z/OS is a 64-bit mainframe operating system from IBM. It is the successor to the operating system OS/390, which in turn combined 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 1960s, z/OS also offers many of the attributes and elements of other modern operating systems. So while CICS, IMS, DB2, RACF, SNA, and similar features are still available and in daily use, they by no means define the entirety of z/OS.

z/OS now also runs 64-bit Java, supports UNIX (Single UNIX Specification) APIs and applications, and communicates directly with TCP/IP. A complementary IBM operating system, z/VM, provides the management of multiple Linux systems (guests) on the same physical mainframe. These new functions within z/OS and z/VM, and the Linux support, have encouraged development of new applications for mainframes. Many of them utilize the WebSphere Application Server for z/OS middleware.

Starting April 1, 2007, z/OS is only supported on 64-bit (z/Architecture) mainframes. z/OS V1R5 was the last version to support ESA/390, a previous hardware architecture having 31-bit addressing. Old applications are still supported in the same binary form, whether they use 31-bit or even 24-bit addressing.

IBM markets z/OS as a 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 the COBOL and PL/I compilers. z/OS.e was available for IBM z800, z890, and z9 BC mainframes, and customers could choose whether to run with this setting or not (and with the resulting license charges). However, z/OS.e V1.8 was the last release of z/OS.e and was withdrawn from marketing in October, 2007. IBM has replaced z/OS.e with System z New Application License Charges (zNALC). zNALC offers an alternative, lower price for the full z/OS product if employed to serve new applications ("new workloads")

Source: Wikipedia