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===Virtual machine===
A [[virtual machine]] is an operating system that runs as an application on top of another operating system.{{sfn|Anderson|Dahlin|2014|p=11}} The virtual machine is unaware that it is an application and operates as if it had its own hardware.{{sfn|Anderson|Dahlin|2014|p=11}}{{sfn|Silberschatz et al.|2018|pp=701}} Virtual machines can be paused, saved, and resumed, making them useful for operating systems research, development,{{sfn|Silberschatz et al.|2018|pp=705}} and debugging.{{sfn|Anderson|Dahlin|2014|p=12}} They also enhance portability by enabling applications to be run on a computer even if they are not compatible with the base operating system.{{sfn|Anderson|Dahlin|2014|p=11}}
===Library===
A library operating system (libOS) is one in which the services that a typical operating system provides, such as networking, are provided in the form of [[Library (computing)|libraries]] and composed with the application and configuration code to construct a [[unikernel]]<ref name="Unikernels">{{cite magazine
|last1=Madhavapeddy |first1=Anil
|last2=Scott |first2=David J
|date=November 2013
|title=Unikernels: Rise of the Virtual Library Operating System: What if all the software layers in a virtual appliance were compiled within the same safe, high-level language framework?
|magazine=Queue
|volume=11 |issue=11 |pages=30–44
|___location=New York, NY, USA |publisher=ACM
|issn=1542-7730
|url=https://doi.org/10.1145/2557963.2566628
|doi=10.1145/2557963.2566628
|access-date=2024-08-07}}</ref>: a specialized, [[Single address space operating system|single address space]], machine image that can be deployed to cloud or embedded environments.
The operating system code and application code are not executed in separated [[Protection ring|protection domains]] and OS services are accessed via simple library calls, without the usual overhead of [[Context switch|context switches]]<ref name="rise-of-libOS">{{cite web
|url=https://www.sigarch.org/leave-your-os-at-home-the-rise-of-library-operating-systems/
|access-date=2024-08-07
|title=Leave your OS at home: the rise of library operating systems
|date=<!--Sep 14, -->2017-09-14
|publisher=[[ACM SIGARCH]]
|archive-date=2024-03-01
|archive-url=https://web.archive.org/web/20240301072916/https://www.sigarch.org/leave-your-os-at-home-the-rise-of-library-operating-systems/
|url-status=live}}</ref>, in a way similarly to embedded and real-time OSes.
==History==
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