Computers

• Computers


  Devices that can carry out a variety of tasks
• Stored program concept


  The idea that computers should be able to be easily and quickly reprogrammed, developed by Alan Turing and John von Neumann in the 1940s
• Computer architecture


  The structure of a computer system - the hardware components it has and how they work together to execute programs
• Von Neumann architecture


  • A new computer architecture for a programmable machine, devised by Von Neumann
• Main memory


  Short-term, working memory that holds the program instructions and data the CPU is currently using
• Main memory


  • Consists of a collection of storage locations, each with its own unique address
  • Storage locations can hold a program instruction or an item of data
  • Often referred to as RAM (random access memory)
  • Is volatile and needs power to retain its contents
• Modern laptops, tablets and phones typically have between 4 and 32 GiB of RAM
• Central processing unit (CPU)


  The hardware component that decodes and executes program instructions
• Components of the CPU

  • Fetch program instructions from main memory one at a time, decode them and direct the operations of the other parts of the system to execute them
  • Synchronise the actions of the CPU
  • Perform arithmetic and logic operations on data
  • Provide direct-access storage for instructions, intermediate results and data within the CPU
• Clock


  A tiny quartz crystal that vibrates at a constant rate, triggering the CPU to carry out one action per tick
• Modern CPUs have clock speeds of around 3 gigahertz (GHz)
• Buses


  Collections of wires used to transfer data between components of a computer system
• Types of buses


  • Control bus carries signals between the CPU and other parts of the computer system
  • Address bus holds the address of the memory location the CPU will read from or write to
  • Data bus transfers program instructions and data between memory and the CPU
• The number of wires in the address bus determines how much addressable memory there is
• Fetch-decode-execute cycle


  1. Fetch the next instruction from memory

  2. Decode the instruction by looking it up in the CPU's instruction set

  3. Execute the instruction, coordinating the actions of the CPU components
• Secondary storage


  Provides long-term storage for programs and data when they are not in use
• Differences between primary and secondary storage

  • Primary storage is volatile and power dependent, secondary storage is non-volatile
  • Primary storage provides fast, direct access to the CPU, secondary storage requires transfer to memory first
  • Primary storage has limited capacity, secondary storage has much larger capacity
• Types of secondary storage

  Magnetic (hard disk drives), optical (CDs, DVDs), solid-state (SSDs, USB drives)
• Magnetic storage


  • Data is stored by magnetising areas on metal platters
  • Read/write heads detect/change the magnetic state to read/write data
• Optical storage


  • Data is stored as reflective 'lands' and less reflective 'pits' on a disk
  • A laser is used to read the lands and pits and translate them into 1s and 0s
• Solid-state storage


  • Data is stored by trapping electrons in transistors, with full pools representing 0 and empty pools representing 1
• Solid-state storage is very robust, unlikely to be damaged, and consumes very little power
• Disk surface


  A point reflects light more than a line
• Light reflected off surface

  Detected by a light sensor and translated into 1s and 0s
• Storage technologies


  • Magnetic (internal/external hard drives, tape drives)
  • Optical (CD, DVD, Blu-ray drives)
  • Solid-state (solid-state drives, USB sticks)
• Magnetic storage


  • Has moving parts that will eventually fail
  • Noisy
  • Slow to access
  • Prone to scratches
  • Relatively expensive
• Optical storage


  • Very fast data access
  • No moving parts
  • Low power
  • Quiet
• Solid-state storage


  • Has a limited number of read-write cycles
• An embedded system is a small computer on a chip, that performs a dedicated task within a larger system
• Most embedded systems work in real-time - they must respond immediately to a triggered event or request
• Features of embedded systems

  • Single purpose-designed for a specific task
  • Customised hardware and software
  • Limited memory and processing power
  • Low power consumption
  • Some connectivity
  • Minimal or no user interface
  • Small physical size
  • Responsive to their environment
  • Low maintenance
  • Usually operate in real-time
• Microcontroller

  The brain of an embedded system
• Embedded system operation


  1. Sensors receive inputs from the outside world

  2. Microcontroller processes the inputs

  3. Actuators convert the output commands into mechanical actions
• Embedded system software - the program that tells the device what to do - is installed on the device and does not need to be modified
• Devices that use embedded systems

  • Washing machine that regulates water usage
  • Drinks dispenser that sends reorder request
  • Insulin pump that monitors blood glucose levels
  • Anti-lock braking system
  • Burglar alarm that sends message to homeowner
• The Internet of Things (IoT) is a network of physical objects that use embedded systems and wireless technology to collect and exchange data with little or no human interaction
• There are several privacy and security issues associated with the IoT, such as devices being dispatched with default passwords that can be easily hacked
• Operating system


  The program that acts as an interface between hardware and other software, enabling them to communicate
• Main tasks of an operating system


  • Process management
  • File management
  • User management
  • Peripheral management
• Process management

  Overseeing execution of programs by CPU and allocating memory and CPU time