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block length record length

tape recording technique (seven-track only) tape density

record format

to the OS/390 system. This information was coded in VSE programs. However, it is strongly recommended that this data be removed from VSE programs as they are being converted to OS/390 versions. Place the above data in the DCB subparameter of the DD JCL statement specifying the tape file.

5.4.2.1 Standard User Labels

Differences between VSE and OS/390 in the area of standard user labels exist at the application program level.

With VSE you must supply a routine to check or build standard user labels. Specify the symbolic address of your label routine in the DTFMT, DTFSR, or DTFPH entry in LABADDR=name. The storage address of this routine is available in register 15. The IOCS OPEN and CLOSE routines branch to your label routine after processing the standard labels for a file. At the end of the routine, return to IOCS by issuing an LBRET instruction.

Under OS/390, you can specify the address of the standard user label processing routines through the EXLST (exit list) parameter of the DCB associated with the tape data set. An additional specification is made in the job control DD statement.

Code the LABEL parameter of the DD statement associated with the data set as LABEL=(,SUL) if the tape contains (or the tape is to be created with) both standard and user labels. If either SUL or EXLST is omitted, user label processing is bypassed. As under VSE, user labels are processed during OPEN and CLOSE.

Processing of standard user labels on input tapes is skipped if you don′t specify the name of a user routine in the DTF or DCB. Standard user labels are optional on output tapes and are created only if defined in your label exit logic.

5.4.3 No Labels

An unlabeled tape contains only data records and tapemarks. Unlabeled tapes created by VSE generally have a tapemark preceding the first file;

OS/390-created unlabeled tapes do not. See Figure 7 on page 107 for a representation of VSE and OS/390 unlabeled tapes.

Unlabeled output tapes produced by VSE assembler language programs do not have a leading tapemark before the first file if the parameters TPMARK=NO and FILLABL=NO are included in the DTFMT used to create the tape. Unlabeled output tapes produced by VSE PL/I programs do not have a leading tapemark before the first file if NOTAPEMARK and NOLABEL are included in the ENVIRONMENT options list of the VSE PL/I file declaration. All output tapes produced via VSE programs written in other source languages do have a tapemark in the first record.

On OS/390 systems, to position a tape at the desired data set, you must specify the correct data set sequence number in the LABEL parameter of the DD

Chapter 5. Disk and Tape Storage Considerations 105

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IBM OS/390 manual No Labels, Standard User Labels
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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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