SECONDARY STORAGE DEVICES
Secondary storage devices are also called backup storage because it is used to store data. Volume of data on permanent basis which can be partially transferred to the primary storage, when required for data processing. Afterwards these devices are comparatively cheap and provide greater space to store the data /instructions are stored on secondary storage devices in the same binary codes as in primary storage.
The storage capacity of the primary storage of today’s computer is not sufficient. To store a large volume of data as a result additional memory called secondary storage is needed with most of the computer system.These devices also provides the fast communication than I/O devices. The internal memory of a computer is a volatile memory. Therefore, we cannot save the data permanently. In that case we require secondary storage device which provide the facility to store the data for future use.
RANDOM ACCESS DEVICES
Random Access Devices are those devices on which we can directly access the data. These devices are comparatively provide the fast communication.For example, hard disk, floppy disk, optical disk.
A floppy disk, also called simply a diskette or disk, is a small flexible Mylar disk coated with iron oxide on which data are stored. The floppy disk has been around since early 1970s, today it is available in three 3½ inch, 5¼ inch and 8 inch sizes. The 5¼ and 8 inch diskettes are covered by stiff protective jacket with different holes. The central big hole called hub ring which is used to hold by disk drive during rotation. The elongated read write window is used to read and write data through read/write head. The small hole next to the hub ring is called index hole which is used to locating data through computer. The cut out on the side of the floppy disk is called write protect notch. If we cover this opening with a piece of paper then we can’t write data on to disk.In small diskette a hard plastic cover and protective metal is used to protect disk. Before using a disk we have to format a disk in which disk is divided into tracks and sectors for storing the data. Diskettes may be double sided and single sided while the storage capacity become less or more.Floppy diskettes are more convenient to use with microcomputers. A floppy disk which is a random access device can access data fast than magnetic tape.
Magnetic tape is a sequential access device about one half or one fourth inch in size and made of Mylar (a plastic material) coated with a thin layer of iron oxide. Data can be read and write through a device which is called tape drive. The read/write head of tape drive which is an electromagnetic component read, write and erase data from magnetic tape. Magnetic tape is divided into nine separate strips or tracks in which eight tracks are used to store data and ninth track is used for error checking bit.Magnetic tape can store large quantities of data therefore they are erasable, usable and durable secondary storage device. But it can use with large computers.
Hard disk is a thin circular metal plate coated both side with a magnetic material. A hard disk pack consist of a number of disk mounted on central shaft which rotate at a speed of 2400 rpm or more. In a hard disk information is stored on both the surfaces of each disk plate except the upper and the lower surfaces of the bottom plate which are not used. Information is recorded on the track of the disk surfaces in the form of invisible tiny magnetic spot. The presence of a magnetic spot represent 1 bit and its absence represents 0 bit.Hard disk are potentially very high capacity storage devices typically in the range of 20 megabyte to 1 gigabyte. Data are recorded on the tracks of a spinning disk surface and read from the surface by one or more read/write heads. There are two basic types of disk system
.1. Moving head 2. Fixed head
- MOVING HEAD
The moving head consist of one read/write head for each disk surface mounted on an axis and which can be moved in and out. In this system each read/write head moves horizontally across the surface of the disk. So that it is able to access each track individually. Information stored on the tracks which constitute a cylindrical shape through the disk pack are therefore accessed simultaneously.
- FIXED HEAD
In the fixed head system the axis are non-movable. A large number of read/write heads are distributed over the disk surfaces. One head for each track as a result no head movement is required and therefore information is accessed more quickly.
CD-ROM (Compact Disc, Read Only Memory) is an adaption of the CD that is designed to store computer data in the form of text and graphics, as well as hi-fi stereo sound. It is a non-volatile optical data storage medium that is physically the same as an audio CD and is capable of storing large amounts of data up to 1 GB, althrough the most common size is 650 MB about 12 million bytes per pound weight. All CD-ROMs confirm to a standard size and format, so you can load any type of CD-ROM into any CD-ROM player. In addition CD-ROM players are also capable of playing audio CDs. A standard CD is 120mm (4.75 inches) in diameter and 1.2 mm (0.05 inches) thick.
Difference in the Capacity Between Hard Disk and Floppy Disk
A hard disk contains several disk platters stacked on a single rotating spindle. Data are stored on all recording surfaces. For a disk with four platters, there are eight recording surfaces on which data can be stored. A floppy disk has only one platter on which data is written, that is why the storage capacity of a hard disk is more as compared to the floppy disk. Hard disks or fixed disks are also called Winchester disk are permanently installed or fixed with in the system unit.