document is organized by source language type and goes into great detail at that level and includes the I/O considerations.

The conversion of the CICS applications consists of two steps. First, the VSE version of the CICS application subsystem is replaced with an OS/390 version. The two different versions of CICS contain the interfaces to the respective control programs. The second step deals with the application source code itself.

In general, the interfaces provided to the applications by the two versions of CICS are the same, the source programs do not change and need only to be recompiled with a corresponding OS/390 compiler. However, consideration should also be given to the fact that certain application level interfaces available in VSE may not be available in OS/390. The macro level API is one example. Applications written with this interface will have to be changed to use the command level API. Any access to system level control blocks should also be reviewed. Additional considerations will be required if the CICS application programs are interfacing with more than the CICS subsystem. Also, there are some source language restrictions. This document contains a section describing the CICS, DB2 and DL/I subsystems in great detail.

In summary, when comparing online and batch programs, the effort required to convert batch applications is much greater than online applications using application subsystems such as CICS and DL/I. By using application subsystems the differences in control program application interfaces become transparent to the application programmer. The installation only needs to be concerned with the common interface provided by the subsystem in situations where a VSE version and an OS/390 version are both available.

2.1.2.2 Job Control Language

All VSE JCL must be converted to OS/390 JCL. Because VSE and OS/390 differ significantly in JCL structure and syntax, this is normally one of the most complex tasks of any migration. As in the case of batch and online source programs, the considerations are more significant with the batch applications. There are, however, aids available to reduce the effort required.

2.1.2.3 Files

The impact of file conversion can be reduced by positioning the VSE production system with file formats and access methods that are compatible with both VSE and OS/390.

VSAM files are generally compatible files. One section of this document is dedicated exclusively to VSAM files and VSAM catalogs. Additional information regarding VSAM file considerations can be found in the different source language sections.

Direct Access Method (DAM) files require individual evaluation because each can have unique characteristics. Each of the language sections has a description on accessing DAM files. It is recommended that these file structures be converted to relative record VSAM files where possible.

Sequential tape files are compatible between VSE and OS/390. There can be differences in the format of the labels and how they are processed. There is a chapter in this publication that deals exclusively with tape files.

Chapter 2. Sizing the Effort

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IBM OS/390 manual Job Control Language, Files

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.