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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, or OpenMVS). While retaining much of the functionality originating in the 1970s and (in some cases) 1960s, z/OS also offers many of the same attributes and elements of currently-available open systems. So while CICS, IMS, RACF, SNA, and similar features are still available and in daily use, they are less prominent 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 support of current standard of functionality within z/OS and concurrent Linux support have allowed the operating system to garner continued use. z/OS supports pre-z/Architecture mainframes using a 31-bit (ESA/390) mode, and earlier hardware using a 24-bit addressing model. 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, suited for continuous, high volume operation with high security and stability. A lower-cost version of z/OS, z/OS.e, is identical in code, 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, z890, and z9 BC mainframes, and customers choose whether to run with this setting or not (and with the resulting license charges).
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