RDF—Record Definition Field

The RBAs of records or relative record numbers of slots in a control interval ascend from left to right. RDFs from right to left describe these records or slots or a segment of a spanned record. RDFs describe records one way for key-sequenced data sets, entry-sequenced data sets, and variable-length RRDSs, and another way for fixed-length RRDSs.

In a key-sequenced or entry-sequenced data set, records might vary in length and can span control intervals. In a variable-length RRDS, records vary in length but do not span control intervals.

A nonspanned record with no other records of the same length next to it is described by a single RDF that gives the length of the record.
Two or more consecutive nonspanned records of the same length are described by a pair of RDFs. The RDF on the right gives the length of each record, and the RDF on the left gives the number of consecutive records of the same length.
Each segment of a spanned record (one segment per control interval) is described by a pair of RDFs. The RDF on the right gives the length of the segment, and the RDF on the left gives its update number. (The update number in each segment is incremented by one each time a spanned record is updated. A difference among update numbers within a spanned record means a possible error in the record.)

In a fixed-length RRDS, records do not vary in length or span control intervals. Each record slot is described by a single RDF that gives its length and indicates if it contains a record.

An RDF is a 3-byte field that contains a 1-byte control field and a 2-byte binary number, as the following table shows.

Offset	Length and Bit Pattern	Description
0(0)	1	Control Field.
	x... ..xx	Reserved.
	.x.. ....	Indicates whether there is (1) or is not (0) a paired RDF to the left of this RDF.
	..xx ....	Indicates whether the record spans control intervals: 00 No. 01 Yes; this is the first segment. 10 Yes; this is the last segment. 11 Yes; this is an intermediate segment.
	.... x...	Indicates what the 2-byte binary number gives: 0 The length of the record, segment, or slot described by this RDF. 1 The number of consecutive nonspanned records of the same length, or the update number of the segment of a spanned record.
	.... .x..	For a fixed-length RRDS, indicates whether the slot described by this RDF does (0) or does not (1) contain a record.
1(1)	2	Binary number: When bit 4 of byte 0 is 0, gives the length of the record, segment, or slot described by this RDF. When bit 4 of byte 0 is 1 and bits 2 and 3 of byte 0 are 0, gives the number of consecutive records of the same length. When bit 4 of byte 0 is 1 and bits 2 and 3 of byte 0 are not 0, gives the update number of the segment described by this RDF.