Secondary Storage Devices…. What is a Secondary Storage Secondary Storage Devices….. What is a Secondary Storage? Secondary storage also known as Auxiliary memory is used with every computer system. This section of the computer’s memory is non-volatile and has low cost per bit stored, but it generally has operating speed far slower than of the primary storage. This section of the memory is used to store large volume of data on permanent basis which can be transferred to the primary storage as and when required for processing. Data is stored in binary codes in secondary storage.

The popular one’s used in today’s computer systems are: Magnetic tape, Floppy disk, Zip disk, Disk pack, Winchester disk, CD-ROM, WORM disk and Magneto-optical disk.

Several different devices can be used as a secondary storage device, but the one selected for a particular application mainly depends upon how the stored information needs to be accessed. Basically, there are two methods of accessing information--- Sequential access and Direct or random access

Sequential Access Devices… A Sequential access storage device is one in which the arrival at the location desired may be preceded by sequencing through other locations, so that the access varies according to location. The information on this device can only be retrieved in the same sequence in which it is stored. This processing is quite stable for such applications like preparation of monthly pay slips, or monthly electricity bills, etc..

Direct Access Devices… Secondary storage devices exist which permit access to individual information in a more direct or immediate manner. These direct-access devices are also called random-access devices because the information is literally available at random i.e; it is available in any order. Thus a random-access storage device is one in which any location in the device may be selected at random, access to the information stored is direct, and approximately equal access time is required for each location. Magnetic, optical and magneto-optical disks are some examples of direct-access storage devices.

Magnetic Tape… It is one of the oldest and popular form of secondary storage device. It is the storage medium for large data that are sequentially processed and need to be stored off-line. It is used as backup storage for data stored on on-line storage devices such as a hard disk . It is a plastic ribbon coated with iron oxide. Data is stored in the form of tiny invisible magnetized and non-magnetized spots on the coated surface of the tape.

Magnetic Disk… It is the thin, circular plate made of metal or plastic coated with magnetized material as iron oxide. Data are recorded in the form of tiny invisible magnetized and non-magnetized spots on the disk surface. Magnetic disks can also be erased and reused indefinitely. Data is recorded in a standard binary code, usually 8-bit EBCDIC.

Floppy Disks… A floppy disk is a round, flat piece of flexible plastic coated with magnetic oxide. It is encased in a square plastic or vinyl jacket cover. The jacket gives handling protection to the to the disk surface. It has a special liner that provides a wiping action to remove dust particles that are harmful for the disk surface and the read/write head. These disks are made of flexible plastic plates which can bend.

Hard Disks… Hard disks are the primary on-line secondary storage device for most computer systems today. Unlike floppy disks, which are made of flexible plastic or mylar, hard disks are made of rigid metal. The hard disk platter come in many sizes ranging from 1 to 14 – inch diameter. Types of Hard Disks… Zip Disks… Disk Packs… Winchester Disks…

Zip Disks… In this type, a single hard disk platter is encased in a plastic cartilage. It is commonly of 3½ inch having a storage capacity of about 100 MB. Its disk drive is called a zip drive. The zip disks can be easily inserted into or removed from a zip drive just as we insert and remove floppy disks in a floppy disk drive or a video cassette in a VCR.

Disk Packs… A disk pack consists of multiple hard disk platters mounted on a single central shaft. The disk drive of a disk pack has a separate read/write head for each disk surface excluding the upper surface of the topmost disk and the lower surface of the bottom most disk. Different disk packs can be mounted on the same disk-pack drive at different instances of time. This gives virtually unlimited storage capacity to disk packs.

Winchester Disks… A Winchester disk also consists of multiple hard disk platters mounted on a single central shaft. The main difference between a Winchester disk and a disk pack is that Winchester disk are of fixed type. The hard disk platters and the disk drive are sealed together in a contamination-free container and cannot be separated from each other. It has a limited capacity than that of disk packs.

Optical Disk… An optical-disk storage system consists of a rotating disk which is coated with a thin metal or some other material which is highly reflective. Laser beam technology is used for recording/reading of data on the disk. Due to use of laser beam technology, optical disks are also known as laser disks or optical laser disks. Types of optical disks… CD-ROM, WORM Disk, CD-RW, DVD…..

CD-ROM… CD-ROM stands for Compact Disk Read Only Memory CD-ROM… CD-ROM stands for Compact Disk Read Only Memory. CD-ROM disk is a shiny, silver color metal disk of 5¼ inch(12cm) diameter. It has a storage capacity of about 650 megabytes. It has an enormous storage capacity. The information stored on them cannot be altered. The process of recording information on these disks is called mastering. CD-ROM disks are used for distribution of electronic version of conference proceedings, journals, magazines, books, etc. They are also used for storage and distribution of multimedia applications like text, sound, graphics, video and animation.

CD

WORM Disk… WORM stands for write-once, read many WORM Disk… WORM stands for write-once, read many. WORM disk allow the users to create their own CD-ROM disks by using a CD-recordable(CD-R) drive, which can be attached to a computer as a regular peripheral device. WORM disks, which look like standard CD-ROM disks, are purchased blank and encoded using a CD-R drive. As the name implies, data can be written only once on a WORM disk, but can be read many times. All the data to be recorded have to be written on the surface in a single recording section.

Digital Versatile Disk (DVD) Digital Versatile Disk (DVD)... DVD is a standard format for the distribution and interchange of digital content in both computer based and consumer electronic products. The format is designed to store various types of large multimedia data on a new and improved version of CD, which works on optical disk technology. The DVD standard has two layers of specifications known as physical layer and logical layer. The physical layer apply to the physical media itself, where as the logical layer supports all types of applications.

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