Storage device is used to keep a copy of data or information temporally when it is being processed of permanently for future references. They store the data which is used by the computer and the user. The storage capacity is calculated in terms of bytes where 1byte is made up of 8bits (0s and or 1s).
Storage device are categorized into two main classes: primary and secondary storage hardware.
Primary storage device
They are also known as main memory of a computer system. Mostly they store data temporally or semi-permanent. They are said to be volatile type of memory because they retain information as long as the power is on, when it goes off the content is lost.
Mostly they are characterized by low memory space, high cost but high speed of data transfer. They stores data that is being processed at the moment by the computer. This reduces the time to fetch the information from secondary storage like the hard disk and hence increasing the processing speed.
Primary storage are classified again into 2: Random Access Memory (RAM) and Read Only Memory (ROM)
As the word suggests it kind of memory where the information in it can be accessed at any point, there is no specific order of retrieval. RAM stores information temporally only and when the power goes off so the information.
There are dynamic RAMs which stores data in a manner that it is not static, it keeps on refreshing. Static RAM (SRAM) is another one which is the opposite of DRAM, the data is not refreshed it don’t change.
Read Only Memory (ROM)
They only allow reading of data in the memory but no addition during the normal computer operation. They are also random which means like RAM data can be accessed from anywhere in the device. Mostly they store data semi-permanently or permanently. They are used to carry firmware software from the manufacture of the computer, like the BIOS information.
ROM can be divided again into 3: ROM, PROM, EPROM and EEPROM.
Read Only Memory (ROM): It is the normal ROM that has permanent information in they, they cannot be changed once written on.
Programmable Read Only Memory (PROM): It is type of ROM that can be programmed to change the content of the information in it.
Electrical Programmable Read Only Memory (EPROM): They can be programmed afresh by use of electricity which is used to modify how data in arranged in the device.
Erasable Electrical Programmable Read Only Memory (EPROM): The content can be deleted by use of electrical means to put in it a new programme.
Secondary storage device
They are used for permanent storage of data and they are not accessed directly by the processor. Secondary storage are characterized by high storage capacity relatively low cost and low transfer speed when compared with primary storage.
They are classified again into 3: magnetic, optical and solid state storage.
They store data in terms of magnetic fields like in hard disk, floppy disk, magnetic tape etc. hard disk is the main storage in today`s computer. It is made of platters which are divided into tracks and sector for data storage and a head which reads data from the platters. It retrieves data faster that other magnetic devices and it can hold more data.
They stores information on a reflective surface which require a beam of laser light to read or write on it. Compact Disks were the first optical disk but are being replaces by DVDs which has higher storage capacity. Blu-rays are the latest optical disk and they use blue laser light rather than red used on CD and DVD to read and write. They also have higher capacity than the earlier two.
Optical disk requires an optical disk drive installed into the computer to read and write on them.
They have no movable parts like in magnetic disks. They are small in physical size and low storage space but relatively cheap and convenient for storage of small amount of information. Most of them have flash memory. Examples include memory stick, flash disk smart memory etc.