computer science paper 1 (1)

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

  • ALU (carries out maths and logic)
  • Control unit (coordinates fetch-execute cycle and decodes instructions)
• Cache

  Small, fast memory that holds frequently accessed items (instructions, data, addresses)
• Registers

  Ultra small, ultra fast memory where CPU workings are stored
• 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, fetch data if needed, write results
• Clock speed

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

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

  Computers inside other devices, with one specific purpose
• Primary storage


  • RAM
  • ROM
• Secondary storage

  Non-volatile storage for programs and data, slower than RAM
• Virtual memory

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

  Read-only memory, contains program to load operating system on startup
• Secondary storage types

  • Magnetic (HDDs)
  • Optical (DVDs)
  • Solid state (SSDs)
• Secondary storage evaluation

  • HDDs: reliable, large capacity, not durable
  • Optical: portable, reliable, not durable
  • SSDs: fastest, durable, more expensive
• Binary units

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

  1. Carry 1 to next column for 1+1

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

  1. Left shift: double number

  2. Right shift: halve number, lose least significant bit
• Overflow error can occur if result is too large for storage
• 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 (width x height)
• Color depth

  Number of bits per pixel
• Metadata


  Data about data, essential information to recreate an image
• Digital sound

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

  Number of bits per sound sample
• Compression

  Reducing file size, lossy (quality loss) or lossless
• LAN

  Local area network over a small area
• WAN

  Wide area network with infrastructure owned by multiple entities
• Client-server network

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

  Devices can act as both client and server
• Network topologies

  • Star: central node, easy to add devices

  Mesh: direct connections, high performance
• Network hardware

  WAP: wireless access point

  Router: forwards packets between networks

  NIC: network interface card with MAC address
• Bandwidth

  Maximum data transfer rate, shared by all connected 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

  Reduces the bandwidth available per device, increases collisions and errors that need to be resent
• IP address

  Roughly tells us where a device is geographically, in the world
• There are two types of IP addresses currently used, version 4 and version 6