Editing Data set (IBM mainframe) (section)

{{Short description|Type of computer file existing on IBM mainframe operating systems}}
{{about|computer files|data communications|modem}}

In the context of [[IBM]] [[mainframe computer]]s in the [[S/360]] line, a '''data set''' (IBM preferred) or  '''dataset''' is a [[computer file]] having a [[record-oriented file|record organization]]. Use of this term began with, e.g., [[DOS/360]], [[OS/360]], and is still used by their successors, including the current [[z/OS]]. Documentation for these systems historically preferred this term rather than ''[[computer file|file]]''.

A data set is typically stored on a [[direct access storage device]] (DASD) or [[magnetic tape]],<ref>{{cite web
|url=https://www.ibm.com/support/knowledgecenter/zosbasics/com.ibm.zos.zconcepts/zconcepts_172.htm
|title=What is a catalog?
|website=[[IBM]]
|quote=Cataloging of data sets on magnetic tape ...}}</ref> however unit record devices, such as punch card readers, card punches, line printers and page printers can provide input/output (I/O) for a data set (file).<ref>{{cite web|url=http://publib.boulder.ibm.com/infocenter/zvm/v5r4/index.jsp?topic=/com.ibm.zvm.v54.hcpa7/hcse7b3050.htm|title=IBM Knowledge Center - Home of IBM product documentation|website=publib.boulder.ibm.com}}</ref>

Data sets are not unstructured streams of [[byte]]s, but rather are organized in various logical record<ref>{{cite web
|url=https://www.ibm.com/support/knowledgecenter/zosbasics/com.ibm.zos.zconcepts/zconc_datasetintro.htm
|title=What is a data set? |website=[[IBM]] |quote=data set  .. a file that contains one or more records.}}</ref> and block structures determined by the <code>DSORG</code> (data set organization), <code>RECFM</code> (record format), and other parameters. These parameters are specified at the time of the data set allocation (creation), for example with [[Job Control Language]] <code>DD</code> statements. Within a running program they are stored in the [[Data Control Block]] (DCB) or Access Control Block (ACB), which are data structures used to access data sets using [[access method]]s.

Records in a data set may be fixed, variable, or “undefined” length.<ref>{{cite web
|url=https://www.ibm.com/support/knowledgecenter/zosbasics/com.ibm.zos.zconcepts/zconcepts_159.htm
|title=Data set record formats
|website=[[IBM]]
|quote=Records are either fixed length or variable length in a given data set.}}</ref>