statement. If a tapemark might precede the first data set and you specify the LABEL subparameter LTM, OS/390 tests for and bypasses a leading tapemark, if present. If a tapemark precedes the first data set and you do not specify LTM in the LABEL parameter field, you must add one to the data set sequence number.

If a multivolume data set has a leading tapemark on one or more of the volumes, OS/390 can process it as an unlabeled multivolume data set if the LABEL subparameter LTM is specified. Otherwise, OS/390 cannot process it as an unlabeled multivolume data set.

The presence of a leading tapemark makes each data set the second in sequence on the tape. However, OS/390 always assumes that data sets are first in sequence on the tape. Specify LTM in the LABEL parameter field, so the first

data set on a tape can be accessed whether or not it is preceded by a leading

tapemark, for example code LABEL=(1,LTM).

The specification of LTM in the LABEL parameter field does not make allowances for any other tapemarks. You must make any such adjustments in the data set sequence number.

5.4.4 Nonstandard Labels

Nonstandard labels do not conform to standard label formats. They are designed by the installation, and are written and processed by routines provided by the installation. There are no requirements as to the length, format, contents, and number of nonstandard volume labels. When processing nonstandard labels, you must perform many of the functions that control program or IOCS performs in processing standard labels. All input/output operations, such as reading and writing labels, are performed by using the EXCP macro.

When nonstandard labels are to be created or checked, a user routine is required. Because of the basic differences between VSE and OS/390 in handling nonstandard labels, this portion of VSE programs requires rewriting.

Under VSE, nonstandard label processing is included as part of the user program when the DTFMT contains the parameters, FILABL=NSTD and LABADDR=name. IOCS OPEN and CLOSE routines provide the entry point for user processing of nonstandard labels according to the LABADDR parameter in the DTFIMT. If you omit this parameter with nonstandard labels, IOCS bypasses label processing. It is your responsibility to conform to standard register conventions. Only one nonstandard label routine is supported for each file, but multiple DTFs may share a common routine. No VSE job control language statement is required to define a nonstandard label. Figure 7 on page 107 illustrates nonstandard labels supported under VSE.

5.4.5 Bypass Label Processing Facility in OS/390

Should you run across VSE labels that cannot be processed by OS/390, you can use the OS/390 ²bypass label processing² facility. Specifying BLP in the OS/390 JCL LABEL parameter (LABEL=(,BLP)) requests the system to bypass label processing. When used, the operator must ensure that the correct tape volume is mounted. The BLP parameter should only be used for unique situations, and its

use should be controlled.

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IBM manual Nonstandard Labels, Bypass Label Processing Facility in OS/390, Use should be controlled

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.