What is Storage Device ?
Memory and storage devices are used in computer for keeping the entered data and instructions through the input devices. It is also used to keep the intermediate results produced during the execution process in computer. As well as, the final result after processing is also keep in it, before displaying it on the output unit.
Types of Storage Devices
There are two types of computer memories (as shown in figure below) that are commonly used :
Primary Storage Devices /Memory
Primary memory is also known as internal memory. All computers must have primary memory. Primary memory is volatile in nature. Its design looks like a honeycomb that has many small compartments These compartments are individually known as 'cell' or 'memory cell. These cells can be recognized by unique built-in address that cannot be changed. A fixed number of bits can be stored in each cell. These bits are termed at word length.
Primary memory is in the form of memory chip that is housed on rectangular circuit boards. A special slot is provided over the motherboard for plugging up this rectangle circuit board. Motherboard is the principle circuit board which consists of several electronic components.
Types of Primary Storage Devices
On the basis of some features, primary memory can be categorized into two types :
1) RAM :
RAM stands for Random Access Memory. It is a read and write memory (R/W memory) RAM enables users to write and read information from this memory. It has the facility of random access. It is a volatile memory, i.e., any information will erase when computer's power is turned off.
2) ROM :
ROM stands for Read Only Memory. This memory consists of several built-in programs called micro programs for monitoring the control unit. Computer manufacturers store these programs in ROM in the firmware form (embedded programs into a chip) so that no one can manipulate them. All operations are directed by the control unit in accordance with these programs ROM-BIOS (Read-Only Memory Basic Input and Output Services) is one of the micro program used for monitoring basic computer operations.
Secondary Storage Devices /Memory
Secondary memory is also called as Auxiliary memory, Secondary memory is the component of computer system that can be added additionally according to the user's requirement. They are mainly used to store huge amount of data for long time period. The storage capacity of this memory is in terabytes or more. This memory cannot be addressable by the CPU directly. It is a high speed memory. A high speed data channel is used to connect it with main memory.
Magnetic disks and tapes are most commonly used secondary memory devices. Some of the other devices are magnetic drums, magnetic bubble memory, and optical disks. All these devices have the facility to store huge amount of data. One more feature of this memory is its non-volatile nature. As compared to main memory, its speed is slow. Some more characteristics of secondary storage devices are its access mode, access time, transfer rate, capacity and cost.
Types of Secondary Storage Devices
1) Magnetic Storage Media :
In this media, magnetic material is used for storing data. Several types of magnetic media are as follows :
i) Magnetic Tape :
It is similar to cassette tape which uses cartridge tape for recording data. It is made-up of plastic that is coated with the magnetic material. This tape runs along the entire length of tape, dividing it into two tracks. Data are stored sequentially on these tracks. These tapes are used in situations when data volume or software is very large. The capacity of cartridge tape is 1 GB or more.
ii) Hard Disk (HD) :
In hard disk, data are stored on electromagnetically charged surface. It has the facility to provide quick access of huge data. Presently, hard disks are coming with huge capacity storage (1 TB) and permanently fixed in PC. As compared to floppy disk drive, transfer rate of data from CPU to hard disk is much higher. Hard disk is used by CPU mainly for storing data as well as loading programs and data.
Hard disk is designed as a set of stacked disks like a phonograph record. The tracks or concentric circles are used to record the data electromagnetically. Like the phonograph arm. head is used to write and read the information from. tracks. As the disk rotates, two heads on each side of disk read or write data. Data to be located is the main requirement of each read and write operation of disk. This process is known as "seek" operation. Access time of disk is measured in milliseconds.
Hard disks are present with several rotation speeds. that range from 4500 to 7200 rpm. In present time, Hard disk is mapped with LBA (Logical Block Address) to identify the physical location. LBA works with larger address range on hard disk. Physical location can also be identified with cylinder, track, and sector locations.
iii) Floppy Disk (FD) :
It is a storage medium made up of disk (or diskette) of thin and flexible storage medium. These sets of disks are wrapped in a rectangular plastic carrier. To remove dust particles, these sets are lined with fabric. A FDD (Floppy Disk Drive) is used to read and write the floppy disk. Floppy drive is t connected or installed to the computer and represented by 'A' drive. Files can be stored and transferred from computer to another by floppy disk with the use of floppy drive. After reading the disk by floppy drive, users are able to modify the files stored on disk.
2) Optical Storage Media :
These devices use optical or laser beams in place of magnetic storage. Tiny holes presented on the reflective platter disk are burnt with the use of pinpoint laser beam. Another laser beam installed on the disk, shines on the disk surface at the time of reading data. Reflection shows one binary state. No reflection means a reading of next binary state. As compared to magnetic media, this storage is slower. Following are the some basic types of magnetic media :
i) Compact Disk (CD) :
CDs are also termed as optical disks. It has huge capacity of storage such as up to 800 MB or even more. However, for reading data, it requires a special device CD ROM or CD writer.
ii) Digital Video Disk (DVD) :
These are mainly used for video storage. Like the CDs, they have same dimensions, except that DVD can store much more data approximately seven times that of CD.
Variation of DVD
Variation of DVD implies the way data is stored on the discs :
- DVD-ROM (read only memory) contains: data which can only be read and not written.
- DVD-R and DVD+R (recordable) records data only single time and then functions as a DVD-ROM.
- DVD-RW (re-writable), DVD+RW, and DVD-RAM (random access memory) are all able to record and erase data many times.
The standard wavelength of DVD lasers is 650nm with a red color light, Properly formatted and structured video and audio content are referred to as DVD-Video and DVD-Audio discs respectively. DVD Data discs refer to those with video content.
3) Flash Memory :
It is a solid chip developed from EEPROM by Toshiba in 1984. It does not require any external power to maintain stored data. They are mainly used in portable devices.
Pen Drive
This device is used for transferring files easily between USB compatible systems. Its size is very small like a key ring. Its working is same as floppy. When this device is connected to the system, an icon "removable device" is shown in the drive list. The data stored in pen drive can be modified, i.e., can be edited, deleted or even saved. The storage capacity of pen drive ranges from 512 MB to 128 GB.