3.3 Data storage

### PRIMARY STORAGE

- Main memory inside a computer
- Directly accessed by CPU, faster access than secondary storage
- both volatile and non-volatile

#### Random Access Memory (RAM)
- is a hardware device
- allows information to be stored and retrieved on a computer
- RAM is a volatile memory and requires power to keep the data accessible
- RAM normally has a larger capacity than ROM

### Explain why a computer needs RAM // The purpose of RAM
- To store data temporarily
- To store data currently in use
- To store software currently in use
- Data can be accessed faster (than secondary storage)
- To speed up the fetch stage (of the fetch-execute cycle)
##### 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)

- Is a hardware device
- Stores basic start up instructions for a computer e.g. basic input/output system (BIOS)
- Stores bootstrap loader
- ROM contains nonvolatile data, meaning it keeps it data even if the device loses power
- Data can be read from and written to RAM, ROM can only be read from

### Secondary storage

- For non-volatile/permanent/long-term storage of files/data 
- Storage that is not directly accessed by the CPU An example is HDD / SSD
- To store data that is not currently required by the CPU // Stores data / files that can be accessed at a later stage

#### Explain why a computer needs secondary storage.
- To store data permanently
- otherwise, data would need to be entered every time the computer was used
- To allow software to be installed on the computer
- For the creation of virtual memory
#### Hard Disk Drive (HDD)

- HDD is an electromechanical data storage device that uses magnetic storage
- to store and retrieve digital information using one or more rotating magnetic disks
##### Benefits
- Large capacity storage
	- for videos/films that have large file sizes / require large storage space
- Longevity // Can be read from/written to large number of times
	- the copies will be accessible for a long time // device will not break if accessed regularly
	- …may require large number of read/write to access fields
- Relatively cheap per GB // cheaper than SSD per GB …
	- … there is a large capacity required therefore overall may cost less than other devices
- Device will not be moved regularly so no need for portability/durability
##### the operation of a HDD and how it stores data.
- It has platters
- Platters/disk divided into tracks
- Platter/disk is spun
- Has a read/write arm that moves across storage media
- Read/writes data using electromagnets
- Uses magnetic fields to control magnetic dots of data
- Magnetic field determines binary value

#### Solid State Drive (SSD)

##### how an SSD stores data

- Flash storage
- Uses transistors are used as control gates and floating gates
- Can be NAND/NOR technology // Can use flip-flops
- Stores data by flashing it onto the chips/device
- Controlling the flow of electrons using gates
- The electric current reaches the control gate and flows through to the floating gate to be stored
- When data is stored the transistor is converted from 1 to 0 / 0 to 1

Floating Gate: stores electrons and the presence or absence of charge (electrons) -    represents either 1 or 0 
Control Gate: controls charge (electrons) flow for read/write 
##### Why SSD is the most suitable secondary storage for their mobile device

- It is small in size // compact // lightweight // portable
- It has low power consumption
- It runs quietly
- It runs at a cool temperature
- It is robust when dropped // durable
- Fast access/read/write speed
- High capacity
- Not affected by magnets

##### Benefits
- Large capacity storage
	- …for videos/films that have large file sizes / require large storage space
- No moving parts
	- …can be carried/moved to other locations with limited risk of damage
- Fast access speed
	- … videos are large files that will be stored/accessed in less time
- Cost per GB is not significant (in comparison)
	- …there is a large capacity required therefore overall cost will not be significantly more than others
- Used as a copy of files to may not be accessed regularly
	- … the limited number of read/write times/longevity is inconsequential
- Uses less power // runs cooler

##### Example
- Solid-state drive // SSD
- USB flash memory drive
- SD card

| 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. |
Features of magnetic storage
- Data is stored on platters
- (Platters) are divided into tracks and sectors
- Has components that are spun
- Data is read/written using a read/write arm
- Data is read/written using electromagnets
- Magnetic field determines the binary value
- It is non-volatile
Features of solid-state storage
- Flashes data onto chips
- Uses transistors
- Uses NAND/NOR technology
- Uses control gates
- Uses flow gates
- Controls the flow of electrons
- It can be volatile or non-volatile
#### OFFLINE STORAGE
- non-volatile memory that can be removed from a computer system
- It is not directly accessed by the CPU
- must be physically connected to computer to obtain stored data
- used to store files as a backup
###### Example:
- CD
- DVD
- Blu-ray
- Flash memory // USB storage
- removable / external / portable hard disk drive (HDD/SSD)
- SD card

|              | Compact Disk (CD)                              | Digital Versatile Disk (DVD)     |
| ------------ | ---------------------------------------------- | -------------------------------- |
| similarities | Both need a red laser to read/write data       |                                  |
|              | Both are spun to be read                       |                                  |
|              | Both use spiral 螺旋 tracks for data             |                                  |
|              | Both are optical storage                       |                                  |
|              | Both are off-line storage // both non-volatile |                                  |
|              | Both use pits and lands to store data          |                                  |
| difference   | can only be single                             | can be dual layer                |
|              |                                                | has higher storage capacity      |
|              |                                                | has a shorter wavelength laser   |
|              |                                                | are spun faster                  |
|              |                                                | have a higher data transfer rate |

##### Compact Disk (CD)/Digital Versatile Disk (DVD)
- Laser (red) light is used to read and write data on the surface of the disk.
- Use a thin layer of metal alloy to store data.
- Both systems use a single spiral track that runs from the centre of the disk to the edge
- DVD uses *Dual-Layering,* which increases the storage capacity (two individual recording layers)
#####  Blu-Ray
- Optical storage media
- Uses a blue laser to carry out read-and-write operations
- The wavelength of laser light is less than CD and DVD (stores up to five times more data than DVD)
- Automatically come with secure encryption (prevent piracy and copyright infringement)
- Used as backup systems

##### how the laser is used to store and read data from the disk in optical media
- Laser is shone at the disk
- An (arm/head) moves the laser across the surface of the disk
- The laser burns pits onto the surface of the disk
- The laser is used to read the pits and lands on the surface of the disk
- The reflected light from the laser shining on the disk is captured (by a sensor)

##### Flash Memory / USB Flash
- Nonvolatile memory chip used for storage and for transferring data
- between a personal computer (PC) and digital devices
- It has the ability to be electronically reprogrammed and erased.
- It is often found in USB flash drives, MP3 players, and digital cameras

## Embedded Systems
- Combination of Hardware and Software which is designed to carry out a specific set of tasks.
- Embedded systems may contain -
  - Microcontrollers - CPU, RAM, ROM and other peripherals on one single chip
  - Microprocessor - Integrated circuit with CPU only
  - System on Chips (SoC) - microprocessor with I/O ports, storage and memory
- Process of Embedded Devices -
  - Input from the user is sent to the microprocessor (ADC needed if the data is analogue)
  - Data from the user interface is also sent to the microprocessor
  - Microprocessor then sends signals to actuators which is the output
- Non-programmable devices need to be replaced if they need a software update.
- Programmable devices have two methods of updating -
  - Connecting the device to a computer and downloading the update
  - Updating automatically via a satellite, cellular or Wi-Fi link

Advantages and Disadvantages of using embedded systems

| Advantages                                           | Disadvantages                                                           |
| ---------------------------------------------------- | ----------------------------------------------------------------------- |
| Small in size, therefore can easily fit into devices | Can be difficult to upgrade                                             |
| Low cost to make                                     | The interface can be confusing sometimes                                |
| Requires very little power                           | Troubleshooting is a specialist’s job                                   |
| Very fast reaction to changing input                 | Often thrown away as difficult to upgrade and faults are harder to find |
| Dedicated to one task only                           | Increased garbage as they are thrown away                               |
| Can be controlled remotely                           | Any computerised system is prone to attacks                             |

- Applications of Embedded devices -
  - GPS systems
  - Security Systems
  - Vending Machines
  - Washing Machines
  - Oven
  - Microwave

## Virtual memory

##### The purpose of virtual memory:
- To run programs when there is insufficient RAM to run them
- To allow RAM to store more data when required

Virtual memory is used when the **Random access memory (RAM**) is full. It is created by partitioning the **Hard disk drive (HDD)**. Data is divided into **Pages** that can be sent from **Random access memory (RAM)** to the **Virtual memory // Hard disk drive (HDD)** to be temporarily stored until they are required.

##### Describe how virtual memory could be used in this process to stop the computer crashing:
 - Virtual memory can act as an extension to the RAM .
- The secondary storage device is **partitioned** to create the virtual memory.
- Pages can be **transferred** from the RAM to the **virtual memory** when the RAM is full

##### A computer has pages A, B and C that are stored in RAM. Page D needs to be sent to the RAM but the RAM is full. Page B is not needed immediately:
Explain how virtual memory can be used in this scenario.
- The secondary storage / hard drive can be partitioned to create the virtual memory
-  … and page B sent to the virtual memory …
- … which makes space for page D in RAM
-  … Once page A / C / D / another page is not required / has been processed
- … page B can be sent from the virtual memory back to RAM when it is required

## Cloud storage

- A collection of servers that store data in a remote location
- … that are (normally) accessed using an internet connection

##### Benefits of cloud storage

- Can be accessed anywhere with Internet access
- Do not need to buy specific software/hardware(No need for a client to carry an external storage device with them)
- Do not need to install security / security could be better
- Most cloud data service will have in-built backup/disaster recovery
- Can easily increase capacity
- Data can be shared easily
##### Drawbacks of cloud storage

- May be less secure // by example
- May lose access to them if internet connection lost/not available
- Reliant on a third party maintaining the hardware // by example
- Could incur an extra/ongoing fee/cost