Gcse ocr computer science

• The CPU is the central processing unit, it carries out program instructions using data
• Components inside the CPU

  • ALU (Arithmetic Logic Unit)
  • Control Unit
  • Cache
  • Registers
• ALU

  Carries out maths and logic needed to execute instructions
• Control Unit

  Coordinates the fetch-execute cycle and decodes instructions
• Cache

  Small, fast memory that holds frequently accessed items
• Special purpose registers

  • Memory Address Register (MAR)
  • Memory Data Register (MDR)
  • Program Counter
  • Accumulator
• Fetch-Execute Cycle

  1. Fetch instruction from memory using Program Counter

  2. Decode instruction

  3. Execute instruction, possibly fetching data
• Clock speed

  Rate at which the fetch-execute cycle is carried out, in GHz
• CPU performance factors

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

  Computers built into other devices, with one specific purpose
• Types of storage

  • Primary (RAM, ROM)
  • Secondary (HDD, SSD, optical)
• RAM

  Volatile memory, contents lost when power is off
• ROM

  Non-volatile memory, contains startup program
• Secondary storage types

  • Hard disk drives: reliable, large capacity, fragile
  • Optical discs: portable, reliable, fragile
  • Solid state drives: fast, durable, expensive
• Binary units

  • Bit
  • Nibble
  • Byte
  • Kilobyte
  • Megabyte
  • Gigabyte
• Binary addition

  1. Carry 1 to next column for 1+1

  2. Carry 1 for 1+1+1
• Binary shifts

  1. Left shift: double the number

  2. Right shift: halve the number
• Overflow error

  Result takes up more storage space than is possible
• Character sets

  • ASCII: 8-bit, 256 characters
  • Unicode: 16-bit+, supports many languages
• Pixels

  Blocks of color that make up an image
• Image resolution

  Number of pixels, e.g. 3x3
• Color depth

  Number of bits per pixel
• Metadata

  Data about data, e.g. camera details for an image
• Digital sound

  Samples of sound wave amplitude at regular time intervals (sample rate)
• Bit depth

  Number of bits per sound sample
• Compression

  Lossy: deletes data, reduces quality

  Lossless: rearranges data, no quality loss
• Network types

  • LAN (Local Area Network)
  • WAN (Wide Area Network)
• Client-server network

  Clients request from servers, servers respond
• Peer-to-peer network


  Devices can act as both clients and servers
• 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, complex but high performance
• Network hardware

  • Wireless access point
  • Router
  • Network interface card (NIC)
  • Switch
• MAC address

  Unique hardware address for identifying devices on a network
• Bandwidth

  Maximum data transfer rate, shared between all devices
• MAC address

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


  • A bit like a router but within a single network, uses MAC addresses to make sure packets go to the correct destination
• Bandwidth


  The maximum amount of data that can be transferred at any given time
• Adding more devices to a network


  Requires sharing the limited bandwidth, leading to slower transmission and more collisions
• IP address


  Roughly tells us where a device is geographically, in the world