4.3.1.2 Data Driven Segmentation of Output

An artifact of this sequential processing in VSE is that the spooling system extracts its control statements (JECL) from the input as it is being spooled. When the input processing crosses the input stream position where the JECL statement was located, the spooling system will take the action specified by the JECL statement. The JECL statement can change the output destination of printed or punched data, or other characteristics, such as special forms requirements.

VSE Example

*$$ JOB JNM=DJANDA,CLASS=A

*$$ LST CLASS=A,DEST=*

//JOB DJANDA

//EXEC MYPROG

INPUT DATA CARD 1 INPUT DATA CARD 2 INPUT DATA CARD 3 INPUT DATA CARD 4 INPUT DATA CARD 5

* $$ LST CLASS=J,DEST=DANJ,DISP=H

INPUT DATA CARD 6 INPUT DATA CARD 7 INPUT DATA CARD 8 INPUT DATA CARD 9 INPUT DATA CARD 10 INPUT DATA CARD 11 /* /&

* $$ EOJ

The result would be that output printed by this job and program would be sent to the default system printer, up to the point when the program read INPUT DATA CARD 6. Output generated from that point forward (including that of cards 6 through 11) would be sent to a specific user ID, DANJ, rather than the default printer, and it will be in the HOLD disposition state.

A second type of input segmentation appears when a given program will open an instream data file, read some of its records and close the file. Later, it will open the same instream data file and read additional records. In VSE, the records read in the second group will follow the first group in the input stream. A simple conversion to OS/390 will result in the second file open re-reading the same records read by the first file!

A method to circumvent this problem is to change the program logic or to write a subroutine which traps all the reads on the two input streams and which has one single DCB, so there is only one DDNAME and then the behavior would be similar to the VSE case.

4.3.1.3 JCL Parameter Handling

Another difference seen because of the philosophy and architecture changes between VSE and OS/390 is the fact that VSE JCL parameters and JCL handling can depend on values that are changed at execution time. VSE conditional JCL can test return codes, as MVS JCL can, but in addition, VSE can test parameter values as well.

Also, in VSE, procedure expansion and parameter substitution is done at execution time, so the results of previous job step execution can affect the

Chapter 4. Job Control Language (JCL) Differences and Considerations 75

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IBM OS/390 manual Data Driven Segmentation of Output, JCL Parameter Handling, $$ Lst Class=J,Dest=Danj,Disp=H, $$ Eoj
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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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