For ISO/ANSI tapes, variable-length spanned records must be specified in the DCB RECFM parameter as DCB RECFM=DS or DBS. Format-DS and format-DBS records are similar to format-VS or format-VBS records. The exceptions are described in Converting the Segment Descriptor Word and Processing Records Longer than 32␠760 Bytes.

Figure 1 shows what spanned variable-length records for ISO/ANSI tapes look like.

Figure 1. Spanned Variable-Length (Format-DS) Records for ISO/ANSI Tapes As Seen by the Program

Figure 1 shows the segment descriptor word (SDW), where the record descriptor word (RDW) is located, and where block prefixes must be placed when they are used. If you are not using IBM access methods see z/OS DFSMS Macro Instructions for Data Sets for a description of ISO/ANSI record control words and segment control words.