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A data set is a collection of information organized as a stream of bytes in logical record and block structures for use by IBM mainframe operating systems. The record format is determined by data set organization, record format and other parameters.
The physical structure of each record is nearly the same, and uniform throughout a data set. This is specified in the data control block record format parameter. The fixed-length records eliminate the need for any delimiter byte value for separate records. This means the data may be of any type (binary, floating point or characters) without using a false end-of-record condition.
The alternative to a data set is files, which are an unstructured stream of bytes that are favored by Unix, Windows and Mac OS. A data set typically contains a specific type of data such as names, salaries and sales data that are all numerical and fixed-format. In contrast, files may contain a wide variety of data types, such as text, graphics, audio data and video data which will be of variable format.
Data sets can be organized into a partitioned data set which may hold multiple members that each contain a separate sub-data set. This style of organization is similar to files being organized into directories or folders. PDSs are often used for executable programs and source program libraries. A PDS is analogous to a zip file in a file system, but the data is not compressed.