CS PAPER1

• The CPU is the central processing unit, with one CPU in a computer. Its job is to carry out program instructions using data.
• CPU components

  • ALU (arithmetic logic unit) - carries out maths and logic
  • Control unit - coordinates the fetch-execute cycle and decodes instructions
  • Cache - small, fast memory that holds frequently accessed items
  • Registers - ultra-small, ultra-fast memory where CPU workings are stored
• Memory address register (MAR)

  Holds the address in memory where data/instructions are fetched from or written to
• Memory data register (MDR)


  Holds the actual data/instructions that have been fetched from or will be written to memory
• Program counter


  Holds the address of the next instruction to be fetched
• Accumulator


  Holds the results of ALU calculations
• Von Neumann architecture


  The structure of a CPU that carries out the fetch-execute cycle
• Fetch-execute cycle


  1. Fetch instruction from memory using program counter

  2. Decode instruction in control unit

  3. Execute instruction, usually using ALU and accessing data
• Clock speed


  The rate at which the fetch-execute cycle is carried out, in GHz (billions of cycles per second)
• CPU characteristics


  • Clock speed
  • Number of cores
  • Cache size
• Embedded systems


  Computers built into other devices to perform a specific function
• Types of storage


  • Primary (RAM, ROM)
  • Secondary (magnetic, optical, solid state)
• RAM


  Volatile memory that stores programs and data while the computer is running
• ROM


  Non-volatile memory that contains the program to boot up the computer
• Virtual memory


  Part of secondary storage used as an extension of RAM when it is full
• Types of secondary storage

  • Magnetic (hard disk drives)
  • Optical (DVDs)
  • Solid state (SSDs)
• Characteristics of secondary storage types

  • Magnetic - reliable, large capacity, not durable
  • Optical - portable, reliable, not durable
  • Solid state - fast, durable, expensive
• Binary units


  • Bit
  • Nibble
  • Byte
  • Kilobyte
  • Megabyte
  • Gigabyte
  • Terabyte
• Converting between binary, hexadecimal and decimal

  1. Use place value tables

  2. Binary - each column is 2x previous

  3. Hexadecimal - each column is 16x previous
• Binary addition rules


  • 1 + 1 = 0, carry 1
  • 1 + 1 + 1 = 1, carry 1
• Binary shifts


  • Left shift - doubles the number
  • Right shift - halves the number
• Overflow error - when a result is too big to fit in the available storage space
• Character sets


  • ASCII - 8-bit codes for 256 characters
  • Unicode - 16-bit+ codes for thousands of characters
• Pixels


  Blocks of colour that make up an image
• Image resolution


  The number of pixels in an image, expressed as width x height
• Color depth


  The number of bits used to represent the color of each pixel
• Metadata


  Data about data, stored with an image or sound file
• Analog to digital conversion

  Sampling the amplitude of a sound wave at regular time intervals, determined by the sample rate
• Bit depth


  The number of bits used to represent each sample in a sound file
• Compression


  Reducing file size, either losslessly or with some loss of quality
• Network types


  • LAN (local area network)
  • WAN (wide area network)
• Client-server network


  Devices are either clients (request data) or servers (provide data)
• Peer-to-peer network


  Devices can act as both clients and servers, connecting directly
• Network topologies


  • Star
  • Mesh
• Star topology


  • Central device all other devices connect to, easy to add new devices
• Mesh topology


  • Direct connections between all devices, high performance but complex
• Network hardware


  • WAP (wireless access point)
  • Router
  • NIC (network interface card)
  • Switch
• MAC address


  Unique hardware address used to identify devices on a network
• Bandwidth


  The maximum amount of data that can be transferred at any given time on a network
• MAC address


  A unique address worldwide specific to hardware that enables you to pinpoint the correct device on your network