Secondary Storage Devices
CD-ROM, DVD, ZIP DISK, JAZ Drive, WORM, Magnetic tapes etc used to store data’s are known as Secondary Memory. Secondary memory can also be called as Backup Memory because they are used to store data and information for future use.
Some of the Secondary Storage devices are described below:
(a) Floppy Disk:
- In 1972, IBM lunched Floppy diskette in market to store data data and program.
- Floppy is made up with Mylar plastic or Polly vinyl plastic. It has concentric ring (tracks) and redial sectors to store data.
- Plastic is covered with iron oxide or barium ferrite. When magnetic material is magnetized binary 1 is produced and when it is demagnetized, binary 0 is produced.
- Its storing capacity is 1.44 MB and rotates with speed of 360 rpm (revolution per minute)
- SSSD (Single Sided Single Density)
- SSDD (Single Sided Double Density)
- DSDD (Double Sided Single Density)
- DSDD (D0uble Sided Double Density)
(b) Hard Disk:
IBM unveiled its first hard disk in 1956, it was the size of a dishwasher, with fifty platters, each with a two- feet diameter. These platters could hold 160KB of data. Seagate technology announced the first Winchester 5.25″ hard disk drive with four platters capacity of 5MB data in 1980.
- Platters: There are many aluminum platters are packed in Hard disk. These platters are coated on both side with a special materials designed to store data in the form of magnetic patterns. The read-write surface of disk is coated with magnetic materials. The upper most surface of upper disk and lower most surface of lower disk are not used as read-write surface. The standard size of disk is 3.5 inches. The platters are mounted in a stack on a spindle. Driven by a special motor connected to the spindle, these platters rotate at a high speed. The read/write heads are used either to record to or read from the disk, with their position on the surface of the disk controlled by a device called actuator.
- Winchester technique is employed to seal read / write head and disks.
- Its speed is 5400 rpm to 15,000 rpm (revolution per minutes)
- Its storing capacity is many times larger than floppy diskette and its measuring unit is GB (Giga bytes). The hard disk of 80 GB or more storing capacity is available in market.
- FAT: DOS and Windows Operating system use a File Allocation Table (FAT) to organize files on the drive. The sectors are organized into a group called cluster. Early drive used 12bit number (known as FAT12), but drives today use either 16-bit number or a 32-bit number (called FAT32). By assigning each cluster with its own number, it is possible to store files in available (unused) clusters throughout the drive without worrying about size of file. As file is erased, those clusters become available for use.
- What do you meant by FAT32?
A version of the File Allocation Table (FAT) available in Windows 98 OSR 2 and Windows 98. FAT32 increases the number of bits used to address clusters and also reduces the size of each cluster. The result is that it can support larger disks (up to 2 terabytes) and better storage efficiency (less slack space).
- Slack space: However, a problem with the FAT system is that you can only have as many clusters as can be specified by the number of bits available. For a 12-bits FAT, you can only have 4,096(2 power 12) clusters. For a 16-bit FAT, you can have 65,536(2 power 16) clusters. If the drive is 120MB, each cluster must be about 120MB/65,536=1.8KB (2KB, in actual). If the drive is 500MB, each cluster must be about 540MB/65,536 = 7.6KB (8KB, actually). Since only one file can be assigned to any given cluster, the entire space for the cluster is assigned (even if the file is very small). So, if you were to store a 2KB file in 8KB cluster, you’d waste 6KB. This wasted space is called slack space.
- CHS (Cylinder, Head and Sector): When the PC was developed, design engineers had to decide how many bytes to designate for addressing particular memory locations within the system, including the hard disk drive memory. It was also necessary to specify how the address bytes would be structured to access that memory. Originally, these bytes were divided into Cylinder, Head, and Sector (CHS) address locations that related directly to physical layout of the hard drive.
The first Compact Disk (CD) was developed by Sony and Philips Corporations to store music in early 1980. The same technology is used to store programs, text, and graphics digitally in optical storage system that is called CD-ROM.
- CD-ROM stands for Compact Disk Read Only Memory.
- It is made from polycarbonate and covered with high reflective aluminum.
- Its diameter is 12 cm, thickness is 1mm and weight is 16 grams.
- Information is stored in spiral tracks. Spiral tracks have pits and lands to store information.
- Its storing capacity is 650 MB. It can stores data equivalent to 475 floppies or 24 volume of encyclopedia.
- Laser beam is used to read data from CD-ROM. Laser beam falls on pit and land, and reflected back to photodiode. The small amount of light is reflected from pit and sufficient light from land. The amount of change in light is converted into electrical signals.
(d) DVDs (Digital Video Disk/ Digital Versatile Disks):
In December 1995, nine major electronic companies (Toshiba, Matsushita, Sony, Philips, Time Warner, Pioneer, IVC, Hitachi, and Mitsubishi Electric) agreed to promote a new optical disc technology for distribution of multimedia and feature-length movies called DVD. Its storing capacity is at least 15 times greater than a CD-ROM and it can stores movies of 4 hours duration. The DVD of 4.7 GB, 8.5 GB, 20 GB are available in market. It is also called Super Density Disk (SD). It is available in two formats: (a) Video Format, and (b) DVD-ROM (Read Only Memory) Format. The DVD-Video Format is used for movies and DVD-ROM Format is used to store interactive games, photographs, videos etc.
- DVD-ROM: The first DVD format, or a DVD drive that can read, but cannot write to DVD disc.
- DVD-5: Single layered, single sided DVDs with a capacity of just under 5GB (4.7GB approximately)
- DVD-9: double layered, single sided DVDs with a capacity of 8.5GB.
- DVD-10: Single layered, double-sided DVDs with capacity of 9.4 GB.
- DVD-18: Dual-layered, double sided DVDs with a capacity of about 18GB.
The term single layer means the DVD disc that has only one writeable layer per writable side and dual layer has two writeable layers per writable side.
Zip disk is called super Floppy and inserted into Zip Drive to store large volume of files, programs, images, videos, and backup of systems. It can stores data equivalent to 70 floppies and its storing capacity is 100 MB to 250 MB.
Its size is slightly larger than 3.5’ floppy and thicker than floppy also. The Zip drive comes with a software utility that lets you copy the entire contents of your hard drive to one or more Zip disks.
The Zip-drive was lunched in market in 1994 by IOMEGA which initial capacity was 25MB only. Now a day its capacity is increased 10 times.
(f) Jaz Drive:
Jaz drive is similar to Zip drive. It is used to store large volume of data. It has two types: (i) Internal Jaz drive, and (ii) External Jaz drive. It has capacity to backup 2 GB hard disk and first time introduced in market by IOMEGA. Jaz drives come as external (portable) and internal. A Jaz insider drive fits into any available 3.5” drive bay and will be cheaper. Jaz drives can be used under operating systems like DOS / Windows, Mac OS, OS/2, Windows 95, and Windows NT.
Jaz drives and disks are the ideal solution for storing, transporting and playing multimedia applications. It even can handle full screen, full motion videos. With a Jaz drive, you can backup a 1 GB hard disk in as little as 5 minutes. With the Jaz drive’s speed and the Jaz’s disk’s capacity even larger system backups are easy. A drive will supplement the existing hard disk on computers with expandable storage.
- Write Once, Read Many
- High capacity storage device
- Cannot be erased and rerecord.
- Used to store backup files.
(h) Magnetic Tape:
The magnetic tape is used as storage device since 1950. It is made up of strong plastic, mylar and data is storage in it in sequential fashion.
- Length and width: The magnetic tape is commonly 732 m long by length and 22 mm by width.
- Magnetic materials: The one side of tape is coated with magnetic material. The magnetized dot represents 1 and demagnetized dot is equivalent to 0.
- Channels or Tracks: The tape is divided width-wise in rows. These rows are called tracks or channels. The 7 or 9 tracks tape is very common in beginning but 18 and 36 tracks tape are also available in market. The data density of 18-tracks tapes is 40, 000 characters per inch.
- Frames: The vertical divisions of tape along with length are called frames. In 9-tracks tape, each frame contains 9-bits to store character.
- Storing capacity: The storing capacity of magnetic tape is 500MB, 2GB, 4GB, 10 GB and more.
- Records and Blocks: The group of data stored and arranged in tape is called records. A group of records is called block.
- IBG (Inter Block Gap): A gap between two-consequting block is called IBG.
i. High data storage at cheap rate.
ii. Suitable for batch processing
iii. No complicated software is required for file handling
i. Difficult to recover from parity error.
ii. Slow data transfer rate.
iii. Data are arranged in sequential order, so, specific data cannot be retrieved randomly.
iv. Larger in size than floppy, so, no so easy in handling.
It is a USB (Universal Serial Bus) storage device of 1 GB storing capacity connected to USB port of CPU. When it is compatible with operating system, it works as second hard disk for Operating system.
USE was designed in 1995 to provide high bandwidth data transfer at low cost. USB allows data to travel ten times the speed of the normal parallel port. It is also faster than serial port data transfer. The average parallel port data transfer speed is 1.2MB per second and USB port data transfer speed is 12MB per second. Now a day, many USB based peripherals are lunched in market like as printer, scanner, zip drives, modems, joy sticks, camera, keyboards, and many others.
What is bandwidth?
Bandwidth is directly proportional to the amount of data transmitted or received per unit time. For example, it takes more bandwidth to download a photograph in one second than it takes to download a page of text in one second.
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