zOS

z/OS is IBM's 64-bit operating system for IBM System z mainframes. Introduced in 2000 as the successor to MVS/ESA and OS/390, it is designed for large-scale, mission-critical workloads that require high availability, security, and scalability. It runs on IBM Z hardware and serves as a core platform for many enterprise data centers.

The operating system combines traditional batch and transaction processing with modern subsystems and services. Job processing is handled by Job Entry Subsystem 2 or 3 (JES2/JES3); workload management is provided by z/OS Workload Manager (WLM); and security is managed by RACF. System Management Facility (SMF) collects auditing and performance data. z/OS UNIX System Services offers a POSIX-compliant environment that enables open-source and UNIX-like software to run within the platform. File and data management includes VSAM data sets, sequential and partitioned data sets, and file systems such as zFS and HFS.

z/OS supports virtualization and high availability through features like logical partitions (LPARs) and PR/SM, with Parallel Sysplex enabling parallel operation of multiple system images for improved throughput and disaster recovery. The platform provides deep integration with IBM middleware and applications, including CICS, IMS, Db2, MQ, and WebSphere, and supports development in languages such as COBOL, PL/I, Java, and C/C++.

Key strengths of z/OS include reliability, security, sophisticated workload management, and strong data integrity. It continues to evolve with enhancements in security, storage management, and modernization tools, while preserving compatibility with legacy applications and data formats.