3.1 Computers and their components

# Hardware
## Primary Memory
Random Access Memory and Read Only Memory

| RAM                                                  | ROM                                                          |
| ---------------------------------------------------- | ------------------------------------------------------------ |
| Volatile memory: loses content when power turned off | Non-volatile memory: does not lose content when power turned off |
| Can be read and altered                              | Can only be read                                             |
| Used to store currently executing program            | Used for storing OS kernel and boot up instructions. E.g. Basic input/output system (BIOS) |
### Types of RAM
Dynamic RAM vs. Static RAM

| DRAM                                                         | SRAM                                                         |
| ------------------------------------------------------------ | ------------------------------------------------------------ |
| Has to be refreshed, hence has slower access times and needs higher power | Doesn’t need to refresh hence uses less power and faster access time if accessed at a high frequency, power usage can exceed that of DRAM |
| consists of a number of transistors and capacitors, hence less expensive to purchase | uses flip-flops, more complex circuitry, hence more expensive |
| Each bit stored as a charge                                  | Each bit stored in flip-flop                                 |
| Has higher data density/capacity                             | Has lower data density/capacity                              |
| Used in main/primary memory                                  | Used in cache memory                                         |

### Read-only memory (ROM)

- primary memory unit that can only be read from.

| PROM                                            | EPROM                                                        | EEPROM                                                       |
| ----------------------------------------------- | ------------------------------------------------------------ | ------------------------------------------------------------ |
| Programmable ROM                                | Erasable Programmable ROM                                    | Electrically Erasable Programmable ROM                       |
| Can be programmed only once after it is created | Can be erased by UV light exposure and can then be reprogrammed | Can be erased by an electrical signal and can then be reprogrammed |
| Data cannot be erased or deleted                | Chip has to be removed for reprogramming                     | Can update data without removing chip.                       |

## Secondary Storage/Memory

- to store files/software long term 
- are not directly accessible by the CPU

- non-volatile storage for noncritical data that will not be frequently accessed

#### INTERNAL OPERATIONS OF MAGNETIC HARD DISK

- hard disk drive has one or more platters made of aluminum or glass 
- Each surface of the plater is capable of being magnetized 
- The platters are mounted on a central spindle and rotated at a high speed 
- Each surface of the platter had a read/write head mounted on arm positioned just above the surface 
- Electrical circuits control the movement of the arm and heads 
- The surface of the platter is divided into concentric tracks and sectors 
- One track in one sector is the basic unit called a block 
- The data is encoded as a magnetic pattern for each block 
- When writing to disk, a variation in the current in the head produces a variation in magnetic field on the disk 
- When reading the disk, a variation in magnetic field produces a variation in current through the head

#### NTERNAL OPERATIONS OF SOLID STATE 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

\-    Control Gate: controls charge (electrons) flow for read/write

| Hard disk drives                                             | Solid state drives                                           |
| ------------------------------------------------------------ | ------------------------------------------------------------ |
| Use magnetic properties to read/write data                   | Use electronic properties of NAND/NOR chips                  |
| Heat is produced due to movement of read-write heads moving to fetch data | No moving parts so it runs cooler                            |
| Heavy                                                        | Lighter and used in tablets                                  |
| More power consumption compared to SSD                       | Lower power consumption                                      |
| Noise due to a spinning disc                                 | Silent as no moving parts                                    |
| Less reliable, as the moving parts could break               | Highly reliable, as no moving parts                          |
| HDD can be used for a long time. Widely used in servers where huge numbers of operations take place every day. | SSD has limited read-write operations for a limited period of time. For example: 20 GB write operations in 3 years for early SSDs. Hence, cannot be used in servers. |

| **Feature**           | **Flash Memory**                                                                | **EEPROM**                                                                                 |
| --------------------- | ------------------------------------------------------------------------------- | ------------------------------------------------------------------------------------------ |
| Storage Technology    | NAND                                                                            | NOR                                                                                        |
| Operation Speed       | Slower                                                                          | Faster                                                                                     |
| Cost                  | Relatively Inexpensive                                                          | Relatively Expensive                                                                       |
| Data Read/Erase Mode  | Read/Erase in Data Blocks                                                       | Read/Erase Byte by Byte                                                                    |
| Suitable Applications | Typically for Large Capacity and General Use E.g. USB flash drive Memory sticks | More useful in specific applications where byte-level access or erasure is needed E.g. ROM |

#### INTERNAL OPERATIONS OF OPTICAL DISC

\-    Disc surface has reflective metal layer and is spun

\-    Tracking mechanism moves laser assembly

\-    Lens focuses laser onto disc

\-    Laser beam shone onto disc to read/write

\-    Tracks have sequences of amorphous and crystalline states on the metallic layer

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

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

#### Removable secondary storage:

\-   File backup and archive

\-   Portable transfer of files to second device

#### EMBEDDED SYSTEM

\-    Small computer systems such as microprocessors that are often a part of a larger system

\-    Each embedded system performs one specific task unlike general-purpose computers

| **Benefits**                                                 | **Drawbacks**                                                |
| ------------------------------------------------------------ | ------------------------------------------------------------ |
| small in size and therefore easy to fit into devices         | difficult to upgrade devices to take advantage of new technology |
| relatively low cost to make                                  | troubleshooting faults in the device becomes a specialist task |
| usually dedicated to one task, making for simple interfaces and often no requirement of an operating system | although the interface can appear to be simple, in reality it can be more confusing (changing the time on a cooker clock can require several steps, for example) |
| consume very little power                                    | any device that can be accessed over the internet is also open to hackers, viruses, and so on |
| very fast reaction to changing input (operate in real time)  | due to the difficulty in upgrading and fault finding, devices are often just thrown away rather than being repaired (wasteful) |
| with mass production comes reliability                       |                                                              |

- Buffers: short-term memory storage that stores data right before it’s used, typically in RAM.

#### Principle Operations of Hardware Devices

##### Laser printer

- Data from the document is sent to a printer driver 
- Printer driver ensures that the data is in a format that the printer can understand 
- Check is made by the printer drive to ensure that the chosen printer is available to print 
- Data is sent to the printer and stored in a temporary memory known as a printer buffer 
- A laser beam and rotating mirrors are used to draw image of the page on a photosensitive drum 
- Image converted into electric charge which attracts charged toner such that it sticks to image 
- Electrostatic charged paper rolled against drum 
- Charge pulls toner away from drum and onto paper 
- Heat applied in the fuser to fuse toner to the paper 
- Electrical charge removed from drum and excess toner collected

##### Inject printer

- Data from the document is sent to a printer driver 
- Printer driver ensures that the data is in a format that the printer can understand 
- Check is made by the printer drive to ensure that the chosen printer is available to print 
- Data is sent to the printer and stored in a temporary memory known as a printer buffer 
- a sheet of paper is fed into the main body of the printer. A sensor detects whether paper is available in the paper feed tray – if it is out of paper (or the paper is jammed), an error message is sent back to the computer 
- as the sheet of paper is fed through the printer, the print head moves from side to side across the paper printing the text or image. The four ink colours are sprayed in their exact amounts to produce the desired final colour 
- at the end of each full pass of the print head, the paper is advanced very slightly to allow the next line to be printed. This continues until the whole page has been printed 
- if there is more data in the printer buffer, then the whole process from stage 5 is repeated until the buffer is empty 
- once the printer buffer is empty, the printer sends an interrupt to the processor in the computer, which is a request for more data to be sent to the printer. The process continues until the whole of the document has been printed

##### Two types of printer

- ###### Thermal bubble

- Tiny resistors produce localised heat that vaporises the ink. 
  - The bubble of ink is ejected from the paper head onto the paper. 
  - When the bubble of ink collapses, a vacuum is created to draw fresh ink from the print head.

- ###### Piezoelectric

- A crystal at the back of the ink reservoir is given an electric charge and vibration is created. 
  - This vibration forces the ink to be ejected onto the paper.

##### 3D printer

- The object is designed using computer aided design software 
- The software splits the object into slices 
- The data about the slices is sent to printer 
- The solid plastic is melted and transferred to the nozzle 
- A stepper motor moves the nozzle into position 
- The nozzle extrudes the molten plastic 
- The previous steps are related until the layer is complete 
- A fan cools the layer 
- Each layer is printed by following the same steps

##### Binder 3D printing

- 3D printing method that uses a two-stage pass; the first stage uses dry powder and the second stage uses a binding agent.

##### Direct 3D printing

- 3D printing technique where print head moves in the x, y and z directions. Layers of melted material are built up using nozzles like an inkjet printer.

##### Speaker

- Takes electrical signals and translates into physical vibrations to create sound waves 
- Electric current in voice coil generates an electromagnetic field 
- Change in digital audio signal causes current direction to change which changes field polarity 
- Electromagnet is either attracted or repelled to a permanent magnet, causing a diaphragm that is attached to the coil to vibrate 
- Vibration transmitted to air in front of speaker 
- Degree of vibration determines amplitude and frequency of sound wave produced

##### Microphone

- Incoming sound waves enter wind screen and cause vibrations about a diaphragm 
- Vibrations cause coil to move past a magnetic core 
- Electrical current generated which is then digitized

##### LCD

- a screen back-lit by light-emitting diodes and with liquid crystal cells sandwiched between polarisers

##### OLED

- uses movement of electrons between cathode and anode to produce an on-screen image. It generates its own light so no back lighting required.

##### Touchscreen

- Considered as both an input & output device

- 2 types:

| Aspect                 | Capacitive                                     | Resistive                                 |
| ---------------------- | ---------------------------------------------- | ----------------------------------------- |
| Technology Layers      | Multiple glass layers creating electric fields | Polyester (top) and glass (bottom) layers |
| Touch Detection Method | Changes in electric current when touched       | Completion of circuit when touched        |
| Input Options          | Bare fingers, some support for special stylus  | Bare fingers, gloved fingers, or stylus   |
| Cost                   | Medium cost                                    | Relatively inexpensive                    |
| Screen Visibility      | Good, even in strong sunlight                  | Poor in strong sunlight                   |
| Multi-touch Capability | Supports multi-touch                           | Does not support multi-touch              |
| Durability             | Very durable, resistant to impact              | Fair durability, vulnerable to scratches  |
|                        | Takes a major impact to break glass            | Screen wears out over time                |

##### 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

#### Monitoring and Control Systems

- ##### 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 typically 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