Data set characteristics

The HDR2 label immediately follows the HDR1 label. The system uses the HDR2 label to determine certain data set characteristics, if these characteristics are not otherwise specified by the user. The characteristics that can be obtained from the HDR2 label are:

Record format
Block length
Logical record length
Tape recording technique
Type of control characters
Coded character set identifier (CCSID)

The above information is obtained from the label and recorded in the job file control block (JFCB) and the data control block, provided that the appropriate fields in these control blocks contain zeros. The label information cannot override any characteristics previously specified in the processing program or the DD statement. For an explanation and illustration of this merging process, see Completing the data control block.

Because the HDR2 label is optional with ISO/ANSI interchange tapes, and the HDR2 format may vary if produced by systems other than MVS, the manner of processing HDR2 labels depends on whether the HDR1 label specifies "IBMZLA" as the name of the system producing the tape. If "IBMZLA" is specified for Version 3, the HDR2 label "Reserved for Operating System" field contains fields that are similar to an IBM standard HDR2 label, and is processed accordingly. See Processing the HDR2 label for more information. For Version 4, the "Reserved for Operating System" field also contains the CCSID that the data set was written in. If "IBMZLA" is not specified, the data in the HDR2 label "Reserved for Operating System" and "Record Format" fields cannot be used by the operating system because the contents are unknown. In that case you can specify the missing information in the DCB or DD statement.

The HDR2 label is merged into the vacant fields of the JFCB if a tape data set is being opened for INPUT, INOUT, DISP=MOD, OPEN for OUTPUT, or RDBACK.