An indexed sequential data set is processed sequentially or directly. Direct processing is accomplished by the basic access method. Because you provide the key for the record you want read or written, all device control is handled automatically by the system. If you are processing the data set sequentially, using the queued access method, the device is automatically positioned at the beginning of the data set.

In some cases, you might want to process only a section or several separate sections of the data set. You do that by using the SETL macro, which directs the system to begin sequential retrieval at the record having a specific key. The processing of succeeding records is the same as for normal sequential processing, except that you must recognize when the last desired record has been processed. At this point, issue the ESETL macro to ends sequential processing. You can then begin processing at another point in the data set. If you do not specify a SETL macro before retrieving the data, the system assumes default SETL values. See the GET and SETL macros in z/OS DFSMS Macro Instructions for Data Sets.