# Group Control System

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{{Short description|IBM operating system}}
{{More citations needed|date=November 2024}}
{{Infobox OS
| name                   = Group Control System (GCS)<!-- Name of program or distribution -->
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| developer              = [IBM](/source/IBM)<!-- Name of main developer or sponsor-->
| family                 = <!-- "Unix-like" or "Microsoft Windows" -->
| working state          = <!-- "Current", "Discontinued" (operating systems), or "No longer supported" (releases) -->
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| marketing target       = [IBM mainframe computers](/source/IBM_mainframes)
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{{History of IBM mainframe operating systems}}
'''Group Control System''' ('''GCS''') is an [operating system](/source/operating_system) made by [IBM](/source/IBM), meant to run as a guest of [VM](/source/VM_(operating_system)).<ref>{{Cite web |title=What GCS Is | website=[IBM](/source/IBM) |url=https://www.ibm.com/docs/en/zvm/7.2?topic=overview-what-gcs-is}}</ref> GCS is an integral component of the discontinued [VM/SP](/source/VM%2FSP) (since VM/SP 4), VM/XA SP, [VM/ESA](/source/VM%2FESA) and current [z/VM](/source/z%2FVM) IBM System product offerings.

==Overview==
GCS's purpose is to provide an environment to run some specific [OS/MVS](/source/MVS)-based applications and networking [middleware](/source/middleware) under VM. To this end, GCS provides a limited simulation of the OS/MVS APIs. While [CMS](/source/Conversational_Monitor_System), the usual VM guest, already has OS simulation, it is not extensive enough to run some applications such as [VTAM](/source/VTAM).

Specifically, GCS provides [OS multitasking](/source/Computer_multitasking) support. In order to be able to spread the load over several virtual machines, GCS also implements a notion of group where each group member virtual machine can interact with one another.

To implement this, GCS uses several techniques:
* A writable shared segment
* [IUCV](/source/IUCV) communications between virtual machines
* A recovery virtual machine, designed to clean up locks in the shared segments when virtual machines unexpectedly leave the group

GCS is not designed to be a multi-purpose user operating system such as CMS. For example, it does not allow for file editing.

Examples of programs designed to run under GCS are:
* VM/VTAM : The VM implementation of the VTAM stack
* [RSCS](/source/RSCS) : The Remote Spooling Communication Subsystem, which, when associated with an instance of VM/VTAM, can use SNA resources to communicate with other SNA/NJE hosts
* [NETVIEW](/source/NETVIEW) : A networking monitoring service

== References ==
{{reflist}}

== External links ==
* [https://archive.today/20121210084740/http://publibz.boulder.ibm.com/cgi-bin/bookmgr_OS390/BOOKS/HCSF5B20/CCONTENTS?SHELF=EZ2ZVM01&DN=SC24-6098-02&DT=20070419173550 z/VM 5.3 Group Control System]

{{IBM operating systems}}

{{DEFAULTSORT:Group Control System(Vm)}}
Category:IBM mainframe operating systems
Category:VM (operating system)

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{{mainframe-compu-stub}}

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Adapted from the Wikipedia article [Group Control System](https://en.wikipedia.org/wiki/Group_Control_System) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Group_Control_System?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.