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 Some of the file attributes coded in the VSE JCL are superseded by the disk or tape manager: the proper file attributes must be retrieved in the tape or disk manager¢s catalog or in the VTOC listings.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, ²reverse engineering² 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:

1Understanding of application data flows (from enterprise-wide cross-references between files and steps)

2Classification 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

3Definition 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 ²&&² work files for step-level temporary files

and so on

4Generation 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 ²VSE files² which might (although not always) have the same record length or record layout, but are true separate data

34VSE to OS/390 Migration Workbook

Page 58
Image 58
IBM manual VSE to OS/390 Migration Workbook

OS/390 specifications

IBM OS/390, a versatile operating system, was a cornerstone in enterprise environments and played a pivotal role in mainframe computing. Released in the mid-1990s, OS/390 combined the strengths of IBM's MVS (Multiple Virtual Storage) with new features and enhancements, targeting scalability, reliability, and performance in demanding business applications.

One of the key features of OS/390 was its robust support for multiple users and processes. The system allowed thousands of concurrent users to access applications and data, ensuring high availability and minimizing downtime—a critical requirement for many large organizations. This scalability was supported through various enhancements in memory management and processor scheduling, enabling optimal resource allocation across diverse workloads.

OS/390 was known for its superior workload management capabilities. The Workload Manager (WLM) component allowed administrators to define service policies, specifying how system resources would be allocated according to the priority of tasks. This ensured that critical business processes received the necessary resources while less critical tasks were managed more flexibly.

Another significant characteristic of OS/390 was its commitment to security. The operating system provided comprehensive security features, including user authentication, data encryption, and auditing capabilities. This focus on security was vital for organizations handling sensitive data, ensuring compliance with regulations and safeguarding against unauthorized access.

OS/390 also supported advanced technologies that facilitated integration and development. The system included features like the IBM CICS (Customer Information Control System) for transaction processing and IMS (Information Management System) for database management. These technologies allowed organizations to build robust, high-performance applications tailored to specific business needs.

The ease of network integration was another strength of OS/390. With the advent of the Internet and global connectivity, OS/390 systems could easily interface with various network protocols, enabling businesses to operate in a connected world. This inclusion paved the way for many organizations to expand their capabilities and offer new services, driving digital transformation.

In conclusion, IBM OS/390 represented a significant advancement in mainframe technology, combining scalability, security, and robust workload management. Its rich feature set and support for critical enterprise applications solidified its role as a vital component of many organizations' IT infrastructures, ensuring they could meet their operational challenges head-on while supporting future growth. As technology continues to evolve, the legacy of OS/390 remains influential in the realm of computing.