Types of storage devices and their characteristics

The storage device is used to keep a copy of data or information temporally when it is being processed, or permanently for future reference. They store the data which is used by the computer and the user. The storage capacity is calculated in terms of bytes where 1 byte is made up of 8 bits (0s and or 1s).

The main categories of storage devices are RAM, ROM, cache, magnetic, optical, cloud, and solid-state storage methods. Primary storage, they are volatile while secondary stores data permanently.

The table below shows the most common unit to measure storage memory.

Memory units in a computer
Representation (Notation)	Memory unit description
0 OR 1	bit
4 bits	nibble
8 bits	byte
1 KiloByte (KB)	1024bytes (2^10)
1 MegaByte (MB)	1024KB
1 GigaByte (GB)	1024MB
1 TeraByte (TB)	1024GB
1 PetaBtye (PT)	1024GB
NB: Most of today’s ordinary user devices reach a TB capacity but memory size is increasing every day for future devices.

Storage devices are categorized into two main classes: primary and secondary storage hardware.

Primary storage devices and their characteristics

They are also known as the main memory of a computer system. Mostly they store data temporally or semi-permanent.

Mostly they are characterized by low memory space, and high cost but high speed of data transfer. They store 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 increases the processing speed.

Characteristics of Main Memory

1. They are volatile (they store data only when they are powered, specifically RAM)
2. Data is stored temporarily (RAM) and semi-permanently for ROM
3. They have high access speed compared to secondary storage.
4. Main memories have a small storage capacity compared to secondary storage.

Primary storage is classified again into 2: Random Access Memory (RAM) and Read-Only Memory (ROM)

Random Access Memory (RAM):

They are the working memory of the device and they are used to store data that is being processed. RAM stores information temporally until it is stored in secondary storage like a hard disk or flash disk.

RAMs are volatile memory since they only store data where they are supplied with power. When power is switched off the data is lost.

There are 2 types of RAMs.

1. Dynamic RAM (DRAM): for this memory to keep data they need to refresh many times hence the term dynamic. They require extra circuits to enable frequent refreshing. DRAM is commonly used in computers since they are cheaper although they are slower. They leak data which necessitates refreshing.
2. Static RAM (SRAM): they can store data without refreshing. They are more expensive than DRAM and hence not commonly used in most computers. SRAM is also faster than DRAM since they don’t lose data refreshing.

Read-Only Memory (ROM)

They only allow the reading of data in the memory but no addition during normal computer operation.  Mostly they store data semi-permanently or permanently. They are used to store firmware software from the manufacturer of the computer, like the BIOS information.

ROM can be divided again into 3:

1. Programmable Read-Only Memory (PROM): It is a type of ROM that can be programmed to change the content of the information only once.
2. Erasable Programmable Read-Only Memory (EPROM): They can be programmed by exposing them to ultraviolet light. During programming, you need to delete all data to add more.
3. Electrical Erasable Programmable Read-Only Memory (EEPROM): The content can be deleted by use of electrical means. Users can delete only a portion of the memory and leave the data in other parts. they can be reprogrammed many times without a problem.

What are cache memory and its importance?

A cache is a special type of memory that is sometimes inbuilt within the CPU. The computer processor operates at a very high speed hence it requires to access data faster. RAM is the main memory where processors access data for processing, but compared to processor speed it is slow. You can read more on functions, architecture, types, and advantages of cache memory.

To increase the access speed cache memory is used. It is faster than the RAM and the first place processor looks for data, if it misses in the cache then it moves to RAM.

There are two levels of cache depending on the proximity of the memory to the processor.

1. Level 1 Cache memory: This is the closest to the processor and the first place where it checks data. It is inbuilt within the processor.
2. Level 2 cache memory: It is a chip close to the processor and it is the second place where the processor checks for data after Level 1.

Characteristics of cache memory

1. It is the fastest type of memory
2. It stores data temporally
3. The technology used to manufacture is expensive.
4. Inbuilt within the CPU or motherboard
5. Stores data that is frequently required.

Types of secondary storage devices

They are used for bulky permanent storage of data. Secondary storage is characterized by high storage capacity, relatively low cost, and low transfer speed when compared with primary storage.

Characteristics of secondary storage devices

1. They have a high storage capacity. They can be used for bulky storage of backup.
2. Devices are relatively cheap compared to main memory.
3. Secondary storage devices have low data transfer speeds when compared with primary storage.
4. Data is stored permanently or until the user decides to delete it.

Secondary storage is classified into 4 types:

• Magnetic storage devices

They store data in terms of magnetic fields like hard disks, floppy disks, magnetic tape, etc. A hard disk is the main storage in today`s computer. It is made of platters that are divided into tracks and sectors for data storage and a head that reads data from the platters. It retrieves data faster that other magnetic devices and it can hold more data.

Some latest computers mostly portable computers are using solid-state storage devices. This offer even fast data access since there are no moving parts like on a hard disk.

• Optical storage devices

They store information on a reflective surface which requires a beam of laser light to read or write on it. Compact Disks were the first optical disk but are being replaced 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 CDs and DVDs to read and write. They also have a higher capacity than the earlier two.

Optical disk requires an optical disk drive installed into the computer or an external drive to read and write on them.

• Solid-state storage devices

They have no movable parts, unlike magnetic disks. They are small in size and have low storage space but are relatively cheap and convenient for the storage of a small amount of information. Most of them have flash memory. Examples include memory sticks, flash disk smart memory, etc.

They don’t consume a lot of power hence they are powered by simply connecting them to a computer via USB.

• Cloud storage

With fast internet speed, most organizations are moving operations to the cloud where all data is stored and accessed. The storage is sourced from companies that offer storage space online for different purposes.

The main advantage of this technology is that data can be accessed from any location. It is also very simple to expand the storage capacity since you don’t need to buy any hardware just to pay for extra space. Google Cloud is an example of cloud storage.

It should be noted that still, cloud storage providers use physical storage where their data centers are located.

Comparison between primary and secondary storage devices

S/No	Primary memory	Secondary storage
1	Fast data access speed.	Low access speed.
2	Volatile.	Non-volatile.
3	Data is stored temporally.	Data is stored permanently or until deleted by the user.
4	Working memory of the computer.	Backup storage of the computer.
5	Low storage capacity.	High storage capacity.
6	Examples are RAM and ROM.	Hard disk Tape disk, Flash memory, DVD/CD, SD card.
7	They are expensive.	Relatively cheap.