Relatively Cheaper: These storage devices are relatively cheaper and cost effective than primary memory of the CAP]. 4. Portable: These devices act as a portable media for transferring data from one system to another. The access to data stored can be in serial order or random depending upon type of secondary storage devices. Sequential and direct access Devices: Editorial at the desired storage location may be preceded by sequencing through other locations Data can only be retrieved in the same sequence in which it is stored C]Decades time varies according to the storage location of the information being accessed
Diamagnetic tape is a typical example of such a storage device Direct-Access Devices: C]Devices where any storage location may be selected and accessed at random Deposits access to individual information in a more direct or immediate manner Diplomatically equal access time is required for accessing information from any storage location Suitable for direct processing applications such as on-line ticket booking systems, on-line banking systems Accident, optical, and magneto-optical disks are typical examples of such a storage device Best example to differentiate b/w sequential and direct access is the audit asset and compact disk…
In audio cassette if there are 10 songs recorded in the cassette and you need to play 6 the song… Len that case you need to listen all the five songs or fast forward these songs…. You cannot jump directly to the sixth song which is the benefit of random access Magnetic Tapes: [Discommode used sequential-access secondary storage device Biophysically, the tape medium is a plastic ribbon, which is usually h inch or 1/4 inch wide and 50 to 2400 feet long C]Diamagnetic tapes are made of thin plastic coated with magnetic materials like iron oxide.
Which can be magnetized in either of two directions-D ACID One polarity is used for representing binary digit ‘1’ and other for representing ‘O’. These tapes are similar to that of the tapes used in audio tape recorders. Magnetic Tape drive used to read and also to write data on the magnetic tape. The advantages of this tape is it can be reused and the disadvantage is that access being sequential so searching becomes difficult. How Data is Stored in Magnetic Tapes: The Information is recorded on the tape in binary form, with two different strengths of signal representing 1 and O.
These Tapes have their own coding formats to store data. Usually Magnetic Tapes are divided into seven to nine tracks. The 9 track arrangement is called Extended Binary Coded Decimal Interchange Code(Secede). Here the data is stored in eight tracks and the 9th track is used for error-checking bit. Each Track is used to store a bit. The EPIC code which uses 9 bits out of which one bit is set aside for checking purposes. This is called parity bit. This bit is used as a checking bit for minimizing the risk of errors in transmission of data.
This bit is usually called even parity bit, it checks that there are always even number of bits in one storage area, if there are odd number of bits, the rarity bit is turned on to make the bits even else if the bits are even then the parity set is turned off. Thus the procedure says that when there are even number of bits, the parity bit is turned off, else if there are odd number of bits it is turned on. This parity bit is used as a measure to ensure that data is transferred without any error.
If it will found any kind of discrepancy I. E if it find any kind of suspected error then the computer will retransmit data, so to ensure proper transmission. The magnetic tapes are extremely compact in size and one tape can be equal to 500000 cards. A Tape stores millions bits of information. The number of characters that can be stored in an inch of tape is defined as the density of the tape. The density of the tape can be measured as bits per inch I. E bpi.
The most commonly tapes having varied densities like 800 dip, 1600 dip, 3250 dip, 6000 dip. What Type Of Data stored in Magnetic Tapes: In magnetic tapes one can store: 1. Fixed length records I. E all the records are of same length. 2. Variable Length records I. E where the record sizes/Length is variable. While Storing these records on the magnetic tape in order to separate each cord from other, these system use an Inter Record Gap(large) which is also known as Inter Block Gaps(lbs.), so that the tape drive recognizes two different records.
Data Retrieval: When the data has to be retrieved from the magnetic tape, the data will be retrieved in sequential order. The read heads read a record and it Stops when the inter record/ block gap comes, till the record is processed. After processing the second record is read in the same way and this continues till the total file is processed. The device that reads the tape is the Tape Drive or Tape Unit. There are two common forms of Magnetic Tape: Reel-to-Reel Magnetic Tape Reels Large reels of tape which must be loaded into a reel-to-reel tape drive.
This type of tape is usually used by mainframe computers. Cartridge Magnetic Tape The tape is supplied in a small cartridge rather like a music tape. This type of tape is used on PC’s (microcomputers) and the device used to read/write the tapes is called a tape streamer. Capacities of cartridges vary from KGB to KGB. Limitations: C]Due to their sequential access nature, they are not suitable for storage of those data that frequently require to be accessed randomly
L]Dumdums be stored in a dust-free environment because specks of dust can cause tape-reading errors C]Dumdums be stored in an environment with properly controlled temperature and humidity levels tape ribbon may get twisted due to wrapping, resulting in loss of stored data Outsold be properly labeled so that some useful data stored on a particular tape is not erased by mistake Advantages: Lido applications that are based on sequential data processing Cabbaging up of data for off-line storage Teethed Tape is very economical I. E it is relatively very cheap.
Transferring of data from one computer to another that are not linked gather Magnetic Disks: Codification used direct-access secondary storage device. Accident disk is a thin, circular plate/ platter made Of metal or plastic that is usually coated on both sides with a magnetized recording material such as iron-oxide C]Data are recorded on the disk in the form of tiny invisible magnetized and non-magnetized spots (representing Co’s and Xi’s) on the coated surfaces of the disk Stitched disk is stored in a specially designed protective envelope or cartridge, or several of them are stacked together in a sealed, contamination-free container.
Magnetic Disk – Storage Organization For data recording, the surface of a disk is divided into a number of invisible concentric circles called tracks. As shown in the figure the tracks are numbered consecutively from outermost to innermost, starting from zero. The number of tracks varies greatly between disks, from a few as 40 on some small, low capacity disks, to several. Thousand on large, high capacity disks. Each track is further subdivided into sectors.
For this, in addition to the concentric circles, the disk surface is also divided into invisible pie-shaped segments as shown in figure, Hence if there are eight such pie-shaped segments, each track will be divided into eight parts, and each of these eight portions of a track is called a sector. A sector typically contains 51 2 bytes. A sector is the smallest unit with which any disk can work. That is disk drives are designed to access(read/write) only whole sectors at a time I. E Even if the computer needs to change just one byte, out of the 512 bytes stored on a sector, it rewrites the entire sector.
When a people refer to the number of sectors a disk has, the unit they use is sectors per track- not just sectors. Hence if a disk has 200 tracks and 8 sectors re track, is has 1 600(xx) sectors-not 8 sectors. Each sector of a disk is assigned a unique number. Before a disk drive can access a piece of data (a record) stored on a disk, it must specify the record’s disk address. The disk address is comprised of sector number, track number, and a surface number (when double sided disk is used).
That is disk address represents the physical location of record on the disk. Working: Often multiple disks are stacked and used together to create large capacity disk storage system. In this case a set of magnetic disks is fixed to a central haft, one below the other, to form a disk pack. The disk pack is sealed and mounted on a disk drive, which consist of a motor to rotate the disk pack about its axis. The disk drive also has an access arms assembly, which has separate read/write for each surface of the disk pack, on which data can be recorded.
Normally the upper surface of the top disk and lower surface of the bottom disk are not used in the disk pack, because these surfaces may be easily scratched. The access arms assembly are designed in such a manner that all the access arms for all the disks surfaces move together. For faster access of data from the disk packs, a concept called cylinder is used for data organization on disk packs. Types of magnetic Disks: 1. Hard Disks 2. Floppy Disks 1. Hard Disks: Hard disks are the primary on-line secondary storage devices for most computer system today.
Round, flat piece of rigid metal (frequently aluminum) disks coated with magnetic oxide. Come in many sizes, ranging from 1 to IA-inch diameter. Depending on how they are packaged, hard disks are of three types: Zip/Bernoulli disks ICC]DiSk packs C]Denunciates disks [Zip/Bernoulli disks: Douses a single hard disk platter encased in a plastic cartridge. COCA molly used zip disk is Of 3 1/2 inch size, having a storage capacity Of about 1 00 MS, depending on the formatting style used by a particular computer system.
The later versions increased this to first 250 MBA and then 750 MBA C]Disk drives may be portable or fixed type Oilfield type is part of the computer system, permanently connected to it Deplorable type can be carried to a computer system, connected to it for the duration of use, and then can be disconnected and taken away when the work is done L]Ditz disks can be easily inserted/removed from a zip drive just as we insert/remove floppy disks in a floppy disk drive
Unlike the hard disk drives, here the head comes in contact with the surface of the disk. The disk is accessed from the hole in the disk jacket, which has even an index hole. The following are the steps in data retrieval: 1 . The read/write head moves to the pacified track using the disk address I. E the track number and the sector number. 2. With the help Of index hole, the disk controller locates sector on the disk, The mechanism uses a light sensitive device which detects the light when it passes through the disk. Thus the hole’s location I. E index hole is detected. 3. Data will be read by the disk controller. 4. The data is transmitted to the processing unit. The data is retrieved from a floppy disk using the index hole.
Advantages of Magnetic Disks: C]Démodé suitable than magnetic tapes for a wider range of applications because they support direct access of data C]Tornado access property enables them to be used simultaneously by multiple users as a shared device. A tape is not suitable for such type of usage due to its sequential-access property Suitable for both on-line and off-line storage of data Except for the fixed type Winchester disks, the storage capacity of other magnetic disks is virtually unlimited as many disks can be used for storing very large data sets Due to their low cost and high data recording densities, the cost per bit of storage is low for magnetic disks.