Paper 1

• Fetch Decode Execute Cycle

  1. The address of the next instruction in memory held in the programme counter is copied into the MAR
  2. The address in the programme counter is incremented by one, now storing the address of the next instruction
  3. The processor sends a signal along the address bus to the MAR
  4. The instruction is sent along the data bus from the MAR to the MDR
  5. The instruction is copied to the CIR
  6. The instruction is decoded and then executed
  7. Results are stored in the ACC
  8. The cycle repeats
• CPU components

  • Control Unit - controls Fetch - Decode - Execute cycle
  • Arithmetic Logic Unit - arithmetic and logical operations
  • Cache - high speed RAM in the processor
  • Register - small high speed memory. Eg. in Fetch - Decode - Execute cycle
• Cache memory


  Level 1 - fast but small

  Level 2 - slower but bigger
• Register's purpose


  • Program Counter - address of next instruction
  • Memory Address Register - address of current instruction to be fetched OR address for data to be transferred to
  • Memory Data Register - Holds data found at the address previously stored in MAR OR data to be transferred to primary memory
  • Current Instruction Register - holds the instruction currently being decoded or executed
  • Accumulator - holds data being processed and the results
• Optical Storage

  Lands (reflection) = 1

  Pits (no reflection) = 0

  Types:

  • ROM - Read only
  • R - read / write once
  • RW - read / write
• Common storage sizes

  CD -> 640MB

  DVD -> 4.7GB

  Blu-ray -> 50GB

  USB -> 2GB - 2TB

  HDD -> 500GB - 12TB

  SSD -> 256GB - 4TB
• Character sets


  ASCII

  • 7 bits
  • 128 characters in total
  • 32 control codes for printing
  • 52 letters,
  • 32 items of punctuation
  • 10 numbers

  Extended ASCII

  • 8 bits
  • Adds accents and other special symbols

  Unicode

  • 16 bits
  • All languages
• Sound sampling


  Sample rate - number of samples (snippets of sound amplitude) at a given time in Hz. 44.1KHz is normal

  Bit Depth - accuracy of amplitude within samples. 16 to 24 bits in normal

  Bit Rate - Sample rate * Bit depth = bits/second
• PDF and GIF are lossless file formats
• Typical internet speeds

  • Copper - 10Gb/s
  • Fibre - 100Gb/s
  • (home) Wi-Fi - 600 Mb/s
  • (business) Wi-Fi - 6Gb/s
• Comparing types of internet medium

  COST MAX SPEED MAX DISTANCE

  COPPER cheap 1Gb/s 100m

  FIBRE expensive 40Tb/s+ 2Km+

  Wireless range: 50m
• Internet


  Eg. in bbc.co.uk:

  • Top level domain is .uk - generic
  • 2nd level domain is .co - type of organisation
  • 3rd level domain is bbc - organisation name

  
  

  The DNS converts the web name (the URL) to an address (4 sets of 3 digits)
• Bluetooth runs at 2MB/s
• Asymmetric encryption: public key encrypts; whilst private key decrypts
• Addressing and Protocols


  Addressing Protocols:

  IP:

  • Static IP stays the same whilst a dynamic IP changes allowing more devices to be connected than there are addresses available
  • IPv4 has 4 sets of numbers up to 255 - 4.3bn combinations
  • IPv6 has 8 sets of 4 digit hex - 340 nonillion combinations

  MAC:

  • Media Access Control
  • Only has static addresses
  • Each NIC is assigned an MAC code denoted by a hex string
• Addressing and Protocols

  Other protocols:

  • TCP/IP - enables communication over the internet
  • HTTP/HTTPS - hypertext transfer protocol - governs communications between web server and client
  • FTP - transmission of files
  • SMTP - Simple mail transfer protocol - emails from network to mail server
  • POP and IMAP - Post Office protocol, replaced by Internet message access protocol - emails from mail servers
• Network Layering


  Allows standards and adaption without modifying the entire protocol

  TCP/IP model:

  • Application layer - encodes/decodes message
  • Transport layer - splits and reassembles packets
  • Network layer - adds sender's and recipient's IP address
  • Link layer - enables transfer of packets
• Memory management method:

  Paging:

  • memory is split into equal sizes pages - usually 4KB
  • A programme is allocated certain pages
• Impact on Society - Ethical Issues


  • Ensuring public safety
  • Data security: malware could be used to access data
• Impact on Society - Legal Issues


  • Data could be illegally shared
  • Financial information could be stolen
  • Films etc. could be illegally copied and distributed
  • information could be used to extort/blackmail people
  • Data Protection Act 1998, updated to GDPR in 2018 (general data protection regulation)
  • Computer Misuse Act 1990
  • Copyright, designs and patents Act 1988
  • Creative Commons
  • Freedom of Information Act 2000
• Impact on Society - Cultural Issues


  • Digital divide: old vs young; poor vs rich
  • Those who are less technically literate may struggle to find employment
  • Changing nature of employment: work from home; call centres abroad; automation of low-skilled jobs