Editing Data set (IBM mainframe) (section)

==Record format (RECFM)==
Regardless of organization, the physical structure of each record is essentially the same, and is uniform throughout the data set. This is specified in the DCB <code>RECFM</code> parameter. <code>RECFM=F</code> means that the records are of fixed length, specified via the <code>LRECL</code> parameter. <code>RECFM=V</code> specifies a variable-length record. V records when stored on media are prefixed by a Record Descriptor Word (RDW) containing the integer length of the record in bytes and flag bits. With <code>RECFM=FB</code> and <code>RECFM=VB</code>, multiple logical records are grouped together into a single [[Block (data storage)|physical block]] on tape or DASD. FB and VB are <code>fixed-blocked</code>, and <code>variable-blocked</code>, respectively. RECFM=U (undefined) is also variable length, but the length of the record is determined by the length of the block rather than by a control field.

The <code>BLKSIZE</code> parameter specifies the maximum length of the block. <code>RECFM=FBS</code><ref>{{cite web
|url=https://www.ibm.com/support/knowledgecenter/en/ssw_ibm_i_72/rzatb/rbfvbs.htm
|title=Example: Record format VBS
|website=[[IBM]]
|quote=Variable-length, blocked, spanned (VBS)}}</ref> could be also specified, meaning <code>fixed-blocked standard</code>, meaning all the blocks except the last one were required to be in full <code>BLKSIZE</code> length. <code>RECFM=VBS</code>, or <code>variable-blocked spanned</code>, means a logical record could be spanned across two or more blocks, with flags in the RDW indicating whether a record segment is continued into the next block and/or was continued from the previous one.

This mechanism eliminates the need for using any "delimiter" byte value to separate records. Thus data can be of any type, including binary integers, floating-point, or characters, without introducing a false end-of-record condition. The data set is an abstraction of a collection of records, in contrast to files as unstructured streams of bytes.