Computers

• Previously, computing machines were built to perform just one job
• 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 processing unit to decode and execute program instructions fetched from main memory
  • A memory unit into which program instructions and data are loaded prior to being processed
  • Input and output mechanisms to input programs and data and output the results of processing
• Being able to store programs in memory was a major breakthrough. It enabled computers to be general purpose machines capable of carrying out lots of different tasks
• Main memory

  • Short-term, working memory that only holds the program instructions and data the CPU is currently using
  • Consists of a collection of storage locations, each with its own unique address
  • Referred to as RAM - random access memory because storage locations can be read from and written to in any order
  • Classed as primary storage because the CPU has fast, direct access to it
  • Volatile - needs power to retain its contents, wiped when computer is switched off
  • Modern laptops, tablets and phones typically have 4-32 GiB of RAM
• The von Neumann architecture is still the basis for the design of most modern computers, although invented in the 1940s
• Fetch-decode-execute cycle

  1. Fetch: CPU places memory address of next instruction on address bus, content transferred to CPU on data bus

  2. Decode: Control unit looks up instruction in CPU's instruction set

  3. Execute: Control unit coordinates actions of CPU components to carry out operation
• Modern CPUs carry out billions of fetch-decode-execute cycles per second
• Primary storage

  Volatile, short-term storage directly accessed by CPU, limited capacity
• Secondary storage

  Non-volatile, long-term storage for programs and data when not in use, large capacity
• When a user opens an application

  It is loaded into main memory from secondary storage
• When the user saves a file

  It is transferred from main memory into secondary storage
• When the application is closed

  It is removed from main memory
• Primary storage is needed to provide the CPU with fast, direct access to the program instructions and data it is currently using, but it only retains its contents temporarily
• Secondary storage is non-volatile and is needed to keep programs and data for the long term when they are not in use
• Main memory is limited in size so it can only hold a small amount of data, but there is practically no limit to the number of programs and amount of data that can be accommodated in secondary storage
• Without secondary storage, computers would not be able to swap between tasks quickly, preventing them from being general purpose machines
• Volatile storage


  Storage that loses its contents when power is switched off
• Non-volatile storage

  Storage that retains its contents even when power is switched off
• Types of secondary storage

  • Magnetic (HDDs, magnetic tape)
  • Optical (CDs, DVDs)
  • Solid-state (flash memory)
• Writing data to magnetic storage

  An electromagnet in the read-write head magnetises the surface of the platter to one of two polarities, representing 1 or 0
• Reading data from magnetic storage


  The read-write head detects the magnetic state of the platter
• Writing data to optical storage

  A laser is used to burn the surface of the disk, creating reflective lands and less reflective pits to represent 1 and 0
• Reading data from optical storage

  A laser beam is shone onto the disk surface, and the amount of light reflected is detected and translated into 1s and 0s
• Writing data to solid-state storage


  An electrical current is applied to the transistor, forcing electrons through a barrier and trapping them in pools to represent 1 and 0
• Solid-state storage

  Storage that consists of a grid of transistors
• To write data

  1. Apply an electrical current to the transistor

  2. Forces electrons through a barrier, trapping them in pools

  3. A full pool represents 1, an empty pool 0
• Lands

  More reflective areas on an optical disk
• Pits


  Less reflective areas on an optical disk
• To read data on an optical disk

  1. Shine a laser beam onto the surface

  2. A pit reflects light more dimly than a land

  3. The amount of light reflected is detected and translated into 1s and 0s
• Solid-state storage

  • Very robust and unlikely to be damaged
  • Consumes very little power
• Embedded system

  A small computer on a chip that performs a dedicated task within a bigger system
• Embedded systems


  • Single-purpose, customised hardware and software, limited memory and processing power, low power consumption, wireless connectivity, minimal or no user interface, small physical size, responsive to their environment, low maintenance, usually operate in real time
• Firmware

  The program that tells the device what to do, installed on the device and does not need to be modified
• Devices that use embedded systems

  • Washing machine, drinks dispenser, insulin pump, anti-lock braking system, burglar alarm
• How the embedded system in a car navigation system alerts the driver

  1. Uses satellite signals via GPS to calculate car position

  2. Uses on-board sensors to calculate car speed

  3. Checks speed limit using stored map data

  4. Outputs a warning if speed limit is exceeded
• Internet of Things (IoT)

  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 privacy and security issues with IoT devices, such as default passwords that can be hacked
• How the embedded system in a dishwasher maintains water temperature

  1. Monitors water temperature

  2. Adjusts heating to maintain correct temperature for the selected programme
• Operating system (OS)


  The program that acts as an interface between hardware and software, enabling them to communicate