2.1.2 Areas of VSE and OS/390 Differences

In order to properly assess and size the magnitude of the migration project, it is first necessary to understand some of the basic differences between the two operating systems. Once these differences are understood a realistic or more reasonable project outlook can be determined. The purpose of this section is to put into perspective these differences.

Even though both VSE and OS/390 support the IBM S/390 architecture, there are differences that must be considered at both the subsystem and application program level. When migrating or converting application programs from VSE to OS/390 it is important to identify these differences. The primary differences can be categorized as follows:

1.Source Programs

2.Job Control Language (JCL)

3.Files

4.Operations

2.1.2.1 Source Programs

The significance of the differences when dealing with program source code can vary by many factors. The primary determining factors involved in converting source programs have to do with the interfaces which provide services to the application programs. These application interfaces and corresponding protocols for requesting supervisor services are different in VSE than in OS/390.

The factors involved in converting batch programs that interface directly to the control program and programs that interface with application subsystems are different. Consequently, the effort and the techniques used will vary.

Source Program Inventory

The first step in assessing the scope of any application program conversion is determining the whereabouts of all of the program source code. This task must not be overlooked and needs to be done early in the conversion project. You will need to determine that all executable modules have associated source code and that all source code has associated executable modules. Executable modules missing source code, for example, will have to somehow be recreated or alternate plans developed to provide the program function. Conversion tools are available to assist in this task and are discussed later in this publication. Customers who have completed or are in the process of Year 2000 compliance are most likely aware of this issue.

The impact of source program conversion can be reduced by positioning the VSE production system with source programs compatible with both VSE and OS/390. For example, moving to the Language Environment for VSE will provide language compiler compatibility (for COBOL , PL/I and C for VSE/ESA) between VSE and OS/390.

Batch and Online Program Conversion

The conversion of batch applications must take into account differences in the application interfaces provided by VSE and OS/390. The significance of the changes required in the source programs depends a great deal on the source program language and to some extent the I/O access methods used. This

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IBM Areas of VSE and OS/390 Differences, Source Programs, Source Program Inventory, Batch and Online Program Conversion
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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.
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