Device Support Facilities (DSF). An IBM-supplied system control program for performing operations on disk volumes so that they can be accessed by IBM and user programs. Note: Examples of these operations are initializing a disk volume and assigning an alternate track.

device-independent. Pertaining to a program that can be executed successfully without regard for the characteristics of particular types of devices. Contrast with device-dependent.

DFSMS environment. An environment that helps automate and centralize the management of storage. This is achieved through a combination of hardware, software, and policies. In the DFSMS environment for MVS, this function is provided by MVS/ESA SP and DFSMS/MVS, DFSORT, and RACF. See also

system-managed storage.

DFSMSdfp. A DFSMS/MVS functional component that provides functions for storage management, data management, program management, device management, and distributed data access.

DFSMSdss. A DFSMS/MVS functional component used to copy, move, dump, and restore data sets and volumes.

DFSMShsm. A DFSMS/MVS functional component used for backing up and recovering data, and managing space on volumes in the storage hierarchy.

DFSMShsm-managed volume. (1) A primary storage volume, which is defined to DFSMShsm but which does not belong to a storage group. (2) A volume in a storage group, which is using DFSMShsm automatic dump, migration, or backup services. Contrast with system-managed volume and DFSMSrmm-managed volume.

DFSMShsm-owned volume. A storage volume on which DFSMShsm stores backup versions, dump copies, or migrated data sets.

DFSMS/MVS. An IBM licensed program that together with MVS/ESA SP compose the base MVS/ESA operating environment. DFSMS/MVS consists of DFSMSdfp, DFSMSdss, DFSMShsm, and DFSMSrmm.

direct access. (1) The capability to obtain data from a storage device, or to enter data into a storage device, in a sequence independent from their relative position, by means of addresses indicating the physical position of the data. (2) Contrast with serial access.

direct access storage device (DASD). A device in which access time is effectively independent of the location of the data. Usually disk storage.

directory. (1) A type of file containing the names and controlling information for other files or other

directories. (2) An index that is used by a control program to locate one or more blocks of data that are stored in separate areas of a data set in direct access storage.

disk file. A set of related records on disk that are treated as a unit.

distributed data. In SAA usage, data that is split across two or more linked systems but which can be accessed and processed as if it resided on one.

Distributed Data Management (DDM). A feature of the System Support Program Product that allows an application program to work on files that reside in a remote system.

DL/I. See Data Language One.

DLIB. Distribution library. IBM-supplied partitioned data sets on tape containing one or more components that the user restores to disk for subsequent inclusion in a new system.

double-byte character set (DBCS). A set of characters in which each character is represented by 2 bytes. Languages such as Japanese, Chinese, and Korean, which contain more symbols than can be represented by 256 code points, require double-byte character sets. Because each character requires 2 bytes, the typing, display, and printing of DBCS characters requires hardware and programs that support DBCS. Contrast with single-byte character set.

DSCB. See data set control block.

dsname. data set name. The name of a data set (1 - 40 characters) on the DD statement in the JCL or the dsname operand of the TSO ALLOC command.

dynamic address translation (DAT). In System/390 virtual storage systems, the change of a virtual storage address to a real storage address during execution of an instruction.

dynamic storage. A device that stores data in a manner that permits the data to move or vary with time such that the specified data are not always available for recovery. Magnetic drum and disk storage are dynamic nonvolatile storage. An acoustic delay line is a dynamic volatile storage.

E

emulation. (1) The use of a data processing system to imitate another data processing system, so that the imitating system accepts the same data, executes the same programs, and achieves the same results as the imitated system. Emulation is usually achieved by means of hardware or firmware. (2) The use of programming techniques and special machine

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IBM manual VSE to OS/390 Migration Workbook

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.