Figure 1 contains control information consisting of two types of fields: one control interval definition field (CIDF), and one or more record definition fields (RDFs).

Figure 1. Control interval format

CIDFs are 4 bytes long, and contain the amount and location of free space. RDFs are 3 bytes long, and describe the length of records and how many adjacent records are of the same length.

If two or more adjacent records have the same length, only two RDFs are used for this group. One RDF gives the length of each record, and the other gives the number of consecutive records of the same length. Table 1 shows RDFs for records of the same and different lengths:

Table 1. Record definition fields of control intervals

Record definition fields of control intervals

If a record exceeds the maximum record length, an error message is generated. If a record in an entry-sequenced or key-sequenced data set, or variable-length RRDS is smaller than the maximum record length, VSAM saves disk space by storing the actual record length in the RDF. However, in a fixed-length RRDS, records do not vary in length. The RDF reflects the length of the record slot, and cannot be adjusted.