OS390

OS/390 is IBM's mainframe operating system developed for the IBM Enterprise System/390 (ES/390) and System/390 hardware family. Introduced in 1995, OS/390 is the MVS-based platform used to run traditional MVS workloads on ES/390 hardware. It also included Unix System Services (USS), a POSIX-compatible UNIX environment that enables UNIX applications to run alongside MVS software.

Core components of OS/390 include the MVS base with subsystems such as the Job Entry Subsystem (JES2 or JES3) for batch processing, TSO/E for interactive use, and RACF for security. It supports a range of IBM software, including CICS, IMS, and DB2. Optional features add networking (TCP/IP), data management tools, and extended UNIX capabilities via USS.

OS/390 ran on ES/390 and System/390 platforms and leveraged IBM hardware virtualization features such as processor resource/system management and logical partitions to support large, multi-workload environments. It provided system management and accounting facilities, including SMF (System Management Facility) for data collection and performance monitoring, and could integrate with other mainframe subsystems and middleware.

In the early 2000s, IBM introduced z/OS as the successor to OS/390, combining elements of MVS, VM, and VSE with new capabilities for modern workloads. The OS/390 branding was gradually phased out as migration to z/OS progressed, though its legacy and compatibility features influenced later mainframe operating system development.