Z/OS
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 The z/OS welcome screen as seen through a terminal emulator. This interface is still available and a Web interface is commonly used. |
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| Website: | IBM: z/OS operating system |
| Company/ developer: |
IBM |
| OS family: | z/OS |
| Source model: | Mostly closed source |
| Kernel type: | Monolithic kernel |
| License: | Proprietary monthly license charge (MLC) |
| Working state: | Current |
- The correct title of this article is z/OS. The initial letter is shown capitalized due to technical restrictions.
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, DB2, 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 communicates with TCP/IP and the Web. A complementary IBM product, z/VM, assists the management of Linux guests on the same physical mainframe. These new functions within z/OS and z/VM, and the "multiple Linux image" support have encouraged new applications to appear on the z/OS operating system.
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).
