SEQUENTIAL ACCESS, DIRECT ACCESS STORAGE

SEQUENTIAL ACCESS

Access through sequential access storage is non-addressable that is an operator cannot refer directly to the contents of a particular storage location such as that used with magnetic tape. This involves examining sequentially all recorded data. This form of storage necessitates tape searching by starting at the beginning of the tap and continuing to search through all records until the desired information area in found.

DIRECT ACCESS STORAGE

Direct Access Storage is addressable that is a given item can be selected from any where in storage by simply specifying the address where it is located Direct Access devices such as magnetic disks provide immediate access to individual records and do not require reading from the beginning of a file to find a particular record.

Auxiliary storage is also called the backing storage because it is used to supplement the capacity of main storage. Backing storage is essential not only due to the limited size of the random access memory (RAM) but also because RAM is only a temporary memory and moreover it is expensive a well. Auxiliary storage provides massive capacity for storing large number of programme and vast amount of daa of the order of hundreds of megabytes. Hence the name mass storage is also given to backing storage. These permanent storage devices come in the following forms:

• Magnetic Tapes
• Magnetic Disks
• Laser Holographic Storage

MAGNETIC TAPE DEVICES

A popular form of secondary storage for both mainframe and personal computers is magnetic tape. Megnetic tape is a thin Mylar tape from 1/4 inch to 1 inch wide. The tape is coated with ferrous oxide, on which information is recorded in binary form. Magnetic tape is available in two primary forms, reel-to-reel tap and data-cassett/cartridge tape.

The information is transferred to from the tape with a read/write head, which encodes the magnetic signals on the ferrous oxide tape coating as the tape moves across the read/write head. Most computer centers have a tape library run by special librarian who manages tape storage. These tapes are cheap and reliable but they take considerable time to load and to loacte a programme.

MAGNETIC DISKS

A magnetic disk is a metal or plastic disk coated with ferrous oxide. A bit pattern may be magnetically coded onto the ferrous oxide. Because these disks spin in the disk drives, the read/write heads can reach any data on the disks directly rather than sequentially. Because of its random and sequential processing capabilities, magnetic disk storage is the overwhelming choice of computer users. A variety of magnetic disk drives (the hardware devices) and magnetic disks (the media) are manufactured for computer use. There are two fundamental types of magnetic disks, interchangeable disks calles the floppy disks and fixed disks called the hard disks.

Nearly all PCs sold today are configured with at least one fixed disk drive and one interchangeable disk drive. Having two disks increases system flexibility and throughput. The interchangeable disk drive provides a means for the distribution of data and software and for backup and archival storage. The high capacity hard disk storage has made it possible for today's PC users to enjoy the conenience of having all data software readily accessible at all times.

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