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The primary operating system used in IBM mainframes. OS/390 was originally the MVS/ESA operating system renamed and repackaged in 1996 with an extensive set of utilities. Although the name MVS was still used to refer to the base control program of OS/390, enhancements in usability and workload balancing made OS/390 stand apart from its MVS heritage. OS/390 is upward compatible from MVS/ESA 5.2.2, but downward compatibility is not ensured. See System/390.


The IBM mainframe line that superseded System/370. Introduced in 1990, the 32-bit System/390 was an evolution of the Enterprise System Architecture introduced with the System/370. The bipolar-based ES/9000 models, which came both water cooled and air cooled, incorporated fiber-optic channels, clustering (see Parallel Sysplex) and other advancements.

From Bipolar to CMOS
In 1994, IBM introduced the next generation System/390s known as Parallel Enterprise Servers and abbreviated the name to S/390. These machines contained single-chip CMOS CPUs that used less power and dissipated less heat than the bipolar-based ES/9000s. In 1995, IBM introduced models with up to 10 CPUs, and as many as 32 10-way systems could be combined to make a multiprocessing system with 320 CPUs. As requirements increased, customers migrated from ES/9000s to clusters of the CMOS-based S/390s. See System/360, System/370 and IBM mainframes.

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