2.7.2 Program Conversion

The conversion of VSE application code to OS/390 is often (but falsely) believed to be the center, most challenging, most labor consuming and most critical part of the conversion, but it is not. With few exceptions (see VSE positioning), it is a simple code modification which does not change program logic, and can nearly always be applied with a simple two-pass translation tool.

VSE COBOL code must also be upgraded to the latest (COBOL for OS/390) compiler level. But this upgrade too, requires no program logic change, and can be applied with a simple two-pass translation tool.

In technical terms, these OS/390 and COBOL upgrade modifications are simple code ²re-engineering² which fall into one of the following categories:

Syntax modification: replace a syntax pattern on one or several statements by a similar OS/390 compatible syntax pattern.

Device independence: eliminate block sizes and other device dependencies from the converted code. Under OS/390, device dependent file attributes are either coded in the JCL or determined by the system managed storage (DFSMS) components of OS/390.

Elimination of VSE-only features: features such as COMREG, UPSI, DATE and USER can be replaced by calls to user-developed ad-hoc subroutines that simulate the feature under OS/390. Some other VSE-only features, such as the usage of VSE system macros and VSE supervisor calls from Assembler may be more complex to convert.

The difficulty level when converting COBOL code to OS/390 is fairly similar from one VSE installation to another, but it is not so with Assembler. The conversion of Assembler code can be fairly easy, if VSE standard application coding was used, or very complex, if system-dependent non-standard coding was used. In some cases, the conversion of an Assembler program may start with a complete redesign, in which one must identify what function or feature will still be performed by the program, and what function or feature will be handled by the OS/390 system software and utilities. This leads to partial or complete rewrite. Fortunately, those situations are becoming rarer, as VSE installations progressively eliminate their non-standard and system-dependent coding practices.

The conversion of VSE code to OS/390 and COBOL code upgrade may last two to four months and represent 10 to 15% of the total application conversion effort, unless there is a significant inventory of technical non-standard Assembler programming.

2.7.3 JCL Conversion

JCL conversion is nearly always underestimated in both duration and labor. It is the central, most challenging, most labor consuming and most critical part of the VSE to OS/390 conversion.

VSE JCL streams alone are not sufficient to define the flow of the associated job streams. The sequence of steps is evident, but the file references are not always visible. Some are hidden in the standard or partitioned labels. Some are passed and reused from one step to the next. File reference statements (TLBL and DLBL) coded in the JCL are not necessarily used in VSE; the program might not open that file. It is accepted practice in VSE and doesn¢t trigger any syntax or execution error. The file open mode (input, output) is not visible from the VSE

Chapter 2. Sizing the Effort

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IBM OS/390 manual Program Conversion, JCL Conversion

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.