Format-D, format-DS, and format-DBS records are used for ISO/ANSI tape data sets. ISO/ANSI records are the same as format-V records, with three exceptions:

• Block prefix
• Block size
• Control characters.

Block Prefix. A record block can contain a block prefix. To specify a block prefix, code BUFOFF in the DCB macro. The block prefix can vary in length from 0 to 99 bytes, but its length must remain constant for all records in the data set being processed. For blocked records, the block prefix precedes the RDW for the first or only logical record in each block. For unblocked records, the block prefix precedes the RDW for each logical record.

To specify that the block prefix is to be treated as a BDW by data management for format-D or format-DS records on output, code BUFOFF=L as a DCB parameter. Your block prefix must be 4 bytes long, and it must contain the length of the block, including the block prefix. The maximum length of a format-D or format-DS, BUFOFF=L block is 9999 because the length (stated in binary numbers by the user) is converted to a 4 byte ASCII character decimal field on the ISO/ANSI tape when the block is written. It is converted back to a 2 byte length field in binary followed by two bytes of zeros when the block is read.

If you use QSAM to write records, data management fills in the block prefix for you. If you use BSAM to write records, you must fill in the block prefix yourself. If you are using chained scheduling to read blocked DB or DBS records, you cannot code BUFOFF=absolute expression in the DCB. Instead, BUFOFF=L is required, because the access method needs binary RDWs and valid block lengths to unblock the records.

When you use QSAM, you cannot read the block prefix into your record area on input. When using BSAM, you must account for the block prefix on both input and output. When using either QSAM or BSAM, you must account for the length of the block prefix in the BLKSIZE and BUFL parameters.

When using QSAM to access DB or DBS records, and BUFOFF=0 is specified, the value of BUFL, if specified, must be increased by 4. If BUFL is not specified, then BLKSIZE must be increased by 4. This permits a 4 byte QSAM internal processing area to be included when the system acquires the buffers. These 4 bytes do not become part of the user's block.

When you use BSAM on output records, the operating system does not recognize the block prefix. Therefore, if you want a block prefix, it must be part of your record.

The block prefix can contain only EBCDIC characters that correspond to the 128, seven-bit ASCII characters. Thus, you must avoid using data types (such as binary, packed decimal, and floating point), that cannot always be converted into ASCII. For DB and DBS records, the only time the block prefix can contain binary data is when you have coded BUFOFF=L, which tells data management that the prefix is a BDW. Unlike the block prefix, the RDW must always be binary. This is true whether conversion or no conversion is specified with CCSID for Version 4 tapes.

Block Size. Version 3 tapes have a maximum block size of 2048. This limit can be overridden by a label validation installation exit. For Version 4 tapes, the maximum size is 32␠760.

If you specify a maximum data set block size of 18 or greater when creating variable-length blocks, then individual blocks can be shorter than 18 bytes. In those cases data management pads each one to 18 bytes when the blocks are written onto an ISO/ANSI tape. The padding character used is the ASCII circumflex character, which is X'5E'.

Control Characters. Control characters, if present, must be ISO/ANSI control characters. For more information about control characters see z/OS DFSMS Macro Instructions for Data Sets.

Figure 1 shows the format of nonspanned variable-length records for ISO/ANSI tapes, where the record descriptor word (RDW) is located, and where block prefixes and control characters must be placed when they are used.

Figure 1. Nonspanned Format-D Records for ISO/ANSI Tapes As Seen by the Program