3.7.4.2 - Secondary storage devices Flashcards
What is secondary storage and why is it needed?
Secondary storage is the name give to a range od devices that offer permanent storage for data files and application
It is needed because the main memory, the other form of memory, is volatile and low capacity(so more expensive per data), so data will be lost when the computer is shut off
What is secondary storage and why is it needed?
Secondary storage is the name given to the devices that be used to permanently store data files and applications
main memry is volatile and low capacity(so more expensive) so a non volatile long term high capacity storage is needed
What is a HDD?
Hard disk drives are a type of secondary storage that use magnetism and magnetised disks to represent 1 and 0s, and so to store data
How do HDDs work?
A series of concentric disks are stacked on each other.
An actuator is connected to an actuator arm which connects the read write head to be able to brush against the spinning platter
The actuator moves the read write head laterally while the platter spins so that all parts of the disk can be accessed.
How are the platters organised and addressed?
The platter is split into a series of concentric rings called tracks
Tracks are divided into sectors
A few contiguous track sectors are called a cluster, which is the smallest addressable unit of memory on the HDD
What is optical disc storage?
A type of secondary storage that uses laser technology to store information on a disc.
It consists of one large spiral track unlike HDD which consists of a series of concentric tracks
How do read only disks work?
Read-only - Pits are burned into the disk by a high powered laser permanently. A low powered laser is then used to read from this disc.
Light is reflected from the laser onto a photodiode which detects light.
When the light reflects of a pit, it is scattered, when it reflects of a land, it is reflected straightback
How do recordable disks work?
They used a photosensitive dye rather than pits and lands. A high powered laser can change the dye from opaque to transparent in order to represent information(the photodiode will recieve more light when it is transparent to represent a 1)
A low powered laser is used to read from the disc which does not affect the dye
How do rewriteable disks work?
A rewriteable disk also uses a special dye where the chemical burning process can be reversed, allowing it to be written to multiple times
What is SSD?
SSDs(solid state drives) are a type of non volatile secondary storage that use millions of NAND flash memory cells to store data and controllers to manage them.
It stores data in floating gate transistor, which work without power by trapping electrical charge to represent info between oxide layers
Data is organised into blocks which consists of a table of pages.
Downsides and upsides of SSd
It has no mechanical parts making it less prone to failure
it has very high read write access speeds
lower latency
when data from a page must be re written to, tee whole block has to have its data erased
it is high cost and has less capacity thant magnetic
