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to the OS/390 system. This information was coded in VSE programs.
However, it is strongly recommended that this data be removed from VSE
programs as they are being converted to OS/390 versions. Place the above
data in the DCB subparameter of the DD JCL statement specifying the tape
file.
5.4.2.1 Standard User Labels
Differences between VSE and OS/390 in the area of standard user labels exist at
the application program level.
With VSE you must supply a routine to check or build standard user labels.
Specify the symbolic address of your label routine in the DTFMT, DTFSR, or
DTFPH entry in LABADDR=name. The storage address of this routine is
available in register 15. The IOCS OPEN and CLOSE routines branch to your
label routine after processing the standard labels for a file. At the end of the
routine, return to IOCS by issuing an LBRET instruction.
Under OS/390, you can specify the address of the standard user label processing
routines through the EXLST (exit list) parameter of the DCB associated with the
tape data set. An additional specification is made in the job control DD
statement.
Code the LABEL parameter of the DD statement associated with the data set as
LABEL=(,SUL) if the tape contains (or the tape is to be created with) both
standard and user labels. If either SUL or EXLST is omitted, user label
processing is bypassed. As under VSE, user labels are processed during OPEN
and CLOSE.
Processing of standard user labels on input tapes is skipped if you don′t specify
the name of a user routine in the DTF or DCB. Standard user labels are optional
on output tapes and are created only if defined in your label exit logic.
5.4.3 No Labels
An unlabeled tape contains only data records and tapemarks. Unlabeled tapes
created by VSE generally have a tapemark preceding the first file;
OS/390-created unlabeled tapes do not. See Figure 7 on page 107 for a
representation of VSE and OS/390 unlabeled tapes.
Unlabeled output tapes produced by VSE assembler language programs do not
have a leading tapemark before the first file if the parameters TPMARK=NO and
FILLABL=NO are included in the DTFMT used to create the tape. Unlabeled
output tapes produced by VSE PL/I programs do not have a leading tapemark
before the first file if NOTAPEMARK and NOLABEL are included in the
ENVIRONMENT options list of the VSE PL/I file declaration. All output tapes
produced via VSE programs written in other source languages
do
have a
tapemark in the first record.
On OS/390 systems, to position a tape at the desired data set, you must specify
the correct data set sequence number in the LABEL parameter of the DD
Chapter 5. Disk and Tape Storage Considerations 105