Z/OS

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z/OS is a 64-bit server operating system from IBM. It is the 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).

While assiduously retaining many features, functions, and interfaces harking back to the 1970s and (in some cases) 1960s, z/OS also offers many of the same attributes and elements of so-called open systems. So while CICS, IMS, RACF, SNA, and the like are still available and in daily use, they are less the focal point for new applications than in years past. z/OS now also runs Java, supports UNIX (Single UNIX Specification) APIs and applications, and easily communicates with TCP/IP and the Web. A complementary IBM product, z/VM, improves Linux support on the same system. This combination of cutting edge functionality within z/OS and concurrent Linux support has thoroughly modernized the mainframe.

z/OS is also capable of running in 31-bit (ESA/390) mode on pre-z/Architecture mainframes. However, starting with z/OS V1R6, introduced on September 24, 2004, z/OS requires a 64-bit zSeries or System z9 server. (IBM will support z/OS V1R5 until at least March 31, 2007.)

z/OS is IBM's flagship operating system, well-regarded for its ability to support mission-critical, continuous, high volume business and government operations with the utmost in security and reliability.

A lower cost version of z/OS, z/OS.e, is code identical but runs with a startup setting that prevents execution of "classic" workloads such as most COBOL and PL/I. z/OS.e is available for IBM z800 and z890 mainframes, and customers choose whether to run with this setting or not (and the resulting license charges).

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