Editing Z/OS (section)

==Major characteristics==
<!-- No, z/OS does not implement, e.g., CICS, COBOL DB2, IMS, MQ, PL/I. -->
z/OS supports<ref group=NB>Some, e.g., TSO/E, are bundled with z/OS, others, e.g., CICS, are separately priced.</ref> stable mainframe facilities such as [[CICS]], [[Common Business-Oriented Language|COBOL]], [[IBM Information Management System|IMS]], [[PL/I]], [[IBM Db2]], [[IBM Resource Access Control Facility|RACF]], [[Systems Network Architecture|SNA]], [[IBM MQ]], record-oriented data [[access methods]], [[REXX]], [[CLIST]], [[SMP/E]], [[Job Control Language|JCL]], [[Time Sharing Option|TSO/E]], and [[ISPF]], among others.

z/OS also ships with a 64-bit [[Java (programming language)|Java]] runtime, [[C (programming language)|C]]/[[C++]] compiler based on the [[LLVM]] open-source [[Clang]] infrastructure,<ref>{{Cite web |title=IBM z/OS XL C/C++ {{!}} IBM |url=https://www.ibm.com/products/xl-cpp-compiler-zos |access-date=2023-12-04 |website=IBM |language=en-us}}</ref> and UNIX ([[Single UNIX Specification]]) APIs and applications through [[UNIX System Services]]&nbsp;– [[The Open Group]] certifies z/OS as a compliant UNIX operating system&nbsp;– with UNIX/Linux-style hierarchical [[Hierarchical File System (IBM MVS)|HFS]]<ref group="NB">Not to be confused with the [[Classic Mac OS|Macintosh]] [[Hierarchical File System (Apple)|HFS]]</ref><ref group="NB">IBM has dropped HFS starting with z/OS Version 2 Release 5.</ref> and [[zFS (z/OS file system)|zFS]]<ref group="NB">Not to be confused with [[ZFS]]</ref> [[file systems]]. These compatibilities make z/OS capable of running a range of commercial and [[open-source software]].<ref>{{Cite web |url=http://www.oss4zos.org/ |title=Main Page - Oss4zos |access-date=2009-12-21 |archive-date=2008-07-04 |archive-url=https://web.archive.org/web/20080704064024/http://www.oss4zos.org/ |url-status=dead }}</ref> z/OS can communicate directly via [[Internet protocol suite|TCP/IP]], including [[IPv6]],<ref>{{cite web |url=https://www.ibm.com/products/zos-communications-server |title=IBM z/OS Communications Server |website=IBM|date=24 April 2024 }}</ref> and includes standard [[HTTP server]]s (one from [[Lotus software|Lotus]], the other [[Apache Foundation|Apache]]-derived) along with other common services such as [[SSH]], [[FTP]], [[Network File System (protocol)|NFS]], and [[CIFS]]/SMB. z/OS is designed for high [[quality of service]] (QoS), even within a single operating system instance, and has built-in [[IBM Parallel Sysplex|Parallel Sysplex]] clustering capability.

z/OS has a [[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 goals. This capability inherently supports multi-tenancy within a single operating system image. However, modern [[IBM mainframes]] also offer two additional levels of virtualization: [[LPAR]]s and (optionally) [[z/VM]].

From its inception z/OS has had tri-modal addressing ([[24-bit computing|24-bit]], [[31-bit computing|31-bit]], and [[64-bit computing|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 the ability to run 64-bit applications 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, and since then, z/OS is supported only on z/Architecture mainframes and runs exclusively in 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 a 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 its flagship operating system,<ref>{{cite web |url=http://www-03.ibm.com/systems/z/integration/advantages.html |website=IBM |title=Why System z for Business Integration? |archive-url=https://web.archive.org/web/20060513124158/http://www-03.ibm.com/systems/z/integration/advantages.html |archive-date=2006-05-13 |url-status=dead}}</ref>  suited for continuous, high-volume operation with high security and stability.

z/OS is available under standard license pricing and via IBM Z New Application License Charges (zNALC) and "IBM Z Solution Edition", two lower-priced offerings aimed at supporting newer applications ("new workloads").<ref>{{cite web |url=https://www.ibm.com/docs/en/zos/3.1.0?topic=pricing-more-about-znalc |title=More about zNALC pricing |website=IBM}}</ref> U.S. standard commercial z/OS pricing starts at about [[US$]]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.

z/OS introduced Variable Workload License Charges (VWLC)<ref>{{cite web|title=Workload License Charges (WLC)|website=[[IBM]] |url=https://www.ibm.com/docs/en/zos/2.5.0?topic=metrics-workload-license-charges-wlc}}</ref> and Entry Workload License Charges (EWLC)<ref>{{cite web|title=Entry Workload License Charges (EWLC)|website=[[IBM]] |url=https://www.ibm.com/docs/en/zos/2.5.0?topic=metrics-entry-workload-license-charges-ewlc}}</ref> 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 computing|64-bit]] mode (which requires [[z/Architecture]] hardware), must have 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.<ref>{{cite web |url=https://www.ibm.com/docs/en/zos/3.1.0?topic=level-zos-planning-sub-capacity-pricing |title=Abstract for Planning for Sub-Capacity Pricing |website=IBM}}</ref> 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.