Storage – Computer Science OCR – Unit Three

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Storage:  (The Information Is Sourced From Seneca And CGP Computer Science)

Secondary storage – The long term, non-volatile storage of data

The need for secondary storage: 

As RAM is volatile, any data or programs currently being stored there will disappear once the power is lost. This is when secondary storage comes in. Secondary storage is used to store a copy of programs and data that need to be kept long term.

Data Capacity: – This is how much of data a device can store.

-The Bit: – This is the smallest unit of data that can be stored. It is often referred to as a ‘Binary digit’

The value of a bit can be either 0 or 1.

Ranking Order of Capacity:


-Nibble (4 Bits)

-Byte (8 Bits)

-Kilobyte (1024 bytes)

-Megabyte (1024 Kilobytes)

-Gigabyte (1024 Megabytes)

-Terabyte (1024 Gigabyte)

-Petabyte (1024 Terabyte)

Optical Storage:

Optical storage uses laser light to retrieve data from the surface of optical media.

Optical storage is often used to deliver multimedia content such as video.

Storing a bit: Each 1 or 0 is represented as a microscopic hole, which called a pit, or a flat surface, called a land, on the surface of the disk.

Accessing data: A laser is shone on the disk surface.

-If the laser hits a land, it will be reflected into a sensor.

-If the laser hits a pit, then it will be reflected elsewhere.

-This can be used to represent 1 and 0.

Properties of Optical Media:

-Low Capacity – Disks are small and fairly cheap, so it can be easy to gain a high capacity of storage by buying multiple disks. (Low capacity per disk)

-Slow Speed – the parts in the optical media drive have to move to find the right bit of data on the disk, this means that the access times can be quite slow and writing to a disk is a slow process.

-Great Reliability – Optical media is waterproof and shockproof. (robust) This makes it ideal for portability (transporting) However, moving parts can cause the disk drive to break and can be scratched.

Types of Optical storage:

-Blu-Ray Disks (BD) – These are used to store high definition movies or games. There store around 25GB of data.

-Compact Disk (CD) – CDs are the original optical media standard. A single CD can store about 700MB of data. CDs are very cheap to produce.

-Digital Versatile Disk (DVD) – These are often used to store games or standard-definition movies for home viewing. A single DVD can store around 4.7 GB of data.

Form of Optical storage:

-Re-writable (RW) – A CD-RW, DVD-RW, OR BD-RW is a disk which can be overwritten whenever the user would to. These are often used for backups which might be updated in the future.

-Write-once ( R ) – A CD-R, DVD-R, OR BD-R s a disk which can be written once by the user at home. This means that once the user has written data to the disk, they cannot overwrite it.

-Read-Only Memory (ROM) – A CD-ROM, DVD-ROM, or BD-ROM is a disk which can be read only. This is because the manufacturer writes the initial data, and this cannot be changed. This is often used in content distribution so that people cannot accidentally overwrite the content.

 Magnetic Storage:

Storing a bit: The value of each bit of data (0 or 1) is represented as a positively or negatively charged magnetic particle.

These particles are part of a magnetic disk called the platter.

Accessing Data: Data is stored magnetically in small areas called sectors within circular tracks. A magnetic read/write head is used to access sectors while the platter spins.


-The Hard Disk Drive has moving parts.

-Although the HDD is generally reliable, any device with moving parts will be subject to wear and tear.

-Also, they can be damaged by shocks and vibrations. They are not robust.

Properties of Magnetic Media:

-Slow Speed. Magnetic media have moving parts. This makes magnetic media slow to access.

-Good Reliability. Magnetic media is reliable and lasts for a long time. However, it is not robust, it tends to break on impact or during vibrations.

-Low cost. Magnetic media is very low cost per gigabyte (or low cost per unit capacity)

-High capacity. Magnetic media can store a large quantity of data in a compact way. This makes them an ideal choice for archives and home computers.

Example of magnetic storage:

-Hybrid drives: Combination of magnetic and solid state storage is knowing as a hybrid drive. This offers the benefits of both systems. (Been told that this will not be asked in the exam, but you never know. No harm in not knowing)

-Hard Disk Drives (HDD) – Main form of secondary storage in a computer system. Stores the operating system, applications and user data.

-Magnetic tapes:

These are used for archiving due to the fact they have a much greater storage capacity than HDDs.

They come in plastic cassettes which contains reels of tape. A cassette requires a special tape-drive for read/writing.

Tape is read and written sequentially. This means that it reads/writes from the beginning to the end. (or stopped by the computer) This means tape is very slow when finding specific data stored on it. However, it has a fast read/write speed once it is in the correct place to begin reading/writing.

Solid state storage:

Storing a bit: Solid state drives are made of flash memory. Flash memory stores the value of each bit in a semi-conductor chip.


-Solid state drives are robust. They have no moving parts. This means they are not easily damaged by shocks or vibrations.

-SSD have a limited number of read/write cycles. They can only be flashed (rewritten) a certain number of times before they start to degrade.

Accessing Data: Data can be assessed randomly. This means that any piece of data can be accessed in a constant amount of time. This is achievable because there are no moving parts.

Properties of solid state media:

-High speed – Solid state media have very fast read/write times compared to magnetic and optical storage media.

-High cost – Solid state media has a higher cost-per-GB compared to magnetic media. A person may pay more for the increase in performance. For example, faster data access speeds.

-Good capacity – They have a good capacity but not on the scale compared to magnetic media.

-Good reliability: Solid state drives are robust. They have no moving parts. This means they are not easily damaged by shocks or vibrations.

Advantages of Hard Disk Drives: Advantages Of Solid State Drives:
HDDs are cheaper. SSDs are faster.
Both are high capacity, but HDDs are higher. SSDs don’t need fragmenting.
HDD have longer read/write life than SSDs as SSDs can only be written a certain number of times before they begin to degrade. SSDs are more shock-proof than HDDs.
HDDs make some noise while SSDs are slient.

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