Record Descriptor Word (RDW)
A variable-length logical record consists of a record descriptor word (RDW) followed by the data. The record descriptor word is a 4 byte field describing the record. The first 2 bytes contain the length (LL) of the logical record (including the 4 byte RDW). The length can be from 4 to 32␠760. All bits of the third and fourth bytes must be 0, because other values are used for spanned records.
For output, you must provide the RDW, except in data mode for spanned records (described under Controlling Buffers). For output in data mode, you must provide the total data length in the physical record length field (DCBPRECL) of the DCB.
For input, the operating system provides the RDW, except in data mode. In data mode, the system passes the record length to your program in the logical record length field (DCBLRECL) of the DCB.
The optional control character (a) can be specified as the fifth byte of each record. The first byte of data is a table reference character (b) if OPTCD=J has been specified. The RDW, the optional control character, and the optional table reference character are not punched or printed.