13.2.6.12 Track and Record Addressing

Track Addressing

In VSE and MVS, you can make track references by using either the actual or relative addressing technique. The track reference field for actual addressing in VSE and MVS is of the form MBBCCHHR. The contents of the M byte is different in VSE and MVS. In VSE, M represents volume number, starting with zero for the first volume. This must be increased by one for each subsequent volume.

In the MVS data set control block (label), M contains the extent sequence number, which always starts with zero, on each volume. This must be increased only for extents within the same volume. In the MVS data extent block, M is a pointer to extent information. Therefore, if actual addressing is required, you must change your calculation routine to meet the MVS requirements. However, you should use relative addressing instead of actual addressing because of the simplicity in calculating the proper track number.

In the case of relative track addressing, the correct disk addresses are generated by the control program. If you use actual addressing, you must check the extents on that volume to make sure nothing else is written there. Actual addressing does not allow you to take advantage of some of the MVS facilities and may impair the performance of the system. Relative addressing allows the control program to place the data set where it is most convenient. It does all the necessary checking of extents. With relative addressing, the system keeps track of each data set, thus making programming easier and system use more efficient.

Record Addressing

Within a track, records may be addressed either by their record number (ID) or by key.

Record Addressing by ID

Provide the record number in the R byte of the track reference field.

VSE

READ

filename,ID

 

 

 

MVS

READ

ecbname,DI,...,blockaddress

 

 

 

Blockaddress points to a field containing the complete identification of the record.

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IBM OS/390 manual Track and Record Addressing, Track Addressing, Record Addressing by ID

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.