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 is still used to refer to the base control program of OS/390, enhancements in usability and workload balancing have 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.