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There are three different methods which can be used to access (locate) data stored on a backing storage device. They are direct access, serial access and sequential access.
When direct access is used the head that reads data from the storage medium can move directly to any point on the storage medium. If a particular record or file must be loaded or saved then the head can move directly to the record/file's position on the storage medium and read the data. Records and files can therefore be located very quickly.
Magnetic disks such as hard disks and floppy disks and optical disks such as CD-ROMs use the direct access method. Direct Access is required if transaction processing is taking place.
When a serial access medium is being used, the head that reads data from the storage medium has limited freedom of movement. The only serial access medium is magnetic tape.
To read a particular record/file from a serial access medium, all of the data that comes before the record/file must be scanned through. The tape head can not move to a particular record/file on the tape without reading through all of the other records that come before it.
Because of this it can take a very long time to locate a record/file on a tape and so tapes are only used for specific applications such as backup and batch processing. For these applications the speed of locating data is not important and the other advantages of magnetic tapes outweigh the slow access speed.
Sequential access is a slight modification of the serial access method. This method is identical to serial access except that the records are stored on the storage medium in a particular order, e.g. by customer number. Sorting the data into an order may speed up operations such as searching the tape. The sequential access method is usually used by batch processing systems.
GCSE ICT Companion 04 - (C) P Meakin 2004