link-edit. To create a loadable computer program by means of a linkage editor.

load module. An application or routine in a form suitable for execution. The application or routine has been compiled and link-edited; that is, address constants have been resolved.

load module library. A partitioned data set used to store and retrieve load modules. See also object module library, source module library.

local area network (LAN). (1) A computer network located on a users premises within a limited geographical area. Communication within a local area network is not subject to external regulations; however, communication across the LAN boundary may be subject to some form of regulation. See also wide area network. See figure 88. ñNote: A LAN does not use store and forward techniques. (2) A network in which a set of devices are connected to one another for communication and that can be connected to a larger network. See also token ring.

lock. (1) A serialization mechanism by means of which a resource is restricted for use by the holder of the lock. See exclusive lock, shared lock. (2) The means by which integrity of data is ensured by preventing more than one user from accessing or changing the same data or object at the same time.

log data set. A data set consisting of the messages or message segments recorded on auxiliary storage by the ACF/TCAM logging facility.

logical device. (1) A file for conducting input or output with a physical device. (2) A file for mapping user I/O between virtual and real devices.

logical record. (1) A set of related data or words considered to be a record from a logical viewpoint. (2) A record from the standpoint of its content, function, and use rather than its physical attributes, that is, a record defined in terms of the information it contains.

(3)In CICS/VS, a data record sent by one transaction program to another. The length of the record is contained in a two-byte field immediately preceding the record. (4) In VSAM, a unit of information normally pertaining to a single subject; a logical record is the user record requested of or given to the data management function. (5) In COBOL, the most inclusive data item.

M

main task. In VSE, the main program within a partition in a multiprogramming environment.

maintenance. Any activity intended to retain a functional unit in, or to restore it to, a state in which it can perform its required function. Maintenance

includes keeping a functional unit in a specified state by performing activities such as tests, measurements, replacements, adjustments, and repairs.

MAS . Multi-Access Spool facility. A loosely connected complex of JES2 members.

mass conversion. An automated process that includes program translation, JCL conversion, and file transfer. This process is what makes possible the rapid switchover from VSE to MVS.

master console. In a system with multiple consoles, the basic console used for communication between the operator and the system.

MCS . (1) Multiple Console Support. A feature of MVS that permits selective message routing to up to 32 operators consoles. (2) modification control statement. An SMP/E control statement used to package a SYSMOD. These statements describe the elements of a program and the relationships that program has with other programs that may be installed on the same system.

menu. A list of options displayed to the user by a data processing system, from which the user can select an action to be initiated.

message data set. A data set on disk storage that contains queues of messages awaiting transmission to particular terminal operators or to the host system.

migrate. To move to a changed operating environment, usually to a new release or version of a system.

migration (VSE/MVS). The entire process of transition from a VSE environment to an MVS environment. Migration includes training, project planning and management, system and configuration setup, conversion design, and the conversion itself.

minidisk. Synonym for virtual disk

module. A program unit that is discrete and identifiable with respect to compiling, combining with other units, and loading; for example, the input to or output from an assembler, compiler, linkage editor, or executive routine.

multiprocessor. (1) A computer including two or more processors that have common access to a main storage. (2) A system of two or more processing units, ALUs, or processors that can communicate without manual intervention.

multitasking. A mode of operation that provides for concurrent performance, or interleaved execution of two or more tasks.

multithreading. Pertaining to concurrent operation of more than one path of execution within a computer.

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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.