expansion for subsequent steps. In general, this is not possible in MVS JCL in the OS/390 environment.

4.3.2 JCL Expansion

In VSE, JCL is expanded at execution time. The most current changes, even those changed two seconds before the job begins execution are included. The first step could change a procedure that is used by the third step.

In OS/390, JCL is expanded all at once when the job is submitted. This may be hours or days before it is executed. All the procedures, all of the jobs, all of the includes that are required are expanded at the same time. You can not change variables during execution in OS/390 using symbolic names in JCL.

If a job is submitted today to be run tomorrow and overnight one of the included members is changed and not reflected in the original JCL that was submitted, the job will fail.

4.3.2.1 Early Error Detection

An advantage of expanding the JCL when the job is read in OS/390 is that many of the JCL errors will be detected early and the job will fail. This removes the ability to correct the job on the fly as in VSE. In OS/390 many errors are detected before the job starts. In VSE a card has to be processed before errors are found and the operator can act on it. However, because of this, you must have a syntactically correct JOB statement to get JCL errors from OS/390.

4.3.2.2 Overrides

The OS/390 Overridesoccur at the step level. A procedure can only be overridden in OS/390 through the addition of a DD Statement to a specified step. This is different from VSE, where statements in PROCs and SLIs are overridden using each statements sequence number in positions 73-80.

4.3.3 Operator Flexibility and Intervention

Another difference between VSE and MVS JCL is the flexibility created by requiring operator intervention. For example, the operator may correct invalid JCL syntax.

4.3.3.1 Correcting Invalid Syntax

A syntax error in a JCL statement will cause a message to be posted on the operator console. The operator will respond to the message by typing in the correct JCL statement and processing will continue. This facility is not available with OS/390. The job will fail with a JCL error, and must be corrected and resubmitted.

4.3.3.2 Operator Data Entry

From a VSE point of view this flexibility is a feature. Installations depend on this flexibility to address situations where it is necessary to have the operator retype the JCL or command. For example, a programmer may purposely put in a syntax error in the JCL to ensure it comes to the operators attention. The experienced operator retypes the string and allows the job to continue.

This technique is illustrated in the following example, where the syntax of the ASSGN statement is invalid and causes the operator to be prompted for action:

76VSE to OS/390 Migration Workbook

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IBM OS/390 manual JCL Expansion, Operator Flexibility and Intervention

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.