Secondary Storage Devices.
Secondary storage devices are non-volatile devices that hold data until it is deleted or overwritten. When data processing is going on the program and data ( maybe partial ) are stored in the internal memory of computer known as RAM.
Since the RAM is volatile therefore this program and data has to be erased when power is turned off, it is necessary to save program and data on any storage media for huge amount of data because a big amount of data cannot be loaded into ram at a time several types of data storage units are available in the market some commonly used data storage units are available in market some commonly used data storage units are described in the following sections.
It is very important to know the difference between device and media the data is physically recorded on media while the equipment used to store data is called device, for example, the magnetic tape is a type of media while magnetic tape unit is a device Same as a floppy disk is a media and disks drive unit is a device.
Backing Storage is the nonvolatile memory that is external to the main memory of a computer. A secondary storage medium is usually used for the storage of great totals of data or for permanent or long –term storage of data or programs.
Secondary storage media are also used for storing backups or copies of data and program secondary storage media are also used for storing backups or copies of data and programs. An Input/ Output (I/O) device is the hardware that allows you to store data and instructions on secondary storage media and to access them.
The secondary storage devices are based on two concepts direct access and sequential access.
Direct and Sequential Accessing.
To explain these concepts, we can take the example of a CD player (For Direct Accessing) in which any song can be played just by pressing the required song track number. For sequential accessing consider an audio cassette.
We at the beginning of the cassette the time taken to start song number 1 will be the shortest in case we want to listen to song number 5 the accessing time will be the longest that means the position of the object changes the accessing.
Secondary Storage devices are categorized as.
- Direct access storage devices.
- Sequential Access Devices.
The secondary storage devices can also be categorized as:
Magnetic Technology uses.
Optical (LASER) Technology.
- MAGNETIC TECHNOLOGY.
Media, the technology uses media, a surface of which is coated with the iron oxide (magnetic particles) and just the by an arrangement of those particles the data is stored. Magnetic devices help us to store and retrieve data from the magnetic media.
Here, we are discussing the DASDs and SADs based on magnetic technology.
Direct Access Storage Devices.
media and equipment are most commonly used secondary storage devices used in modern computers. These provide fast access and high storage capacities at the reasonable cost. Two types are hard (metal) disks and floppy 9plstic) disks. Several types of magnetic disk peripheral are used as Direct Access Storage Devices (DASDs) in both small and large computer systems.
The media in these magnetic disks is divided into concentric circles, called Tracks and cuts of pie, called Sectors through a process called Formatting. The number of tracks and sectors differs for different disks.
The storage capacity of the disk system depends on the bits per inch (bpi) of the track and tracks per inch (TPI) of a surface.
TYPES OF MAGNETIC DISK.
Following types of the magnetic disk, arrangements are available. Users use any of these types of magnetic disks depending on their requirements and both are famous in Secondary Storage Devices.
1. Floppy Disks.
2. Hard Disks.
1- Floppy Disk.
Floppy disk is another secondary storage media, which has many advantages over magnetic tape. The first floppy disk was developed in 1972 by IBM. A floppy disk is a circular piece of mylar or other plastic-like material coated with iron oxide.
This flexible disk is housed in a protective jacket. The unit which reads or writes the floppy disk is called floppy disk drive. The movable heads mounted inside the floppy disk drive are called read/ write heads. These heads have direct contact with the disk surface.
These heads can move back and forth on disk and records or retrieves information from the magnetic surface. Data is recorded in the form of magnetic dots or spots on tracks. These tracks are formed as concentric circles on the surface of a disk.
The outermost circle is called “Zero Track” the subsequent inner circle is tracked 1 then track 2 and so on, though the innermost circle has the highest track number usually track number 3979. All of these tracks are further divided into some sectors (usually8/9) for better accessing of data.
Originally the size of floppy disk was 8 inch, at present 5 ¼ inch and 3 ½ inch disks are also available. All of these disks have different storage capacities, because of a different number of tracks and sectors.
The disks could be single sided means that data can only be written on one side, or disks could be double sided means that data may be written on both sides of the disk.
All these disks are double sided, means the data is written and read from both the sides of the disk.
Floppy disk of 8 inches, was used with old mini and computers, it is obsolete now. The 5.25 inches disks are also becoming obsolete, because of their large size low capacity more flexibility and open media which can easily get dust. The dust can destroy the disk and the drives.
You can easily calculate the capacity of their large size low capacity of a disk if the number of tracks and sectors are known. An example is given of a disk which has 80 tracks and 9 sectors:
- 1 track = 9 sectors.
- 80 tracks = 80×9 =720 sectors.
- As the disks are double sided so multiply by2.
- 720 x 2 = 737, 280 bytes, so multiply by 512.
- As sector stores, 512 bytes, so multiply by 512.
- 1440 x 512 = 737 280 bytes.
- To convert into KB, Divide by 1024 as 1 KB= 1024.
- The capacity of this disk = 720 KB.
2- Hard Disk.
These disks combine magnetic disks, access arms and read/write heads into a sealed module Removable disk cartridges are also available.
Several disks may be mounted on the vertical shaft rotating the disks at a speed of 2,400 to 5,000 revolution per minute (rpm), 3,600 being the most common.
Electromagnetic read/write heads are positioned by access arms. Accessing of data is very fast because in hard disks the disks are rotated at a high speed, so the data comes under the head quickly.
Another reason of hard disk’s faster working is, the data is not stored in the form of track, that is stored in an imaginary form of cylinders, in this method thousands of bytes of data can be stored or read with any head movement.
The reason behind the development of hard disks was the requirement of large storage capacities and more faster-accessing capability. Two forms of hard disks are available.
a) Disk packs.
b) Fixed Disks.
a) Disk Packs.
These are easy to handle removable devices. It normally consists of 11 hard disks of 14-inch diameter, packed in the single plastic case. It weighs around 20 pounds and about 6 inches in height.
It can store 500 MB of data higher capacities are possible. Mainly used with old main and minicomputers.
b) Fixed disks.
These are non-removable more than one magnetic disk sealed in a box. They allow higher speeds, greater data- recording densities and closer tolerances within a sealed, more stable environment.
Today, fixed disks up to 9 GB capacity are available, the accessing time is measured in miles seconds.
USB Flash Drive:
A flash drive is the small ultra-portable storage device. Flash drive is connected to the computer and other devices via a built-in USB Type. Flash drive is often referred to as pen drive.
Most of the storage devices mentioned previously, these are no longer used with today’s computers, which primarily use a hard disk drive or SSD to store information and have the options for USB flash drives and access to cloud storage.
2. OPTICAL STORAGE DEVICES (Secondary Storage Devices).
An optical disk is a removable disk on which data is written and read through the usage of laser beams.
These are no mechanical arm, as with diskettes and hard disk.
The most familiar type of optical disk is the one used in the music industry. An audio CD used a digital code and looks like a miniature phonograph record. Such a CD holds up to 74 minutes (2billion bits’ worth) of high-fidelity stereo sound.
A single optical disk of the type used on computers, called CD-ROM can hold up to about 700 megabytes of data. This works out to about 269,000 pages of text, or more than 7500 photos or graphics, or 20 hours of speech, or 77 minutes of video.
Although some disks are used strictly for digital data storage many are used to distribute multimedia programs that combine text, visuals, and sound.
In the principal types of optical disk technology, a high-power laser beam is used to present data by burning tiny pits into the surface of hard plastic disks.
To read the data a low power laser light scans the disks surface: pitted are not reflected and are interpreted as 0 bits; smooth areas are reflected and are interpreted as 1 bit so tiny, great deal more data can be represented than is likely in the same quantity of space on a diskette and many hard disks.
The optical disk technology used with computers consists of four types:
- CD-ROM disks.
- CD-R disk.
- Erasable disks (CD-E).
- DVD/DVD-ROM disks.
For microcomputer users, the best-known type of optical disk is the CDROM. CD-ROM, which stands for compact disk read only memory, is an optical disk format that is used to contain prerecorded text, graphics, and sound like music CDs, a CD-ROM is a read-only disk.
The means of Reading only is that the disk’s content is recorded at the time of production and cannot be written on or erased by the user.
CD-R Disk: Recording Your Own CDs.
CD-R which stands for compact disk recordable is a CD format that allows users to use a peripheral.
CD recorder to write data (Only once) onto a spectrally manufactured disk that can then be read by any compatible CD-ROM drive.
Erasable Optical Disks.
An erasable optical disk (CDE) or rewritable optical disk 9CD-RW) allows users to record and erase data so that the disk can be used over and over again.
The most common type erasable or rewritable optical disk is probably the magneto-optical (MO) disk, with uses aspects of both magnetic disk and optical disk technologies:
DVD: The digital conjunction” disk.
The DVD denotes a new generation of high-density CD-ROM disks, which is read by the laser and have rewritable capabilities. Giving to the various industries supporting it DVD stands for either digital video disk”.
The side, layer DVD has a size of 4.7 gigabytes per side. Dingle sided dual layer DVDs hold 8.5 gigabytes per side. Double sided DVDs hold 9.4 gigabytes if they are the single layer and 17 gigabytes if they are the dual layer.
SEQUENTIAL ACCESS DEVICES. (Secondary Storage Devices)
In sequential access storage media, magnetic tape up of ½ or ¼ inches ribbon of Mylar, with the coating of iron oxide is used in tape drives. Data is stored in the form of magnetized spots on the iron oxide coating of the plastic tape by the read/write head of the magnetic tape drive.
Read/write head is an electromagnetic component. It stores data sequentially and can be used with all sizes of computers. The speed of some tape units is up to 200 inches per second.
The tape is divided into 9 tracks, 8 tracks represent data while the 9th is reserved for the parity bit. These tracks or channels are also divided into frames perpendicular to the tracks.
Blank spaces are given after every record to separate the records these are called inter-record gaps (IRGs). If the records are stored with IRGs then a large capacity of the tape, as fewer gaps are needed in this form, these gaps are called interblock gaps (IBGs).
They are inexpensive media for a large amount of data, often used as backup storage media. This is erasable, media and can easily be cataloged and stored in tape libraries. The amount of stored data depends on the length of the tape, density and the number of IRGs. Density in some tape systems has gone up to 16,000 bytes per inch (BPI).
The tape is available in three forms:
- Reel to Reel Tape.
- Cassette Tape.
- Cassette Cartridge Tape.
1- Reel to Reel Tape.
This form of tape is used with large computer systems. The magnetic tape is placed on open reels of about 10 ½ inches in diameter.
Many megabytes of data be stored on a tape of the normal length of 2,400 feet and ½ wide.
2- Cassette Tape.
Cassette tapes resemble audio cassettes. It has ¼ inch wide tape, enclosed in a plastic casing, and mainly used with small computers. These cassettes are relatively inexpensive and have the storage capacity of 60 MB to 150 MB.
They were very popular with macros, cut diskettes replaced them. Now they have come back as backup media.
3- Cassette Cartridge Tape.
These are similar to cassette tapes having much greater data storage capacity up to 500 MB). An 8mm cartridge stores up 2.2 GB. Due to networking capabilities in the small computer system, cassette cartridge tapes have become popular in PCs also.
- Four consequent bytes (32 bits) we in 80386 and 80486.
- Kilo Byte: (KB).
- It contains 1024 bytes.
- MegaByte: (KB).
- It contains 1024KB or 1 MB = 1048576 Bytes.
- Giga-Byte: (GB).
- It contains optical and laser storage system.
- 1 GB = 1024 MB.
- or 1 GB = 1073741824 Byte.