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Computer data storage is the name for a number of copmonents of a computer. The main purpose of these components is to store data. The central processing unit is then able to fetch and change the data. In most computers, there is a memory hierarchy: Memory that is "closer" to the CPU is usually faster to access, but it is also smaller; data that is stored in this kind of memory usually needs electrical power to keep the data. Memory that is farther away is usually slower to access, but also bigger. Examples for this kind of data are classical storage media, such as hard drives or USB flash drives. Some media offer even more capacity, but accessing them is very slow. Examples for such media are tape drives. Modern CPUs have registers that can store data, they also often have several levels of cache. Finally, there is the main memory the CPU can access. These three are usually referred to as Primary data storage. Secondary storage is usually not directly accessible by the CPU, and data needs to be transferred to primary storage to be available. Secondary storage includes hard drives. Tertiary storage is mainly used for archival and backup; very often it is copied to secondary storage before use


Optical Storage

Optical storage devices save data as patterns of dots that can be read using light. A laser beam is the usual light source.


The data on the storage medium is read by bouncing the laser beam off the surface of the medium. If the beam hits a dot it is reflected back differently to how it would be if there were no dot. This difference can be detected, so the data can be read.


Dots can be created using the laser beam (for media that is writable such as CD-Rs). The beam is used in a high-power mode to actually mark the surface of the medium, making a dot. This process is known as ‘burning’ data onto a disc.


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Magnetic Storage

In the case of magnetic tape the dots are arranged along the length of a long plastic strip which has been coated with a magnetisable layer (audio and video tapes use a similar technology).


In the case of magnetic discs (e.g. floppy disc or hard-drive), the dots are arranged in circles on the surface of a plastic, metal or glass disc that has a magnetisable coating.


Solid state storage

The term ‘solid-state’ essentially means ‘no moving parts’.


Solid-state storage devices are based on electronic circuits with no moving parts (no reels of tape, no spinning discs, no laser beams, etc.)


Solid-state storage devices store data using a special type of memory called flash memory.