IBM OS/390 User Manual

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JCL: it is hidden inside the code (main or sub-program) associated with the step.
Some of the file attributes coded in the VSE JCL are superseded by the disk or
tape manager: the proper file attributes must be retrieved in the tape or disk
manager
s catalog or in the VTOC listings. In short, it is not possible to
understand the flowchart of the job stream without retrieving and analyzing the
file opening inside programs and sub-programs, and without collecting in
formation from standard labels, partitioned labels, the VSE catalogs and VTOC
listings.
Contrary to VSE, OS/390 JCL streams generally reflect exactly the flowchart of
the job streams. All files opened within a step have a file reference (DD
statement) coded in the JCL. There are no unused file references. The mode of
open (input, output, extend) of the file is coded in the OS/390 JCL (disposition).
Therefore, when converting VSE JCL streams to OS/390, whether manually or
automatically, 
reverse engineering
″ 
techniques are first used to rebuild the job
stream flowcharts from:
VSE JCL streams
program conversion (block sizes, device related information, open mode)
standard and partitioned labels
VSE catalogs and VTOC listings
This is the most complicated part of the JCL conversion, not only because it
requires you to collect and coordinate file reference information coming from
different origins, but also because understanding the application job stream
requires:
1
Understanding of application data flows (from enterprise-wide
cross-references between files and steps)
2
Classification of data flows (that is, files) according to data life cycles:
Permanent
Handoff
Passed temporary file
Work (step-level temporary file)
Backup
External input or output
Edition or report
3
Definition and implementation of file management strategies based on the
file classification, for example:
Usage of GDG for permanent, handoff or backup sequential files
Cataloging of passed temporary files and their deletion after last usage
Usage of OS/390 
″&&″ 
work files for step-level temporary files
and so on
4
Generation of OS/390 JCL, DFSMS constructs and VSE to OS/390 file
migration procedures reflecting the understanding of data flows and their
classification.
To illustrate the complexity of VSE JCL conversion and its underlying
identification and understanding of data flows, many VSE labels and even
physical DASD locations are shared by 
VSE files
″ 
which might (although not
always) have the same record length or record layout, but are true separate data
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VSE to OS/390 Migration Workbook