Computer Files and It's Access

As applications are processed on a computer, the data required for the current computations must be stored in the computer memory. The capacity of memory is limited (although it can go over a trillion bytes on some large machines), and there is not enough space to keep all of the data for all of the concurrently running programs (e.g., Microsoft Excel, Microsoft Word, Adobe Photoshop) in memory at the same time. In addition, memory is volatile; if the computer's power goes off, everything stored in memory is lost. To keep large number of data accessible within the computer system in a nonvolatile medium and at more reasonable costs than main memory, file devices-sometimes called secondary memory or secondary storage devices-have been added to all but the tiniest computer systems. File devices include magnetic tape drives, hard (or fixed) disk drives, removable disk drives, flash drives, and CD or DVD (optical) drives. Optical drives record information on it by magnetizing spots on the surface of the media, using a binary coding scheme.

Data can be moved from particular cells in memory to the file and that data can be retrieved from the file to particular memory cells. The disadvantage of files is that the process of storing data in the file from memory or retrieving data from the file to memory is quite slow relative to the computer's computation speed. Depending upon the type of file, the store/retrieve time may vary from a very small fraction of a second to over a minute. Nevertheless, we are willing to live with this disadvantage to be able to store enormous quantities of data at a reasonable cost per byte.

There are two basic ways to organize computer files: sequential access and direct access. With sequential access files, all of the records that make up the files are stored in sequence according to the file's control key. For instance, a payroll file will contain one record for each employee. These individual employee records are stored in sequence according to the employee identification number. There are no addresses within the file; to find a particular record, the file device must start at the beginning of the sequential file and read each record until it finds the desired one. It is apparent that this method of finding a single record might take a long time, particularly if the sequential file is long and the desired record is near the end. Thus, we would rarely try to find a single record with a sequential access file. Instead, we would accumulate a batch of transactions and process the entire batch at the same time.

Sequential access files are usually stored on magnetic tape. A magnetic tape unit or magnetic tape drive is the file device that stores (writes) data on tape and that retrieves (reads) data from tape back into memory. Even with batch processing, retrieval from magnetic tape tends to be much slower than retrieval from direct access files. Thus, if speed is of the essence, sequential access files might not be suitable. On the other hand, magnetic tapes can store vast quantities of data economically.

Denor Linen is an IT analyst, working with SageNext Infotech. SageNext Infotech is a leading QuickBooks hosting provider along with dealing in all kinds of tax and accounting application hosting. SageNext is the cheapest QuickBooks hosting by offering the excellent service. Rate this Article

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Denor Linen has published 5 articles. Article submitted on September 26, 2013. Word count: 518

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