Editing VM (operating system) (section)

== History ==
[[File:VM370 Rel 6 OSVS1 IPL.png|thumb|OS/VS1 starting under VM/370 ]]
[[File:VM370 DASD Dump Restore.png|thumb|Using DASD Dump/Restore (DDR) to back up a VM/370 system]]

The early history of VM is described in the articles [[CP/CMS]] and [[History of CP/CMS]]. VM/370 is a reimplementation of CP/CMS, and was made available in 1972 as part of IBM's System/370 Advanced Function announcement (which added [[virtual memory]] hardware and operating systems to the [[System/370]] series). Early releases of VM through VM/370 Release 6 continued in [[Open-source software|open source]] through 1981, and today are considered to be in the [[public domain]]. This policy ended in 1977 with the chargeable VM/SE and VM/BSE upgrades and in 1980 with VM/System Product (VM/SP). However, IBM continued providing updates in source form for existing code for many years, although the upgrades to all but the free base required a license. As with CP-67, privileged instructions in a virtual machine cause a program interrupt, and CP simulated the behavior of the privileged instruction.
VM remained an important platform ''within'' IBM, used for operating system development and time-sharing use; but for customers it remained IBM's "other operating system". The OS and DOS families remained IBM's strategic products, and customers were not encouraged to run VM. Those that did formed close working relationships, continuing the community-support model of early CP/CMS users. In the meantime, the system struggled with political infighting within IBM over what resources should be available to the project, as compared with other IBM efforts. A basic problem with the system was seen at IBM's field sales level: VM/CMS demonstrably reduced the amount of hardware needed to support a given number of time-sharing users. IBM was, after all, in the business of selling computer systems.

Melinda Varian provides this fascinating quote, illustrating VM's unexpected success:<ref>Varian, ''op. cit., p.'' 30 – extent of VM use; more VM licenses than MVS licenses</ref>

<blockquote>
The marketing forecasts for VM/370 predicted that no more than one 168 would ever run VM during the entire life of the product. In fact, the first 168 delivered to a customer ran only CP and CMS. Ten years later, ten percent of the large processors being shipped from Poughkeepsie would be destined to run VM, as would a very substantial portion of the mid-range machines that were built in Endicott. Before fifteen years had passed, there would be more VM licenses than MVS licenses.
</blockquote>

A [[IBM PC DOS|PC DOS]] version that runs CMS on the [[XT/370]] (and later on the AT/370) is called VM/PC. VM/PC 1.1 was based on VM/SP release 3. When IBM introduced the P/370 and P/390 processor cards, a PC could now run full VM systems, including VM/370, VM/SP, VM/XA, and VM/ESA (these cards were fully compatible with S/370 and S/390 mainframes, and could run any S/370 operating system from the 31-bit era, e.g., MVS/ESA, VSE/ESA).

In addition to the base VM/SP releases, IBM also introduced VM/SP HPO (High Performance Option). This add-on (which is installed over the base VM/SP release) improved several key system facilities, including allowing the usage of more than 16&nbsp;MB of storage (RAM) on supported models (such as the IBM 4381). With VM/SP HPO installed, the new limit was 64&nbsp;MB; however, a single user (or virtual machine) could not use more than 16&nbsp;MB. The functions of the spool filesystem were also improved, allowing 9900 spool files to be created per user, rather than 9900 for the whole system. The architecture of the spool filesystem was also enhanced, each spool file now had a unique user ID associated with it, and reader file control blocks were now held in virtual storage. The system could also be configured to deny certain users access to the vector facility (by means of user directory entries).<ref name=":0">{{cite manual|url=http://bitsavers.org/pdf/ibm/370/VM_SP/HPO/SC23-0189-3_VM_SP_HPO_Release_5_Guide_Jul1987.pdf|title=Virtual Machine/System Product High Performance Option Release 5 Guide|id=SC23-0189-3|date=July 1987|publisher=[[IBM]]|access-date=August 19, 2021|archive-date=June 17, 2022|archive-url=https://web.archive.org/web/20220617123626/http://bitsavers.org/pdf/ibm/370/VM_SP/HPO/SC23-0189-3_VM_SP_HPO_Release_5_Guide_Jul1987.pdf|url-status=live}}</ref>

Releases of VM since VM/SP Release 1 supported multiprocessor systems. System/370 versions of VM (such as VM/SP and VM/SP HPO) supported a maximum of two processors, with the system operating in either UP (uniprocessor) mode, MP (multiprocessor) mode, or AP (attached processor) mode.<ref>{{cite manual|url=http://bitsavers.org/pdf/ibm/370/VM_SP/Release_5_Dec86/SC24-5237-3_VM_SP_Release_5_Installation_Guide_Dec1986.pdf|title=Virtual Machine/System Product Installation Guide Release 5|id=SC24-5237-3|date=December 1986|publisher=[[IBM]]|access-date=August 19, 2021|archive-date=June 17, 2022|archive-url=https://web.archive.org/web/20220617123631/http://bitsavers.org/pdf/ibm/370/VM_SP/Release_5_Dec86/SC24-5237-3_VM_SP_Release_5_Installation_Guide_Dec1986.pdf|url-status=live}}</ref> AP mode is the same as MP mode, except the second processor lacks I/O capability. System/370-XA releases of VM (such as VM/XA) supported more. System/390 releases (such as VM/ESA) almost removed the limit entirely, and some modern z/VM systems can have as many as 80 processors.<ref>{{Cite web|url = https://www.ibm.com/support/pages/apar/VM66265|title = Vm66265: Z/Vm Support for 80 Logical Processors|website = [[IBM]]|date = August 27, 2020|access-date = August 19, 2021|archive-date = August 19, 2021|archive-url = https://web.archive.org/web/20210819134033/https://www.ibm.com/support/pages/apar/VM66265|url-status = live}}</ref> The per-VM limit for defined processors is 64.

When IBM introduced the System/370 Extended Architecture on the [[IBM 308X|3081]], customers were faced with the need to run a production MVS/370 system while testing MVS/XA on the same machine. IBM's solution was VM/XA Migration Aid, which used the new Start Interpretive Execution (SIE) instruction to run the virtual machine. SIE automatically handled some privileged instructions and returned to CP for cases that it couldn't handle. The [[PR/SM|Processor Resource/System Manager]] (PR/SM) of the later [[IBM 3090|3090]] also used SIE. There were several VM/XA products before it was eventually supplanted by VM/ESA and z/VM.

In addition to [[RSCS]] networking, IBM also provided users with [[VTAM]] networking. ACF/VTAM for VM was fully compatible with ACF/VTAM on MVS and VSE.<ref>{{cite manual|url=http://bitsavers.org/pdf/ibm/sna/vtam/LY43-0047-1_VTAM_V3_R3_Reference_Summary_Sep1990.pdf|title=VTAM Reference Summary Version 3 Release 3 for MVS, VM, and VSE/ESA|id=LY43--0047-1|date=September 1990|publisher=[[IBM]]|access-date=August 19, 2021|archive-date=August 19, 2021|archive-url=https://web.archive.org/web/20210819134043/http://bitsavers.org/pdf/ibm/sna/vtam/LY43-0047-1_VTAM_V3_R3_Reference_Summary_Sep1990.pdf|url-status=live}}</ref> Like RSCS, VTAM on VM ran under the specialized GCS operating system. However, VM also supported TCP/IP networking. In the late 1980s, IBM produced a TCP/IP stack for VM/SP and VM/XA.<ref>{{cite manual|url=http://bitsavers.org/pdf/ibm/370/9370/GG24-3227-0_IBM_9370_LAN_Vol_2_IEEE_802.3_Support_Apr88.pdf|title=IBM 9370 LAN Volume 2 - IEE 802.3 Support|id=GG24-3227-0|date=April 1988|publisher=[[IBM]]|access-date=August 19, 2021|archive-date=August 19, 2021|archive-url=https://web.archive.org/web/20210819134044/http://bitsavers.org/pdf/ibm/370/9370/GG24-3227-0_IBM_9370_LAN_Vol_2_IEEE_802.3_Support_Apr88.pdf|url-status=live}}</ref> The stack supported IPv4 networks, and a variety of network interface systems (such as inter-mainframe channel-to-channel links, or a specialized IBM RT PC that would relay traffic out to a [[Token Ring]] or [[Ethernet]] network). The stack provided support for [[Telnet]] connections, from either simple line-mode terminal emulators or VT100-compatible emulators, or proper IBM 3270 terminal emulators. The stack also provided an FTP server. IBM also produced an optional NFS server for VM; early versions were rather primitive, but modern versions are much more advanced.<ref>{{cite web|url=https://www.vm.ibm.com/nfs/|title=VM TCP/IP NFS Server Support|website=[[IBM]]|date=May 29, 2001 |access-date=August 19, 2021|archive-date=April 26, 2021|archive-url=https://web.archive.org/web/20210426093936/https://www.vm.ibm.com/nfs/|url-status=live}}</ref>

There was also a fourth networking option, known as VM/Pass-Through Facility (or more commonly called, PVM). PVM, like VTAM, allowed for connections to remote VM/CMS systems, as well as other IBM systems.<ref>{{cite manual|url=http://publibfp.dhe.ibm.com/epubs/pdf/c2455571.pdf|title=VM/Pass-Through Facility Administration and Operation Version 2|id=SC24-5557-01|date=June 1993|publisher=[[IBM]]|access-date=August 19, 2021|archive-date=August 19, 2021|archive-url=https://web.archive.org/web/20210819134035/http://publibfp.dhe.ibm.com/epubs/pdf/c2455571.pdf|url-status=dead}}</ref> If two VM/CMS nodes were linked together over a channel-to-channel link or bisync link (possibly using a dialup modem or leased line), a user could remotely connect to either system by entering "DIAL PVM" on the VM login screen, then entering the system node name (or choosing it from a list of available nodes). Alternatively, a user running CMS could use the PASSTHRU program that was installed alongside PVM, allowing for quick access to remote systems without having to log out of the user's session. PVM also supported accessing non-VM systems, by utilizing a 3x74 emulation technique. Later releases of PVM also featured a component that could accept connections from a [[Systems Network Architecture|SNA]] network.

VM was also the cornerstone operating system of [[BITNET]], as the RSCS system available for VM provided a simple network that was easy to implement, and somewhat reliable. VM sites were interlinked by means of an RSCS VM on each VM system communicating with one another, and users could send and receive messages, files, and batch jobs through RSCS. The "NOTE" command used [[XEDIT]] to display a dialog to create an email, from which the user could send it. If the user specified an address in the form of <code>user at node</code>, the email file would be delivered to RSCS, which would then deliver it to the target user on the target system. If the site has TCP/IP installed, RSCS could work with the SMTP service machine to deliver notes (emails) to remote systems, as well as receive them. If the user specified <code>user at some.host.name</code>, the NOTE program would deliver the email to the SMTP service machine, which would then route it out to the destination site on the Internet.

VM's role changed within IBM when hardware evolution led to significant changes in processor architecture. [[Backward compatibility]] remained a cornerstone of the [[IBM mainframe]] family, which still uses the basic instruction set introduced with the original [[System/360]]; but the need for efficient use of the 64-bit [[zSeries]] made the VM approach much more attractive. VM was also utilized in data centers converting from DOS/VSE to MVS and is useful when running mainframe [[IBM AIX|AIX]] and [[Linux]], platforms that were to become increasingly important. The current [[z/VM]] platform has finally achieved the recognition within IBM that VM users long felt it deserved. Some z/VM sites run thousands of simultaneous virtual machine users on a single system. z/VM was first released in October 2000<ref>{{cite web |url=http://www.vm.ibm.com/overview/ |title=IBM: About the z/VM Operating System |website=IBM z/VM virtualization technology |publisher=Vm.ibm.com |access-date=July 2, 2015 |archive-date=July 3, 2015 |archive-url=https://web.archive.org/web/20150703135421/http://www.vm.ibm.com/overview/ |url-status=live }}</ref> and remains in active use and development.

IBM and third parties have offered many applications and tools that run under VM. Examples include [[RAMIS]], [[FOCUS]], [[SPSS]], [[Nomad software|NOMAD]], [[IBM DB2|DB2]], [[REXX]], [[RACF]], and [[OfficeVision]]. Current VM offerings run the gamut of mainframe applications, including [[HTTP]] servers, database managers, analysis tools, engineering packages, and financial systems.