DAM (or BDAM)

Direct Access Method (or Basic Direct Access Method) -- used for disk devices. Still in some use, but often replaced by VSAM functions in many VSE shops. Generally requires complex application handling for processing, and may be dependent upon physical device characteristics. Not supported on Fixed-Block Architecture disks.

In OS/390, BDAM is the functional equivalent.

Libraries VSE Librarian should be considered an access method, as it meets all the criteria specified above. The current VSE Librarian has been available since VSE/SP Version 2, in 1984. The previous implementation will not be discussed.

In OS/390, Partitioned Data Sets (PDS, PDS-E) provide the equivalent functions, together with associated utilities.

5.1.2 Operating System Implementations

In the VSE/ESA system, programs define their intent to use an access method and specify needed parameters through the APIs provided through a set of Define The File macros. These include:

DTFCD Define The File CarD

DTFCN Define The File CoNsole

DTFDA Define The File Direct Access

DTFDI Define The File Device Independence

DTFDR Define The File Document Reader

DTFDU Define The File Diskette Unit

DTFIS Define The File Indexed Sequential access

DTFMR Define The File Magnetic ink character Reader

DTFMT Define The File Magnetic Tape

DTFOR Define The File Optical Reader

DTFPH Define The File for PHysical I/O

DTFPR Define The File for PRinter

DTFSD Define The File for Sequential Disk

In addition to these, additional macros are available for definition of VSAM, Librarian, and some other access method objects or files, including telecommunication terminals or lines.

In the OS/390 environment, most of these DTFs are replaced by an analogous control block definition, the Data Control Block, or DCB. The DCB is not device specific, as the VSE DTFs are, so there is more flexibility for using a single OS/390 program to read data, for example, from the SYSIN stream, from tape or from disk. Only the JCL would be changed to specify the device characteristics at run time to switch from one input or output device type to another.

The VSE application program contains the DTF macro expansion, and at linkage edit or execution time, an operating system component module (referred to as a

²Logic Module²) will be connected to the DTF and used by the application program to handle the functions needed for GET, PUT, or other imperative macro commands.

98VSE to OS/390 Migration Workbook

Page 122
Image 122
IBM OS/390 manual Operating System Implementations, DAM or Bdam

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.