The z/OS welcome screen as seen through a terminal emulator. This classic interface is still available but Web access is common. |
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| Company / developer | IBM |
|---|---|
| Programmed in | primarily PL/X, HLASM, and C/C++ |
| OS family | z/OS |
| Working state | Current |
| Source model | Mostly closed source |
| Initial release | March 30, 2001 (V1R1, announced October, 2000) |
| Latest stable release | Release 13 (V1R13) / September 30, 2011 |
| Marketing target | Enterprise / Mainframes |
| Available language(s) | English and most other languages |
| Available programming languages(s) | Java, C/C++, COBOL, PL/I, HLASM, FORTRAN, REXX, and many others |
| Package manager | SMP/E |
| Supported platforms | z/Architecture |
| Kernel type | Monolithic (uniquely hardware-assisted) |
| Default user interface | ISPF, z/OS Management Facility |
| License | Proprietary monthly license charge (MLC); pricing available based on actual use (VWLC, EWLC, AWLC, EAWLC, IWP); reduced pricing options (zELC, zNALC, "Solution Edition") for many applications |
| Official website | IBM: z/OS |
| History of IBM mainframe operating systems |
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On early mainframe computers: On S/360 and successors: |
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z/OS is a 64-bit operating system for mainframe computers, produced by IBM. It derives from and is the successor to OS/390, which in turn followed a string of MVS versions.[NB 1] Like OS/390, z/OS combines a number of formerly separate, related products, some of which are still optional. z/OS offers the attributes of modern operating systems but also retains much of the functionality originating in the 1960s and each subsequent decade that is still found in daily use (backward compatibility is one of z/OS's central design philosophies). z/OS was first introduced in October, 2000.[1]
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z/OS supports stable mainframe systems and standards such as CICS, IMS, DB2, RACF, SNA, WebSphere MQ, record-oriented data access methods, REXX, CLIST, SMP/E, JCL, TSO/E, and ISPF, among others. However, z/OS also supports 64-bit Java, C/C++, and UNIX (Single UNIX Specification) APIs and applications — The Open Group certifies z/OS as a compliant UNIX operating system — with UNIX/Linux-style hierarchical HFS (not to be confused with the Macintosh HFS) and zFS file systems. As a result, z/OS hosts a broad range of commercial and open source software of any vintage.[2] z/OS can communicate directly via TCP/IP, including IPv6, and includes standard HTTP servers (one from Lotus, the other Apache-derived) along with other common services such as FTP, NFS, and CIFS/SMB. Another central design philosophy is support for extremely high quality of service (QoS), even within a single operating system instance, although z/OS has built-in support for Parallel Sysplex clustering.
z/OS has a unique Workload Manager (WLM) and dispatcher which automatically manages numerous concurrently hosted units of work running in separate key-protected address spaces according to dynamically adjustable business goals. This capability inherently supports multi-tenancy within a single operating system image. However, modern IBM mainframes also offer two additional levels of virtualization: LPARs and (optionally) z/VM. These new functions within the hardware, z/OS, and z/VM — and Linux and OpenSolaris 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 z/OS version (at least at present), releases are normally called "Release n," though more formally they are "Version 1 Release n" or "V1.n". The IBM Program Number for all z/OS Version 1 releases is 5694-A01.
From its inception z/OS has supported tri-modal addressing (24-bit, 31-bit, and 64-bit). Up through Version 1.5, z/OS itself could start in either 31-bit ESA/390 or 64-bit z/Architecture mode, so it could function on older hardware albeit without 64-bit application support on those machines. (Only the newer z/Architecture hardware manufactured starting in the year 2000 can run 64-bit code.) IBM support for z/OS 1.5 ended on March 31, 2007. Now z/OS is only supported on z/Architecture mainframes and only runs in 64-bit mode. z/Architecture hardware always starts running in 31-bit mode, but current z/OS releases quickly switch to 64-bit mode and will not run on hardware that does not support 64-bit mode. Application programmers can still use any addressing mode: all applications, regardless of their addressing mode(s), can coexist without modification, and IBM maintains an unwavering commitment to tri-modal backward compatibility. However, increasing numbers of middleware products and applications, such as DB2 Version 8 and above, now require and exploit 64-bit addressing.
IBM markets z/OS as a flagship[3] operating system, suited for continuous, high-volume operation with high security and stability. It is the most popular mainframe operating system.
z/OS is available under standard license pricing as well as via System z New Application License Charges (zNALC) and "System z Solution Edition," two lower priced offerings aimed at supporting newer applications ("new workloads").[4] U.S. standard commercial z/OS pricing starts at about $125 per month, including support, for the smallest zNALC installation running the base z/OS product plus a typical set of optional z/OS features.[5]
z/OS introduced Variable Workload License Charges (VWLC) and Entry Workload License Charges (EWLC) which are sub-capacity billing options. VWLC and EWLC customers only pay for peak monthly z/OS usage, not for full machine capacity as with the previous OS/390 operating system. VWLC and EWLC are also available for most IBM software products running on z/OS, and their peaks are separately calculated but can never exceed the z/OS peak. To be eligible for sub-capacity licensing, a z/OS customer must be running in 64-bit mode (which requires z/Architecture hardware), must have completely eliminated OS/390 from the system, and must e-mail IBM monthly sub-capacity reports. Sub-capacity billing substantially reduces software charges for most IBM mainframe customers.[6] Advanced Workload License Charges (AWLC) is the successor to VWLC on mainframe models starting with the zEnterprise 196, and EAWLC is an option on zEnterprise 114 models. AWLC and EAWLC offer further sub-capacity discounts.
The primary development center for z/OS is located at IBM's facilities in Poughkeepsie, New York, but numerous development centers in several other countries contribute substantially to z/OS. As one example, IBM's center in Perth, Australia is mainly responsible for z/OS's HLASM and DFSMS components.
Within each address space, z/OS typically only permits the placement of data above the 2GB "bar," not code. z/OS enforces this distinction primarily for performance reasons. There are no architectural impediments to allowing more than 2GB of application code per address space. IBM has started to allow Java code running on z/OS to execute above the 2GB bar, again for performance reasons.
Memory is obtained as "Large Memory Objects" in multiples of 1MB (with the expectation that applications and middleware will manage memory allocation within these large pieces). There are three types of large memory objects:
In 1973, IBM published a unique "Statement of Integrity" for MVS which has been updated for z/OS and is still in force.[7]
IBM reliably introduces new releases of z/OS each year in September. Release 13 became generally available in September, 2011.
IBM supports z/OS release coexistence and fallback on an "N+2" basis. For example, IBM customers running Release 9 can upgrade directly to Release 11 (or Release 10), and both releases can operate concurrently within the same Sysplex (cluster) and without conflict using the same datasets, configurations, security profiles, etc. (coexistence), as long as so-called "toleration maintenance" is installed in the older release. If there is a problem with Release 11, the customer can return to Release 9 without experiencing dataset, configuration, or security profile compatibility problems (fallback) until ready to try moving forward again. z/OS customers using Parallel Sysplex (clustering) can operate N+2 releases (e.g. Release 9 and Release 11, or Release 9 and Release 10) in mixed release configurations, in production, as long as required to complete release upgrades. Supported software release mixing within server clusters is extremely rare among operating systems and middleware generally but routine with z/OS. Supported release mixing within a cluster is one of the key reasons why many z/OS installations deliver continuous application service to their users, avoiding both planned and unplanned service outages.
IBM's standard support period for each z/OS release is three years, so, combined with z/OS's uniquely rigorous coexistence and fallback commitments, IBM's customers routinely maintain z/OS release currency (or near-currency) with few or zero service interruptions and without paying extended support fees. Most z/OS customers take full advantage of the N+2 support and skip every other release. Thus most z/OS customers are either "odd" or "even."[8]
IBM releases individual patches (a.k.a. PTFs) for z/OS as needed, when needed. IBM labels critical PTFs as "HIPER" (High Impact PERvasive). IBM also "rolls up" multiple patches into a Recommended Service Update (RSU). RSUs are released periodically (in the range of every one to three months) and undergo additional testing. Although z/OS customers vary in their maintenance practices, IBM encourages every z/OS customer to adopt a reasonable preventive maintenance strategy, to avoid known problems before they might occur. z/OS customers that install periodic RSUs sometimes include the RSU information in their release description — for example, "We are running z/OS Version 1 Release 10 with RSU0909...."[9]