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• CPU


  Fetch-Decode-Execute cycle
• CPU components


  • ALU (Arithmetic Logic Unit) performs mathematical and logical operations
  • CU (Control Unit) controls the flow of data and the fetch-decode-execute cycle
  • Cache: Very high-speed memory that stores recently used instructions and data
  • Registers: storage locations within the CPU, with specialised roles

  
• Von Neumann architecture


  • PC (Program Counter) stores the address of the next instruction
  • MAR (Memory Address Register) stores the address of the next instruction
  • MDR (Memory Data Register) stores the instruction or data that has just been fetched
  • CIR (Current Instruction Register) stores the instruction that the CU is currently executing
  • Accumulator stores the result of a maths or logical operation

  
• Primary storage (RAM)


  • Stores programs that are running and the data they are using
  • Volatile (loses its contents when the power is turned off)
• ROM


  • Non-volatile
  • Stores start-up programs including BIOS and Bootstrap Loader
• Virtual memory


  • Part of secondary storage/hard disk that behaves like main memory
  • Parts of programs are swapped into main memory/RAM when needed
• Secondary storage


  Long-term, non-volatile storage of files
• Types of secondary storage


  • Optical
  • Magnetic
  • Solid state
• Optical storage


  Uses lasers to write and read to CDs, DVDs and Blu-ray disks
• Magnetic storage


  • Data is stored by magnetising a metal disk
  • Includes hard disk drives (HDDs), floppy disks and magnetic tape
• Solid state/flash storage


  • No moving parts
  • Includes solid state drives (SSDs), USB memory sticks and SD cards
• Magnetic storage devices


  • HDD
  • Floppy disks
  • Magnetic tapes
• Optical storage devices


  • CD
  • DVD
  • Blu-ray disks
• Solid state/flash storage devices


  • SSD
  • USB memory stick
  • SD cards
• Bit


  Smallest unit, just one 0 or 1
• Nibble


  4 bits, half a byte
• Byte


  8 bits
• Kilobyte (KB)


  1000 Bytes
• Megabyte (MB)


  1000 Kilobytes
• Gigabyte (GB)


  1000 Megabytes
• Terabyte (TB)


  1000 Gigabytes
• Binary


  Numerical system using only 0 and 1
• Converting Denary to Binary


  1. Write in the column headings

  2. Add up column headings where the value is 1

  
• Converting Binary to Hex

  1. 8 bits can be written as 2 hex digits

  2. The first hex digit represents groups of 16, the second hex digit represents the units left over

  3. Each group of four bits represents a number 0 to 15
• Binary shifts


  Shifting bits left or right

  
• Character set


  A group of symbols that can be represented by a computer
• ASCII


  A character set that uses 8 bits to encode each character
• Unicode


  A character set that uses at least 16 bits to encode each character
• Pixels


  The image is made up of pixels, each with a unique colour number stored in binary
• Colour depth


  The number of bits used to represent the colour of a pixel
• Resolution


  The concentration of pixels, the number of pixels across x the number of pixels down
• Analogue sound wave


  Sampled at regular intervals, the height of the wave is measured and stored as binary numbers

  
• Bit depth


  The number of bits used to store the value of each sound sample
• Sample rate


  The number of times per second that the sound wave is measured
• Compression


  Reduces the file size to save storage space and make transmission quicker
• Lossy compression


  The file size is reduced and some detail is lost, the original cannot be restored
• Lossless compression


  The file size is reduced but no detail is lost
• Factors affecting network performance


  • Number of devices connected
  • Bandwidth
  • Distance from wireless access point
  • Obstacles in the way of wireless signals
• Hardware needed for a LAN


  • WAP (Wireless Access Point)
  • Router
  • Switch
  • NIC (Network Interface Card)
  • Transmission media (copper cables, fibre optic cables, Wi-Fi, Bluetooth)
• The Internet


  • A global WAN using the TCP/IP protocol stack
  • DNS (Domain Name Server) converts URLs to IP addresses
  • Hosting is when a website is on a third-party webserver
  • The Cloud is remote servers owned by a service provider