Chapter 4: Computer Systems

• NOT gate

  Reverses the value of a single input
• Boolean data type

  Has two possible values: TRUE or FALSE, often represented as 1 or 0, and ON or OFF in an electrical circuit
• Boolean logic can be applied to both data and ON or OFF states in an electrical circuit
• ON


  Enough electrical voltage is present
• OFF

  Not enough electrical voltage is present
• Switch ON


  Represented as input of 1
• Truth table


  Shows possible values of outputs for all possible values of inputs
• Logic gates

  Transform INPUT values into an OUTPUT value
• If the input is ON then the output will be OFF. If the input is OFF then the output will be ON.
• Boolean algebra

  Can be used where the input is unknown
• AND gate

  If both inputs are ON then the output will be ON. If any of the inputs are OFF then the output will be OFF.
• The symbol for AND is . (full stop)
• OR gate

  If either input is ON then the output will be ON. If both inputs are OFF then the output will be OFF.
• The symbol for OR is +
• XOR gate

  If only one input is ON then the output will be ON. If both inputs are ON then the output will be OFF because the inputs are not exclusive.
• The symbol for XOR is Ꚛ
• Logic operators have precedence rules, similar to BIDMAS in maths
• Precedence rules for logical operators

  • Brackets
  • Not
  • And
  • Xor
  • Or
• Half adder


  A simple circuit that will add together two binary values
• Full adder


  • Carry in represents the carry being brought in from the previous column's addition
  • Carry out represents the carry value to be carried to the next column's addition
• Application software


  Programs that make the computer useful and enable the user to carry out productive tasks
• Apps


  Applications for a smart phone or tablet
• System software

  A set of computer programs designed to manage the computer system, including the operating system and utility software
• Operating system

  The software used to manage the hardware within a computer system
• The operating system is essential to the smooth running of any personal computer system, but not necessary within some embedded systems
• Functions of an operating system

  1. Managing the human-computer interface

  2. Moving data to and from secondary storage and file management

  3. Sharing memory between software

  4. Scheduling the processor and multitasking of software

  5. Handling input

  6. Handling errors and alerting the user

  7. Sending commands to output devices

  8. Loading and running software

  9. Managing user accounts
• Processors


  The operating system manages the use of the central processing unit (CPU) and multitasking of software
• Memory


  The operating system allocates memory addresses for processes and uses virtual memory when main memory is full
• Input/output devices

  The operating system uses device drivers to communicate with peripheral devices and translate commands
• Applications


  Applications make use of the CPU, memory, secondary storage and peripherals, and rely on the operating system to access them
• Input devices

  Devices that allow the user to input data into a computer system, e.g. mouse
• Operating system translating input device actions

  Translate device input to instructions that move pointer on screen
• Operating system


  Provides access to thousands of device drivers
• Adding new devices to operating system

  New devices made available by downloading driver when device is first connected, known as 'plug and play'
• Applications


  Make use of the CPU, memory, secondary storage and peripherals
• Without the operating system

  Applications could not run
• How applications access hardware

  Applications only need to know how to access the operating system, not the hardware itself
• Services provided by operating system to applications

  • Printing
  • Searching for files
  • Displaying data on screen
• Logging on

  Method for users to gain access to the system
• Authentication methods

  • Username
  • Password
  • Fingerprint
  • Swipe card
  • Facial recognition
  • PIN