There is an additional byte preceding each SDW for DS/DBS records. This additional byte is required for conversion of the SDW from IBM to ISO/ANSI format, because the ISO/ANSI SDW (called a segment control word) is five bytes long. Otherwise, the SDW for DS/DBS records is the same as the SDW for VS/VBS records. The SDW LL count excludes the additional byte.
QSAM or BSAM convert between ISO/ANSI segment control word (SCW) format and IBM segment descriptor word (SDW) format. On output, the binary SDW LL value (provided by you when using BSAM and by the access method when using QSAM), is increased by 1 for the extra byte and converted to four ASCII numeric characters. Because the binary SDW LL value will result in four numeric characters, the binary value must not be greater than 9998. The fifth character is used to designate which segment type (complete logical record, first segment, last segment, or intermediate segment) is being processed.
On input, the four numeric characters designating the segment length are converted to two binary SDW LL bytes and decreased by one for the unused byte. The ISO/ANSI segment control character maps to the DS/DBS SDW control flags. This conversion leaves an unused byte at the beginning of each SDW. It is set to X'00'. See z/OS DFSMS Macro Instructions for Data Sets for more details on this process.
On the tape, the SDW bytes are ASCII numeric characters even if the other bytes in the record are not ASCII.