bits per pixel means more possible points for sound waves to be at so closer to original
• Increasing sound file means

  • Better quality and closer to original
  • Higher file size (Disadvantage)
• Lossy compression permanently removes data, which reduces size more, leading to lower quality. Doesn't work on all files
• Lossless compression temporarily removes data but restores the data when file opened. Reduces size less but better quality and works on all files
• Advantages of Compression:

  • Takes up less file size
  • Less bandwidth used so files can be downloaded faster
  • Websites load faster
  • Match website file restrictions
• Disadvantages of "Lossy" Compression:

  • Can't return to original
  • Can't be used on all files
• Factors affecting Network Performance:

  • Bandwidth is split between users - more users make it worse. Low bandwidth generally is bad as less data is transferred
  • Signal strength - greater distance, makes it worse. Interference makes it worse
  • Connection type - Wired or wireless. Wired has stronger connection
  • Hardware - Fibre optic is better than twisted pair
• • Switch: Used to connect devices on a LAN. Sends incoming data to the correct device based on MAC address
  • Router: Sends data across network in packets
  • NIC: Hardware needed to connect to a network
  • Transmission Media: Wireless (Wifi, bluetooth) or Wired (Fibre Optic etc)
  • Wireless Access Point: Created a wireless network by sending out radio waves
• DNS converts website URL into IP address by:

  • User enters URL into address bar
  • Browser sends request for the URL's matching IP address to the DNS server
  • DNS searches for it
  • DNS returns matching IP address to web browser so you can access it
• The cloud is where servers can be used to store data and programs which can be accessed and used over the Internet
• Pros of Cloud:

  • Access files from any device with WIFI
  • Access files from anywhere
  • No need to buy pricy hardware
  • Cheaper to upgrade storage
• Cons of Cloud:

  • Monthly fee
  • Susceptible to hackers
  • All depends on host for security
  • Need WiFi to access
• Modes of Connection