z/OS is IBM’s flagship 64-bit operating system for IBM Z mainframes (the z/Architecture, known in Linux as s390x). It is the direct successor to OS/390 and descends from the MVS line; its first release appeared in 2001.
The system is designed for mission-critical enterprise environments — banking, insurance, airlines and government — where continuous availability, high transactional throughput and security (including pervasive encryption) are essential. z/OS runs exclusively on IBM Z mainframe hardware and is not supported on IBM LinuxONE systems, which run Linux only.
z/OS 3.1 (2023) achieved UNIX 95 (Single UNIX Specification) certification through its UNIX System Services subsystem and POSIX compliance. z/OS 3.2 (2025) is engineered for the IBM z17 mainframe and introduces embedded AI System Services, an AI-powered Workload Manager and support for AI acceleration via the Telum II processor and the IBM Spyre Accelerator.
The z/OS platform runs applications written in COBOL, PL/I, High Level Assembler, C/C++, Java and REXX, as well as more modern languages — Python, Go and Node.js — available through the IBM Open Enterprise SDK packages. IBM also offers watsonx-based modernization tooling, including watsonx Code Assistant for Z for COBOL-to-Java transformation.
General-purpose or robotics-specialized operating system — Ubuntu, Ubuntu Core, ROS-aware distributions. Provides the platform for middleware, runtime, and robotic applications.
Banks, insurers, airlines, government agencies and large-scale transaction-processing systems worldwide.
COBOL (Common Business-Oriented Language) is one of the oldest high-level programming languages, created in 1959. Designed for business data processing, it remains the backbone of transaction systems in banking, insurance and government. On IBM z/OS it is compiled with tools such as Enterprise COBOL for z/OS.
PL/I (Programming Language One) is a general-purpose procedural language developed by IBM in the mid-1960s, combining features of scientific and business languages. It is still supported on IBM z/OS through the Enterprise PL/I for z/OS compiler.
Java is an object-oriented programming language and a runtime platform based on a virtual machine (JVM). IBM z/OS provides a 64-bit Java runtime (IBM Semeru Runtime / IBM SDK for z/OS), used among other things for mainframe application modernization.
High Level Assembler (HLASM) is IBM’s macro assembler targeting the z/Architecture. It enables low-level programming of IBM Z mainframes with rich macro support and is the standard assembler on the z/OS platform.
REXX (Restructured Extended Executor) is an interpreted scripting language created by IBM in the 1970s. Valued for its readable syntax, it is widely used for task automation, administrative scripting and text processing on the z/OS platform.
Runs exclusively on IBM Z mainframe hardware; no support for x86/ARM architectures (aside from the ZD&T emulator for development purposes).
z/Architecture is the 64-bit processor architecture of IBM Z mainframes, introduced in 2000 as the successor to ESA/390. It features 24/31/64-bit addressing modes and built-in accelerators (including the Integrated Accelerator for AI in the Telum and Telum II processors). In the Linux ecosystem this architecture is known as s390x.
A deployment model in which software runs directly on mainframe hardware located in an organization’s on-premise data center. Typical of mission-critical systems that require high availability, transactional throughput and security.
Engineered for IBM z17. Introduces AI System Services (an embedded AI framework), an AI-powered Workload Manager and AI acceleration support (Telum II, IBM Spyre Accelerator).
First release of the V3 line. Modernized packaging, pervasive encryption, container support (zCX) and UNIX 95 certification.
V2R5 release.
First z/OS release, successor to OS/390.
