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ISAM Databases

ISAM (pronounced "eye sam") stands for Indexed Sequential Access Method. It is a technique for storing and accessing data in a file on secondary storage, for example a disk. ISAM is not a stand-alone program; it is a library of functions that implement an algorithm. It is the heart of many modern database systems. This chapter presents some basic information about the ISAM data access method and provides a brief overview of the supplied libraries.

Originally, the term "access method" referred to the way that data was physically stored on a magnetic tape or disk. It was very specific according to the particular kind of tape or disk drive. Over time, we've come to appreciate the usefulness of shielding the hardware details from the application program. A database program should not be concerned with what type or brand of device is used to store the files.

In the past several years, the term ISAM has grown from its original meaning (an access method dependent on specific hardware) to a more general usage. ISAM now refers to almost any way of indexing a file of data records.

Computers are frequently used, especially in business settings, to store large amounts of information. Related information is collected into a computer file. The file normally resides on secondary storage; that is, on a device outside of the computer's primary memory storage. For microcomputers, secondary storage is usually a floppy disk or a hard disk. All of this information must be organized in some way so that it can be retrieved efficiently.

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