- By Sidra Munir
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1. Secondary Memory
This is also called Mass Storage, Auxiliary Memory and External Memory. This memory is slower than the Main memory as it involves mechanical motion techniques during storage and retrieval of data. This memory is larger in size than Main memory but the processor is unable to access it directly due to its offline link with the processor. This means that the data from secondary storage must be loaded into RAM before the processor starts processing it. The main memory links the secondary memory to the processor.
2. Types of Secondary Memory
2.1. Magnetic Disks
Speedy access to data, relatively low cost, and the ability to erase and rewrite data make magnetic disks the most widely used storage media on today’s computers. With magnetic disk storage systems, data are written by read/write heads magnetizing the particles a certain way on a medium surface. The particles retain their magnetic orientation so they can be read at a later time, and rewriting to the medium is possible. There are two main types of magnetic disks:
2.1.1. Floppy Disk:
Floppy Disk is a round, flat piece of Mylar coated with ferric oxide, rust like substance containing tiny particles capable of holding a magnetic field, and encased in a protective plastic cover, the disk jacket. Data is stored on a floppy disk by the disk drive's read/write head, which alters the magnetic orientation of the particles. Orientation in one direction represents binary 1; orientation in the other, binary 0. Typically, a floppy disk is 5.25 inches in diameter, with a large hole in the center that fits around the spindle in the disk drive. Depending on its capacity, such a disk can hold from a few hundred thousand to over one million bytes of data. A 3.5-inch disk encased in rigid plastic is usually called a microfloppy disk but can also be called a floppy disk.
2.1.2. Hard Disk:
Hard Disk composed of one or more platters that are permanently sealed within a hard metallic casing. These hard disks are fixed in the computer CPU and are seldom transferred from one computer to another. For the better use of the hard disk space, a hard disk can be divided into any number of partitions like C: D: E: etc. however making too many partitions is not a good management practice for the memory of hard disk.
Now days up to 1000 GB hard disks are available in the market. For the better use of disk space a hard disk can be divided into a number of partitions like C: D: E: etc.
2.2. Magnetic Tapes:
Magnetic tape and the tape drives are analogous to a home tape recorder system. It uses the same reading and recording techniques as that of the magnetic disk as the medium used in it is a flexible tape that is coated with magnetic oxide.
Since sequential access device means that for n records, where n = 0, 1, 2, 3, ……… if the tape head is positioned at record number 1 then in order to read the nth record, it is necessary to read all the physical records from 1st to nth records one at a time. If the head position is beyond the desired record, it is necessary to rewind the tape for a specific distance and begin reading forward.
In contrast to the magnetic disk, which is a direct access device, a tape is sequential in nature. A disk drive doesn’t read all the sectors on a disk sequentially to get to the desired record, where as magnetic tape drive read all the sectors b/w the starting and the desired location of data. Magnetic tape was the first kind of secondary memory and is still widely used for its lowest cost, however it is very slow in speed than all of the secondary storage devices.
Secondary Storage is magnetic in nature and therefore magnetic mechanisms are used to store data permanently. Data or information is stored in the form of files. A file is an area of the secondary memory where data or information is permanently stored. Each file has its unique file name through which it is accessed. The storage of data in secondary memory follows some file organization techniques such as Sequential, Indexed Sequential and Random/Direct access file organizations. Sequential access file organization is adopted for Magnetic Tape while Random/Direct access file organization is more suitable for Hard Disk or Floppy Disk.
- Data remains permanently stored even when the computer is switched off.
- This data remains in the memory until deleted by the computer user.
- Very high volumes of data can be recorded for long time and is updated and retrieved efficiently.
- Transfer of data from one computer to another is performed through this memory like through Floppy or CDs.
- System files associated with any Operating System are permanently stored in this memory. These files are loaded into RAM at the time of booting the computer system.
- To prevent any damage and loss of data, Backup and Recovery procedures are facilitated through Secondary Memory.
4. Optical Memory
Optical memory is used for storing large volumes of data like sound, text, graphics, and videos. An optical disk is a removable disk that uses laser to read and write data. With an optical disk, there is no mechanical arm, as with floppy disks and hard disks. Instead a high-power laser beam is used to write data by burning tiny pits into the surface of a hard plastic disk. To read the data, a low-power laser light scans the disk surface: pitted areas are not reflected and are interpreted as 0 bits; smooth areas are reflected and are interpreted as 1 bits. Because the pits are so tiny, a great deal more data can be represented than is possible in the same amount of space on hard disks. An optical disk can hold over 4.7 gigabytes of data, the equivalent of 1 million type-written pages.
The optical memory devices are:
4.1. Compact Disk (CD)
CD is a non-erasable disk that stores the digitized audio information. The standard system uses 12 cm disks and they can record more than 60 minutes of playing time without any interruption.
Optical disk form of secondary storage that is used to hold prerecorded text, graphics and sound. Like music CDs a CD-ROM is a read-only disk. Read Only means the disk’s content is recorded at the time of manufacture and can not be written on or erased by the user. A CD-ROM disk can hold up to 650 MB of data, equal to 300,000 pages of text.
CD-RW (Compact Disk-Rewritable) also called as Erasable Optical Disk allow users to record and erase data so that the disk can be used over and over again. Special CD-RW drives and software is required.
4.3. DVD (Digital Versatile Disk) : the “Digital Convergence” Disk
The DVD represents a new generation of high density CD-ROM disks, which are read by laser and which have both write-once and rewritable capabilities. According to the various industries sponsoring it, DVD stands for either “Digital Video Disk” or “Digital Versatile Disk”, and it is a CD type disk with extremely high capacity, able to store 4.7-17 GB.
DVD disks that allow one time recording by the consumer. Two types of reusable disks are DVD-RW (DVD Rewritable) and DVD-RAM (DVD Random Access Memory), both of which can be recorded on and erased more than once.
4.5. Write Once Read Many (WORM)
WORM is a disk that is more easily written than CD-ROM thus making single copy disks commercially feasible. After performing the write operation the disk is read only. The most popular size is 51/4 “that can hold data from 200 to 800 MB.
4.6. Magneto-Optical Disk
There are a few other types of storage systems that use a combination of magnetic and optical technology – the magneto-optical disk is one of them. M-O disks can store up to 5.2 GB of data.
A very common application of Optical Memory especially CD-ROM is that it can store Multimedia Encyclopedia that contains all 21 volumes of the Academic American Encyclopedia. This encyclopedia comprises full text of about 33000 as well as a comprehensive index of titles, words, pictures and maps. In addition there are thousands of pictures, hundreds of sounds and animations along with dozens of video clips.