Paper 1 all notes

• Binary


  The form of data that computers use to represent all forms of data
• Any form of data needs to be converted to binary to be processed by a computer
• Data is processed using logic gates and stored in registers
• Hexadecimal

  A shorter representation of binary that is easier for humans to understand
• Overflow error


  Occurs if the value is greater than 255 in an 8-bit register
• Overflow error


  • A computer or device has a predefined limit that it can represent or store
  • Occurs when a value outside this limit should be returned
• Logical binary shift


  1. Bits shifted from the end of the register are lost and zeros are shifted in at the opposite end

  2. The positive binary integer is multiplied or divided according to the shift performed

  3. The most significant bit(s) or least significant bit(s) are lost
• ASCII


  Text is converted to binary to be processed by a computer
• Unicode

  • Allows for a greater range of characters and symbols than ASCII, including different languages and emojis
  • Requires more bits per character than ASCII
• Sound representation


  • A sound wave is sampled for sound to be converted to binary
  • The sample rate is the number of samples taken in a second
  • The sample resolution is the number of bits per sample
  • The accuracy of the recording and the file size increases as the sample rate and resolution increase
• Image representation


  • Data storage is measured in bits, nibbles, bytes, kibibytes, mebibytes, gibibytes, tebibytes, pebibytes, exbibytes

  
• Calculating file size


  1. Image file size = image resolution (in pixels) x colour depth (in bits)

  2. Mono sound file size = sample rate (in Hz) x sample resolution (in bits) x length of sample (in seconds)

  3. Stereo sound file size = sample rate (in Hz) x sample resolution (in bits) x length of sample (in seconds) x 2

  
• Data compression


  Reduces bandwidth, storage space, and transmission time
• Lossless compression


  Reduces file size without permanent loss of data, e.g. run length encoding (RLE)
• Lossy compression


  Reduces file size by permanently removing data, e.g. reducing resolution or colour depth, reducing sample rate or resolution
• Packet structure


  • A packet contains a header, payload, and trailer
  • The header includes the destination address, packet number, and originator's address
• Packet switching


  1. Data is broken down into packets

  2. Each packet could take a different route

  3. A router controls the route a packet takes

  4. Packets may arrive out of order

  5. Once the last packet has arrived, packets are reordered
• Serial transmission


  Single wire, one bit at a time, up to 100 meters, cheaper, safer
• Parallel transmission

  Multiple wires, multiple bits transmitted all at once, quicker than serial, shorter distances of around 5 meters, less safe
• Simplex


  One direction
• Full-duplex


  Both directions at the same time
• Half-duplex


  Both directions but only one direction at a time
• USB

  • Devices plugged into the computer are automatically detected and drivers are automatically loaded
  • Supports a maximum cable length of 5m, beyond that USB hubs are needed
  • Connections can only fit one way preventing incorrect connections
  • Supports different data transmission rates (from 1.5mbps to 5gbps)
  • Protocol notifies the transmitter to re-transmit data if any errors are detected, leading to error-free data transmission
  • Relatively easy to add more USB ports using hubs
  • Backward compatible
• Checksum


  A value calculated from the data using an algorithm, transmitted with the data, and recalculated by the receiver to detect transmission errors
• Check digit


  A digit added when transmitting data from a calculation performed on the digits in the data, used to detect errors in data entry and identify
• Parity checks

  • Uses the number of 1-bits in a byte, even or odd parity
  • Limitations: two bits may change during transmission without being detected, the bit(s) changed wouldn't be identified
• Parity blocks


  Overcome the limitations of parity bits by identifying the location of errors through the rows and columns
• Echo check


  The receiver sends the data back to the sender for verification, but doesn't identify if the error occurred when sending or receiving
• ARQ (Automatic Repeat Request)


  Uses acknowledgement and timeout, sender starts a timer when data is transmitted, receiver checks for errors and sends positive or negative acknowledgement, sender resends if no acknowledgement received
• Encryption


  The process of turning data into an unreadable form (ciphertext) using an algorithm, to prevent hackers and attackers from accessing the original data (plaintext)
• Symmetric encryption


  Uses the same encryption key for both encrypting and decrypting the data
• Asymmetric encryption


  Uses a public key for encryption and a private key for decryption, the public key is available to everyone while the private key is only available to the user
• CPU


  Processes instructions and data that are input into the computer so that the result can be output
• Microprocessor


  A type of integrated circuit on a single chip
• CPU units


  • Arithmetic logic unit (ALU), control unit (CU), program counter (PC), memory address register (MAR), memory data register (MDR), current instruction register (CIR), accumulator (ACC)
• Register

  Stores data, addresses and instructions temporarily
• Core

  Made up of an ALU, a control unit and the registers, the number of cores, size of the cache and speed of the clock can affect CPU performance
• Purpose of a core

  To perform a fetch-decode-execute cycle, to execute an instruction
• More CPU cores

  May increase performance as more instructions can be processed simultaneously
• Buses

  Address bus carries addresses, data bus transmits data, control bus transmits control signals