Infobox OS
name = z/OS

caption = The z/OS welcome screen as seen through a terminal emulator. This interface is still available but Web access is common.
website = [http://www-1.ibm.com/servers/eserver/zseries/zos/ IBM: z/OS operating system]
developer = IBM
family = z/OS
source_model = Mostly closed source
latest_release_version = Release 10 (V1R10)
latest_release_date = September 26, 2008
latest_test_version =
latest_test_date =
marketing_target = Enterprise / Mainframes
language =
kernel_type = Monolithic kernel (uniquely hardware-assisted)
ui =
license = Proprietary monthly license charge (MLC); pricing available based on actual use (VWLC)
working_state = Current
supported_platforms = z/Architecture
updatemodel =
package_manager =

z/OS is a 64-bit operating system for mainframe computers, created by IBM. It is the successor to OS/390, which in turn combined MVS and a number of related products.

z/OS offers many of the attributes of other modern operating systems, but also retains much of the functionality originating in the 1970s and 1960s, that is still often in daily use. This includes CICS, IMS, DB2, RACF and SNA.

z/OS 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 virtual 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.

Because there is only one version, releases are normally called "Release n", though more formally they are "Version 1 Release n" or "V1.n".

Starting April 1, 2007, z/OS is only supported on 64-bit (z/Architecture) mainframes. z/OS Release 5 was the last release 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 [IBM: Why System z for Business Integration? [http://www-03.ibm.com/systems/z/integration/advantages.html] ] operating system, suited for continuous, high-volume operation with high security and stability.

A lower-cost variant 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. But z/OS.e ended with Release 8 and was withdrawn from marketing in October, 2007. [IBM System z New Application License Charges [http://www.ibm.com/common/ssi/fcgi-bin/ssialias?infotype=an&subtype=ca&appname=Demonstration&htmlfid=/897/ENUS207-006] ] It is replaced by the System z New Application License Charges (zNALC), which offers the full z/OS cheaper if used to serve new applications ("new workloads").

IBM reliably makes new releases of z/OS annually. Release 10 became generally available on September 26, 2008 (first announced on August 5, 2008).

See also

* Intelligent Resource Director
* Workload Manager
* Linux on zSeries for a mainframe version of a popular operating system
* OS/360 for an ancestor from 1960s
* z/VSE for another mainframe operating system
* Parallel Sysplex
* z/TPF
* Resource Measurement Facility


External links

* [http://www.ibm.com/servers/eserver/zseries/zos/ IBM: z/OS]
* [http://www.ibm.com/software/shopzseries IBM: Shop zSeries (ShopZ)]
* [http://www-03.ibm.com/servers/eserver/zseries/zos/bkserv/ IBM: z/OS Internet Library]


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