chapter 3

• General-purpose computer system


  Consists of a processor, memory, I/O functionality
• Input


  • Take in data from the outside world
• Output


  • Display data for humans' understanding
• Primary storage


  Computer's main memory which stores a set of critical program's instructions & data
• Secondary storage


  Non-volatile storage for noncritical data that will not be frequently accessed
• Removable secondary storage


  • File backup and archive
  • Portable transfer of files to second device
• Embedded systems


  Small computer systems such as microprocessors that are often a part of a larger system
• Embedded systems


  • Each embedded system performs a few specific functions unlike general-purpose computers
• Benefits of embedded systems

  • Reliable since there are no moving parts
  • Require less power
  • Cheap to mass-produce
• Drawbacks of embedded systems

  • Difficult to program functions since there is either no interface
  • Expensive expert help needed for repair
• Laser printer


  1. Laser beam and rotating mirrors are used to draw image of the page on a photosensitive drum

  2. Image converted into electric charge which attracts charged toner such that it sticks to image

  3. Electrostatic charged paper rolled against drum

  4. Charge pulls toner away from drum and onto paper

  5. Heat applied in the fuser to fuse toner to the paper

  6. Electrical charge removed from drum and excess toner collected
• 3D printer


  1. Process starts from saved digital file that holds the blueprint of object to be printed

  2. Object is then built by sequentially adding layers of a material (e.g. polymer resin) until object created

  3. Object is then cured (e.g. resin-made objects are hardened by UV light)
• Microphone


  1. Incoming sound waves enter wind screen and cause vibrations about a diaphragm

  2. Vibrations cause coil to move past a magnetic core

  3. Electrical current generated which is then digitized
• Speaker


  1. Takes electrical signals and translates into physical vibrations to create sound waves

  2. Electric current in voice coil generates an electromagnetic field

  3. Change in digital audio signal causes current direction to change which changes field polarity

  4. Electromagnet is either attracted or repelled to a permanent magnet, causing a diaphragm that is attached to the coil to vibrate

  5. Vibration transmitted to air in front of speaker
• Magnetic Hard Disk

  • Hard disk has platters whose surfaces are covered with a magnetisable material
  • Platters are mounted on central spindle and rotated at high-speed
  • Surface of platters divided into concentric tracks & sectors, where data is encoded as magnetic patterns
  • Each surface is accessed by read/write heads
  • When writing, current variation in head causes magnetic field variation on disk
  • When reading, magnetic field variation from disk produces current variation in read head
• Solid State (Flash) Memory

  • Most use NAND-based flash memory
  • Consist of a grid of columns & rows that has 2 transistors at each intersection
  • Two transistors: Floating Gate (stores electros and the presence or absence of charge (electrons) represents either 1 or 0) and Control Gate (controls charge (electrons) flow for read/write)
• Optical Disc Reader/Writer


  1. Disc surface has reflective metal layer and is spun

  2. Tracking mechanism moves laser assembly

  3. Lens focuses laser onto disc

  4. Laser beam shone onto disc to read/write

  5. Tracks have sequences of amorphous and crystalline states on the metallic layer

  6. When reading, the reflected light from the different states on the track are encoded as bit patterns

  7. When writing, laser changes surface to crystalline and amorphous states along the track, which correspond to 1s or 0s
• Touchscreen


  • Considered as both an input & output device
  • Resistive (consists of two charged plates, pressure causes plates to touch, completing circuit, point of contact registered with coordinates used to calculate position)
  • Capacitive (made from materials that store electric charge, when touched, charge transferred from finger)
• Virtual (Reality) headset


  • Virtual headsets consist of 2 lenses, (LCD) display, circuit board with sensors, cover and foam padding
  • The display provides a simulation of a 3D environment, generated by a 3D graphics package
  • The user can 'move' in the virtual environment by moving their head or using controllers
• Buffers


  Short-term memory storage that stores data right before it's used, typically in RAM
• Random Access Memory (RAM)

  • Volatile memory: loses content when power turned off
  • Can be read and altered
  • Used to store currently executing program
• Read Only Memory (ROM)


  • Non-volatile memory: does not lose content when power turned off
  • Can only be read
  • Used for storing OS kernel and boot up instructions
• Static RAM (SRAM)


  • Doesn't need to refresh hence uses less power and faster access time
  • More complex circuitry, hence more expensive
  • Each bit stored in flip-flop
  • Has lower data density
  • Used in cache memory
• Dynamic RAM (DRAM)


  • Has to be refreshed, hence has slower access times and needs higher power
  • Only single transistor & capacitor, hence less expensive to purchase
  • Each bit stored as a charge
  • Has higher data density
  • Used in main memory
• Programmable ROM (PROM)


  • Can be programmed only once after it is created
  • Data cannot be erased or deleted
• Erasable Programmable ROM (EPROM)


  • Can be erased by UV light exposure and can then be reprogrammed
  • Chip has to be removed for reprogramming
• Electrically Erasable Programmable ROM (EEPROM)


  • Can be erased by an electrical signal and can then be reprogrammed
  • Can update data without removing chip
• Monitoring System


  • Monitors some state external to computer system
  • No changes made to environment by the system and hence no feedback
• Control System


  • Regulates the behaviour of other devices or systems
  • Event-driven system: the controller alters the state of the system in response to some event
  • Time-driven system: the controller takes action at a specific point in time
• Hardware used in a system

  • Sensor: measures an (analogue) property and transmits it to a processing unit, generally as an electrical or optical signal
  • Actuators: switch on/off heavy appliances (e.g. heater to heat/fan to cool)
  • ADC: converts analogue signals to digital signals
  • Transmission cable: to transfer signals
• Feedback Systems


  • Output from system affects the input of sensors
  • Ensures system operates within the given criteria
  • By enabling the system output to affect subsequent system inputs, it may cause a change in the actions taken by the system
  • Thus enables the system to automatically adjust conditions in a continuous process
• Logic Gates


  Use one or more inputs and produce a single logical output
• AND gate: If both inputs are high, the output is high (A•B)
• OR gate: If either input is high, the output is high (A+B)
• NOT gate: an inverter (A)
• NAND gate: (A•B)
• NOR gate: (A+B)
• XOR gate: (A⨁B)
• Touchscreen


  Considered as both an input & output device
• Types of touchscreen


  • Resistive
  • Capacitive