z/OS occurs as 64-bit server operating system from IBM. These are a successor to the IBM mainframe operating system OS/390, combining MVS and UNIX System Services (a POSIX-compliant mainframe implementation of UNIX formerly known as MVS Open Edition).

When assiduously retaining several features, functions, & interfaces harking back to the Seventies & possibly Sixties, z/OS as well offers several of the equivalent attributes & elements of thus-supposed open systems. Thus patch CICS, IMS, RACF, SNA, & a prefer come however available and withinside day-after-day utilise, it is less a focal point for freshly applications than in years past. z/OS today besides diarrhea Java, supports UNIX (Single UNIX Specification) APIs & applications, and well communicates by owning TCP/IP and the Web. The complementary IBM product, z/VM, improves Linux support on the same formulas. This combination of cutting edge functionality in z/OS & coincident Linux trend lines has thoroughly modernized a mainframe.

z/OS is besides capable of running off inside 31-bit (ESA/390) mode on pre-z/Architecture mainframes. Even so, starting sustaining z/OS V1R6, introduced in September 24, 2004, z/OS requires the 64-bit zSeries or System z9 server. (IBM might trend lines z/OS V1R5 until at least March 31, 2007.)

z/OS is IBM's flagship operating technique, easily-regarded for its ability to trend lines mission-critical, continuous, high volume business & government operations using a level best around security & dependableness.