What is z/OS?

Introduced in 2000, IBM z/OS is a 64-bit mainframe operating system (OS) developed by IBM for its family of enterprise z/Architecture mainframe computers that organizations use for running mission-critical applications.

The mainframe continues to be used by the largest enterprises in the world, and mainframe-based applications are a central part of modern business strategies.

The 64-bit z/Architecture is backward-compatible with older 24- and 31-bit applications. Z/OS supports web and Java-based applications, as well as COBOL (Common Business-Oriented Language). The latest version of z/OS is 2.5.

How is z/OS used?

IBM mainframes with z/OS run complex, mission-critical workloads for large enterprise organizations. Mainframes are well suited for tasks involving large-scale transaction processing and can handle thousands of users and many applications at once.

Mainframes can also simultaneously handle diverse units of work because of their ability to run secure, multi-tenant workloads. Each application runs in its own protected memory space and operates to its own performance goals.

Z/OS can also run modern OSes, like Linux and Unix. It also operates on the following IBM Z servers:

Common enterprise systems that run on mainframes include order entry, finance, payroll, inventory control, manufacturing and enterprise resource planning.

Z/OS features

Version 2.5 of IBM z/OS provides the following critical features and functionality:

  • artificial intelligence (AI) capabilities;
  • security features to fight ransomware;
  • cloud storage integration features;
  • Java and COBOL interoperability features; and
  • z/OS Container Extensions.

IBM z/OS 2.5 offers AI capabilities that are tightly integrated with z/OS workloads. These are designed to give users business insights for more informed decision-making. This next-generation OS for IBM Z also supports hybrid cloud.

Z/OS 2.5 also offers the following features.

Storage and data features

  • Bulk Data Transfer moves large data payloads between computer systems.
  • Distributed File Service runs the Unix high-performance zSeries File System and a hierarchical file system that manages files and directories and provides server message block file and print server support.
  • Network File System provides file server support to workstations, PCs and other systems in a TCP/IP
  • The DFSMSdfp module offers storage, data, program and device management.
  • The DFSMSdss module copies and moves data for backup and recovery and reduces disk fragmentation.
  • The DFSMShsm module automates direct access storage device management to help mitigate data loss.

Language features

  • Z/OS has an alternate library for Restructured Extended Executor that enables programmers to run older compiled REXX programs.
  • High Level Assembler Toolkit offers developer tools to improve application development, debugging and recovery.
  • Interactive System Productivity Facility, or ISPF, provides tools for host-based software development, including Dialog Manager for visual interface components, Program Development Facility to assist developers in editing and compiling, Software Configuration and Library Manager to manage all of the software components of an application, and a client-server
  • Z/OS supports programs generated with C, C++, COBOL, Fortran and PL/I.

Document management features

  • BookManager Build creates documents for BookManager products that display, search and manage online documents.
  • The Graphical Data Display Manager group of features provides presentation capabilities. It uses IBM Hypertext Transfer Protocol Server as a web server for z/OS to support e-business applications.
  • Infoprint Server enables administrators to print files on z/OS printers from any networked workstation.

Systems management features

  • Common Information Model enables system administrators to create applications that monitor system resources in a heterogeneous network.
  • First Failure Support Technology provides immediate alerting and data capture for software failure events.
  • Hardware Configuration Definition defines the OS and processor configurations for a system.
  • IBM Tivoli Directory Server for z/OS offers a Lightweight Directory Access Protocol server, an LDAP client and LDAP client utilities.
  • The Open Systems Adapter/Support Facility monitors and controls network connectivity, local area networks and wide area networks.
  • Resource Measurement Facility gathers data about z/OS resource use and performance.
  • System Display and Search Facility enables administrators to monitor, manage and control z/OS systems.

Communications features

  • Z/OS supports secure TCP/IP; Systems Network Architecture, or SNA; and Unix networking. And zEnterprise Data Compression supports direct data compression using zEC12 and zBC12 zEDC Express adapters.
  • Communications Server provides enforcement of network encryption standards.

Security features

  • Cryptographic Services protects and signs data and manages cryptographic keys as long as 56 bits. Longer keys are possible with optional z/OS Security Level 3 features.
  • Integrated Security Services provides Enterprise Identity Mapping, which maps a user's identity on one system to the user's identity on another system; a network authentication service; and Open Cryptographic Enhanced Plug-ins.

What is the latest version of z/OS?

Z/OS was announced in October 2000 and first appeared as Version 1 Release 1 on March 30, 2001. The current version, 2.5, was released on Sept. 30, 2021. Z/OS is the successor to OS/390 and several versions of MVS, or Multiple Virtual Storage.

IBM z/OS history
IBM z/OS history of release and end-of-service dates.

Z/OS pros and cons

Z/OS has established a long pedigree of reliable operation. IBM's careful attention to workload management, scheduling and security can be vital attributes for busy enterprises running critical applications.

With z/OS, companies can also run guest OSes, like Linux, Unix and Windows. However, z/OS and the mainframes it runs on can be an expensive investment that can include a proprietary monthly license charge, pricing based on usage and reduced pricing options for some applications.

IBM native OSes, like z/OS, are also closed source, so users must rely on IBM to maintain and update the code. Users may be forced into updates and associated costs as support expires on aging OS versions.

The demand for mainframe skills post-COVID-19 pandemic has forced organizations to tackle the issue through reskilling and internal hiring.

This was last updated in June 2022

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