Paper 1

• CPU


  Central Processing Unit - the main component of a computer that takes data, executes it, and helps make the system work
• Components of a computer

  • CPU
  • Memory
  • 1+ input/output devices
• Purpose of CPU


  To take data and execute it to help make the system work
• Common CPU components and functions

  • Arithmetic Logic Unit (ALU) - carries out logical operations and calculations
  • Control Unit (CU) - coordinates the CPU actions mainly in fetch-execute cycle
  • Cache - sends and receives signals from other devices
  • Registers - smaller and faster than cache memory, temporarily store memory address, instructions, data
• Fetch-execute cycle


  1. Fetch instruction - copy memory address from PC to MAR, copy instructions stored in MAR to MDR, increase PC

  2. Decode instruction - instructions in MDR are decoded by CU

  3. Execute instruction - instructions are performed
• Von Neumann architecture


  Stored program architecture created by John Von Neumann, consists of 4 registers: MDR, Accumulator, PC, MAR
• MDR


  • Holds/stores data when fetched from memory, waits to be written
• Accumulator


  • Stores all the calculations ALU makes
• PC


  • Holds the memory for the next instructions
• MAR


  • Holds address of current location/piece of data which needs to be fetched or stored
• CPU performance characteristics


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


  Determines the fetch-execute cycles per second (one cycle = 1Hz)
• Cache levels


  • Level 1 - quickest, less capacity
  • Level 2 - slower, more capacity
  • Level 3 - slowest, most capacity
• Number of cores


  More cores allow more operations to be performed in parallel
• Types of computers


  • General purpose
  • Embedded system
• General purpose computer


  Has many different tasks e.g. tablet
• Embedded system


  Has only one main task e.g. coffee machine
• Embedded systems


  • Don't need a full operating system, performance is limited, controlled with switches or buttons, reliable, easier to make, programs mainly held in ROM
• Primary storage (memory)


  Stores the program of current instructions, enables quick access
• Types of primary storage

  • RAM (Random Access Memory) - volatile, read/write
  • ROM (Read Only Memory) - non-volatile, read only
• RAM


  Stores current instructions temporarily, high speed, faster than secondary storage
• ROM


  Stores instructions to boot up the computer (BIOS), read only
• Virtual memory


  Non-physical sections of hard disk that act like RAM when there is not enough space in RAM
• Secondary storage


  Provides long-term storage of files and data, non-volatile
• Types of secondary storage

  • SSD (Solid State Drive) - no moving parts, low power, shock proof
  • HDD (Hard Disk Drive) - common, high capacity, reliable, moveable arm access
  • Optical disk - useful for archiving, read only/write only/rewritable
• SSD is faster than HDD, but HDD has higher capacity and is cheaper
• Optical disks are portable, durable, and inexpensive, but slower than SSD and HDD
• Units of data


  • Bit
  • Nibble
  • Byte
  • Kilobyte
  • Megabyte
  • Gigabyte
  • Terabyte
  • Petabyte
• Number systems


  • Denary (base 10)
  • Binary (base 2)
  • Hexadecimal (base 16)
• ASCII


  American Standard Code for Information Interchange, uses 7 bits to represent 128 characters
• Unicode


  Uses 16 bits to represent 65,536 different characters, can represent most international languages
• Pixels


  Smallest identifiable area of an image, represented by a square with a colour
• Colour depth


  Number of bits used to represent the colour of a pixel (1 bit = black and white, more bits = more colours)
• Image resolution


  Number of pixels in an image (width x height)
• Metadata


  Data about the data stored in a file
• Analogue signal


  Continuously changing data
• Sampling


  Converting an analogue signal to digital by measuring the amplitude at regular intervals
• Sample rate


  Frequency at which the amplitude of an analogue signal is measured (in Hz)
• Types of compression


  • Lossy
  • Lossless
• Lossy compression


  Reduces file size significantly but permanently loses some data quality