CS-iGCSE-CIE-Notes

Data Storage Devices

Used to store applications, data, and files.

Primary and Secondary Storage | 3.3.1, 3.3.2

Primary storage is directly accessible by the CPU, meaning data can be read very quickly. However, their capacity is typically much lower, and often volatile. This includes RAM, ROM, and cache memory.Secondary storage cannot be directly accessed by the CPU. It can be both internal and external to the CPU. It is necessary for storing permanent and large amounts of data as primary storage is often volatile meaning that it will lose its data. Examples of secondary storage include HDD (hard disk drives), SSD (solid state drives), and DVD (digital versatile discs).

Magnetic, Optical, and Solid-State Storage | 3.3.3

**Optical storage - laser technology:

It is a form of data storage that stores digital data on a disc. This is done via laser to read and write information onto the surface of a disc. * ROM - Read only memory

• Stores factory settings of the computer

• Stores start-up sequence and initiated the computer

• Stores set routines

• Non-volatile, permanent memory

• Cannot be changed or written to, read only

• Bios and bootstrapRed laser/blue lasers can either read or write to DVDs, CDs, and Blu-rays. They can be set to write once only or both read and write, depending on the manufacturer. * DVD - Digital versatile disc, CD 

• Thin layer of metal alloy that stores data

• Only a single spiral track runs throughout the disc from the centre to the outer edge

• These disc are also separated in sectors so that the head can directly access data

• When read, the optical head moves to the surface of the disc and reads along the spiral track as the disc spins, using a red laser

• Data is stored in pits and lands where lands represent 1 and pits represent 0

• Lands are flat, non-indented surfaces and pits are small indents in the reflective surface of the disc

• These optical discs can be written once only, or read and written to multiple times

• DVDs can store more data than CDs but they can also be dual-layered, which significantly increases the storage capacity - CDs track widths are larger as well therefore it cannot fit as much data as a DVD.In dual-layering, two disks are joined together and separated with a transparent spacer. The laser accesses the second layer through the first layer (and transparent spacer).* Blu-ray

• A blue laser is used instead of a red laser

• The wavelength of blue light is much smaller than red light, so that the pits and lands can be smaller, allowing for more storage capacity (5 times more than a DVD)

• Blu-rays can be dual-layered as well

• They come with secure encryption

• Can do much more than DVDs: high definition television, quickly skip to any part of the disc, create playlists off of recorded media, automatically search for empty spaceSolid state storage:* SSD - Solid state drive

• Has no moving parts (therefore ‘solid’) which eliminates latency

• Instead of relying on magnets, SSDs store data by moving electrons around within NAND or NOR chips

• Data can also be read throughout the entire chip at the same time rather than one bit at a time in HDDs as the state of all the transistors can be known at the same time

• Overall SSDs are much more efficient than HDDs as they are more reliable, lighter, faster, and uses less power

• However SSDs have a relatively lower endurance and cannot perform as well when writing a lot of data

• How it works:

  • The drive is arranged in a matrix

  • At each intersection there are two transistors, a floating and a control gate

  • A dielectric coating acts as an insulator which allows the transistor to keep their charges

  • To assign charges, a voltage is applied to the control gate and attracts electrons to the corresponding floating gate

  • The insulating layer locks the charge in

  • When the floating gate is charged, its value is 1, and when it has no charge, its value is 0

  • If charges up a ‘floating gate’ to represent either 0 or 1. The way it does this is by attracting electrons to charge a floating gate via a ‘control gate’ when a voltage is applied

• BIOS

  • This is stored in EEPROM - electrically erasable programmable read only memory. This means that its contents can be changed electrically

  • Flash memory is used allowing it to maintain its memory without power

  • However the settings of the BIOS is stored in a CMOS, which is battery powered

  • These settings can be changed by the user

  • Restarting or reconnecting the CMOS to power will reset the BIOS settings to factory defaultsTRANSISTORS‘Used to amplify or switch electrical signals’In SSDs, floating and control gates are transistorsIn RAM, they read and write to capacitors by allowing current or discharging them* USB Flash drive/Memory sticks

  • Uses the same technology as solid state drives

  • They are small, lightweight, and portable

  • They can be used as backup devices, transferring files between devices, or as dongles for expensive software (additional files that a software needs to run - this is a security method that prevents unauthorised use of the software as well as copying the software)Magnetic storage:Data is stored magnetically* HDD - Hard disk drive

    https://www.youtube.com/watch?v=NtPc0jI21i0

  • Data is stored magnetically as magnetic bits

  • Has spinning platters (aluminium, glass, ceramic material) thinly coated with a magnetic substance (can spin 7000 times per second)

  • ‘Read-write’ transducers have electromagnets that can read or write data to the platters

  • The platter is divided into many many tracks and sectors where sectors hold a specific amount of data (bytes)

  • The heads read and write individual bits off of tracks at a designated address - however they need to look for the correct track and sector which may cause latency

  • It takes time for an entire block of data to rotate to a head, that is latency

  • Fragmentation can occur overtime as data is not always stores in adjacent sectors.This means that the heads will have to look for scattered files which makes accessing data slower, and will damage the arms as it has to constantly move

  • HDDs have a coil that generates a magnetic field by spinning

  • Very slow data access compared to RAM* RAM - Random access memory

  • Volatile, temporary memory

  • Memory can be accessed at any location at any time, independent of which memory location was last used or its order

  • When in use, memory from secondary storage is temporarily moved to the RAM so that it can be easily accessed by the CPU

  • Can be written to or read from by the computer

  • Can store data, files, applications, or part of the operation system that is in use by the computer

  • Disadvantages: not as fast as the cache, expensive, cannot handle large amounts of data compared to secondary storage - SRAM

  • Uses flip flops to hold bits

  • Expensive

  • Lower storage capacity because it uses more transistors

  • Lower power usage

  • Advantages: doesn’t need to be recharged, faster data access, CPU cache makes use of SRAM because it is faster than DRAM

• DRAM

  • Uses transistors (switch, allows chip to read or change the value of capacitors) and capacitors (stores the bits)

  • Advantages: cheaper, more memory capacity

  • Disadvantages: needs to be constantly refreshed (recharged every 15ms to maintain its charge, otherwise the 1 charge would be lost and every capacitor would be 0)* Cache

  • This is the fastest possible access to memory the CPU has

  • L1 is the fastest with a larger L2 cache underneath, and sometimes L3

  • The cache is a form of RAM and does not need to be refreshed (recharged) (SRAM)

  • It holds memory that the CPU uses very often

  • When the CPU takes memory from the cache, it’s called a cache hit. When the CPU cannot find necessary files in the cache, it’s a cache miss. 

• Tape

  • Data is stored on a magnetic tape

  • Playback heads touch the tape and read the magnetic bits**

Virtual memory | 3.3.4

Virtual memory is a memory management technique that allows the computer to deal with memory shortages and create an illusion of much more memory than there appears to be.When a computer runs out of space in its RAM, (possibly due to too many programs running) files can be temporarily placed into secondary storage such as a HDD or SSD. Virtual memory is simply memory in the RAM and additional swappable memory on the hard discs or SSDs.Maps:All the programs requesting access to the RAM are sent to a ‘map’. In the case that there is not enough space in the RAM, the map will direct the newly opened programs to the HDD/SSD. Virtual memory will then now ‘map’ the oldest data in the RAM into the HDD/SSD to make space for the program previously moved to the HDD. A process called **paging moves data in and out of the RAM in virtual memory management. A page is essentially a block of data of a fixed-size. When the RAM runs out of space, pages of data are transferred to the HDD/SSD. They can also be moved out and copied over into the RAM. Benefits:* Allows the user to use more memory than physically available

• Does not waste memory on old, unused programs

• Allows computer to handle large amounts of data at a time

• Reduces the need to buy better, more expensive RAMHowever virtual memory requires moving a lot of data in and out of the hard disc, constantly. Overtime, the hard disc will be damaged and break down earlier than expected. Another issue is disc thrashing, where a buildup of data needs to be swapped in and out of the RAM and virtual memory. This is time consuming and will eventually overtake the time it takes to actually execute the programs, and will reach the thrash point. This can be avoided by using SSDs as there are no moving parts.**

Cloud storage | 3.3.5, 3.3.6

In cloud storage, data is stored on remote servers instead of the local primary or secondary storage on the user’s computer. The user’s data is stored on multiple different servers in case of malfunctioning or maintenance. These servers are managed by a hosting company such as Amazon or Google. Systems for cloud storage* Private

• The client and storage provider are one single entity

• For example, Google keeping their own data on their servers

• Public

  • The client and storage provider are two different entities

  • Example: a website using Amazon’s servers

• Hybrid

  • Stores sensitive, private data on the users individual storage device(s) and less sensitive information on the cloudAdvantages: * Negates the need to carry an external storage device while also being able to access the information from any device at anytime

• Unlimited storage capacity for users

• Cloud storage can backup lost data on local storageDisadvantages:* Without internet or if the internet is slow, data on the cloud cannot be accessed

• Expensive if large storage capacity is required

• External facilities may malfunction and the client’s data is lost

• Questionable security - hackers may steal or read private information of public serversSecurity responsibility:Provider:* Encryption to protect the privacy of the user’s data

• Secure infrastructure, resistance to natural disasters, resistance to break-ins

• Policies preventing insider attacks, authorisation codes

• Backup procedures

• Up to date security software to prevent malware, hacking scandalsUser:* Strong passwords

• Not leaving the computer unattended

• Not sharing credentials 

• Not using automatic login

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