Primary memory is volatile, costly and of less capacity. A second type of memory which one can store for a long time and which can also be modified, is needed. This would perform like an audiocassette of a tape-recorder, where an already-recorded cassette can be played any number of times. Secondary Storage memory devices can therefore R/W (read and write) data a number of times. Such devices are discussed below:
A magnetic tape is one of the most popular storage mediums for storing a large volume of data that is to be accessed and processed sequentially. It is a plastic ribbon usually half-an-inch wide, coated on one side with ironoxide which can be magnetized. the tape ribbon itself is stored in reels of 50 to 2,400 feet in small cartridges or cassettes. It is similar to the tape used in tape-recorders. Below are a few important terms related to the magnetic tape.
IBG: there is a gap between two consecutive blocks (group) of records stored on a magnetic tape. this gap is known as IBG (Inner Block Gap).
Blocking Factor: The total number of records in a single block is the blocking factor.
IRG: If the system stores only one record per block, the storage is known as unblocked and IBG is termed as IRG (Inter Record Gap).
The following formulae can be used to determine tape-processing speeds.
1. Transfer rate = tape speed x recording density
2. Length of a block = block size x tape density
3. Time spent per block of data = block size / transfer rate
4. time spent in IBG = IBG size / tape speed
5. Maximum number of = Length of tape
blocks on a tape ---------------------------------
Length of Block + Length of IBG
Limitations of magnetic tapes:
1. Lack of direct access to records: Records must be accessed sequentially
2. Environment problem : The tape unit is susceptible to dust, humidity and high temperature.
Magnetic Disk ( Direct access storage device)
This is also a secondary storage device which provides large storage capabilities. A magnetic disk is circular disk on which a coating of y-ferrite on both sides has been applied. This coating has the property of being magnetized locally ie., it allows the recording of data in the form of magnetised spots. Both sides of the disk are coated for independent recordings.
Data is stored on the disks in a number of concentric circles called tracks. A disk can have 40 to 400 tracks per inch of surface. These tracks begin at the outer edge of the disk and continue towards the centre. Each track has a designated number. Tracks are divided into sectors and the number of sectors per track varies from computer to computer. A motor rotates the disk at a very fast but constant speed. Data are recorded on and read from the tracks of the spinning disk surface in the form of tiny magnetic spots by the disk drives read/write head.
The tracks can be divided into sectors by a software-controlled formatting operation.
the process is known as soft sectoring. If the sectors are permanently marked on the tracks by the manufacturers of the disk then the process is known as hard sectoring. the disks are accordingly known as soft-sectored disks (floppies) and hard-sectored disks (hard disks).
The floppy Disk is a medium used to store valuable data and programs. The floppy disk rotates inside the drive and the drive's read/write head moves onto that to perform read and write operations. Floppy disks are made up of mylar or vinyl plastic material with magnetic coating on one or both sides. These plastic disks coated with magnetic material are further encapsulated permanently in a plastic jacket to protect them against dust and scratches. Floppy disks come in 2 sizes:
5.25 inches with a storage capacity of 1.2 MB
3.5 inches with a storage capacity of 1.44 MB
Each floppy contains a write-protected notch / slide, which can be slided to protect it against being written on or to enable it to write.
A hard disk is a device used for mass storage of data that can be accessed directly. A hard disk is capable of storing a large quantity of data. It is rigid, stable and hermetically sealed in a dust-free environment. a hard disk conntains more than one platter.
A platter is a round disk coated with magnetic recording material. these platters are of different sizes in different disks. Data can be read from and written onto both sides of the platter with the help of R/W heads. A platter can have more than one R/W head to read and write the data. The platter is hard; therefore the R/W head never touches the surface. all these platters are connected onto a single spindle which rotates at a very fast pace and moves back and forth to access the data needed to be read or to be written onto.
These are the hard disks which IBM introduced first in their personal computers. Before that, hard disks were used only in mainframes and mini-computers. They were generally known as disk-drives. Old disk drives are very large in size.