secondary storage
main memory characteristic:
Its contents can be accessed directly by the CPU.
It has very fast access times.
It has a relatively small capacity.
ROM
ROM is memory that cannot be changed by a program or user. ROM retains its memory even after the computer is turned off.ROM is used to store the instructions for the computer to boot up when it is turned on. ROM is also used to store software that needs to be available when the computer is turned on (such as the instructions for booting the computer) or software that will never change, such as the BIOS.
RAM
RAM is a fast temporary type of memory in which programs, applications and data are held.RAM holds things while the computer is on and they are running:
the operating system
applications software.
If a computer loses power, all data stored in its RAM is lost.With von Neumann architecture, the program(s) being run are kept in the computer memory as well as the data that is currently being processed.
Long term storage is called Secondary Storage Hard Disk that stores all the files long term but this term also includes flash memory sticks, floppy disks (seldom used these days), tapes, CDs and DVDs.Secondary storage is usually much larger than the RAM and as we want such a lot of it, it needs to be cheap. Cheaper memory technologies tend to have slower access speeds than main memory.Secondary memory is non-volatile and needs to be robust and reliable.Choosing the right type of storage medium for a particular use is important. You need to consider the following features:Capacity: How much space there is to store files. Compare the size of a floppy disk at 1.44MB to a CD that can store 700MB or 4.7GB on a DVD. Speed: How quickly the computer can read data from a storage device or write data to it.Portability: Can you easily unplug it and carry it away? Does it fit in a pencil case/handbag?Durability: How easily is it damaged? Will it survive dropping or having coffee tipped on it?Reliability: How long will it last? Anything with moving parts is likely to be less reliable.
sequential or random
Memory can be accessed sequentially or randomly.To go from file A to file Z in a sequential-access system, you must pass through all files in order.Sequential access is also called serial access.
Refers to the ability to access data at random. In a random-access system, you can jump directly to file Z.Disks are random access media.
Secondary storage is used to save permanent copies of your files. The majority of secondary storage devices are used for long-term storage. They can also used to:
backup data
add more storage space for files/pictures/videos etc.
transfer files between computers
easily transport files
share files over a network.
magnetic:
A magnetic hard disk drive (HDD) uses moving read/write heads that contain electromagnets.These create a magnetic charge on the disk’s surface which contains iron particles that can be given a magnetic charge in one of two directions.The change in the magnetic particle’s polarity represents 0 (off) or 1 (on). These represent a bit of data that the CPU can recognise.
advantage:
Very large data storage capacity.
Stores and retrieves data much faster than an optical disk.
Data is not lost when you switch off the computer as it is with RAM.
Cheap per MB compared with other storage media.
Can easily be replaced and upgraded.
disadvantage:
Hard disks have moving parts which can fail.
Crashes can damage the surface of the disk, leading to loss of data.
Easily damaged if dropped.
Uses a large amount of power compared with other media.
Can be noisy.
optical storage:
An optical drive uses reflected light to read data.The optical disk’s surface is covered with tiny dents (pits) and flat spots (lands), which cause light to be reflected off them differently.When an optical drive shines light into a pit, the light is not reflected back.This represents a bit value of 0 (off). When the light shines on a flat surface (land) it reflects light back to the sensor, representing a bit value of 1 (on).
advantage:
Easy to store and carry. (portability)
Can be read in a number of devices, such as audio and TV systems.
Very easy to use.
Long lasting if looked after properly, otherwise easily damaged, scratched
disadvantage:
Small capacity
Data on write-once disks (CD-R, DVD-R and DRR) are permanent and cannot be changed.
Require special drives to read/write.
Expensive per GB/TB in comparison with other methods.
There are no standards for longevity tests.
Can easily be scratched and damaged by heat and light.
Easily broken.
solid state disk
Solid state disks contain no moving parts. They are found in three common formats:
Hard disk replacements.
Memory cards.
USB flash drives.
They record data using special transistors that retain their state even when there is no power to them.
advantage
Read speeds are faster than for normal hard drives.
Because there is no moving actuator arm like on a hard disk drive (HDD), they are faster in reading and, in some cases, writing data.
They are also more rugged so are not as easily damaged when dropped.
They have non-volatile memory, which means that data is stable.
They are lightweight.
They are very durable.
They are free from mechanical problems.
They require less power than magnetic drives.
They are silent in use.
disadvantage
They have limited storage capacity when compared with normal magnetic hard drives.
Random write speeds of solid state drives can be four times slower than for normal magnetic hard drives.
The cost per MB stored is higher than for magnetic drives.
cloud storage
Cloud storage means ‘the storage of data online in the cloud’.In cloud storage, data is stored and is accessible from multiple distributed and connected resources that comprise a cloud via the internet.