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((Short description|Type of operating system kernel))
[[File:OS-structure2.svg|right|thumb|580x580px|Structure of monolithic kernel, [[microkernel]] and hybrid kernel-based operating systems]]
A '''hybrid kernel''' is an [[operating system]] [[Kernel (operating system)|kernel]] architecture that attempts to combine aspects and benefits of [[microkernel]] and [[monolithic kernel]] architectures used in operating systems.<ref>((Cite web|title=Hybrid Kernel - OSDev Wiki|url=https://wiki.osdev.org/Hybrid_Kernel|access-date=2021-01-11|website=wiki.osdev.org))</ref><ref>((Cite web|title=What is Hybrid Kernel? - Definition from Techopedia|url=http://www.techopedia.com/definition/27004/hybrid-kernel|access-date=2021-01-11|website=Techopedia.com|date=22 August 2011 |language=en))</ref>((Unreliable source?|reason=OSDev Wiki is user-contributed and Techopedia is not a reliable source|date=August 2022))

==Overview==
The traditional kernel categories are [[monolithic kernel]]s and [[microkernel]]s (with [[nanokernel]]s and [[exokernel]]s seen as more extreme versions of microkernels). The "hybrid" category is controversial, due to the similarity of hybrid kernels and ordinary monolithic kernels; the term has been dismissed by [[Linus Torvalds]] as simple marketing.<ref>((cite web | title=Linus Torvalds | url=http://www.realworldtech.com/forum/?threadid=65915&curpostid=65936 | quote=As to the whole "hybrid kernel" thing - it's just marketing. It's "Oh, those microkernels had good PR, how can we try to get good PR for our working kernel? Oh, I know, let's use a cool name and try to imply that it has all the PR advantages that that other system has.))</ref>

The idea behind a hybrid kernel is to have a kernel structure similar to that of a microkernel, but to implement that structure in the manner of a monolithic kernel. In contrast to a microkernel, all (or nearly all) operating system services in a hybrid kernel are still in [[user space and kernel space|kernel space]]. There are none of the reliability benefits of having services in [[user space and kernel space|user space]], as with a [[microkernel]]. However, just as with an ordinary [[monolithic kernel]], there is none of the performance overhead for message passing and context switching between kernel and user mode that normally comes with a microkernel.

==Examples==

===NT kernel===
((More citations needed section|date = August 2022))
((Main|Architecture of Windows NT#Kernel))

[[Image:Windows 2000 architecture.svg|thumb|275px|The Windows NT operating system family's architecture consists of two layers ([[user mode]] and [[kernel mode]]), with many different modules within both of these layers.]]
One prominent example of a hybrid kernel is the [[Microsoft]] [[Microsoft_Windows|Windows]] [[Architecture of Windows NT#Kernel mode|NT kernel]] that powers all operating systems in the [[Windows NT]] family, up to and including [[Windows 11]] and [[Windows Server 2022]], and powers [[Windows Phone 8]], [[Windows Phone 8.1]], and [[Xbox One]]. 

Windows NT was the first Windows operating system based on a hybrid kernel((cn|reason=Need reliable source that says NT is hybrid|date=August 2022)). The hybrid kernel was designed as a modified [[microkernel]], influenced by the [[Mach (kernel)|Mach microkernel]] developed by [[Richard Rashid]] at Carnegie Mellon University, but without meeting all of the criteria of a pure microkernel. NT-based Windows is classified as a hybrid kernel (or a macrokernel<ref name="Microsoft_2007">((cite web | title = MS Windows NT Kernel-mode User and GDI White Paper | publisher = Microsoft Corporation | year = 2007 | url = https://docs.microsoft.com/en-us/previous-versions//cc750820(v=technet.10) | access-date = 2022-08-24))</ref>) rather than a monolithic kernel because the emulation subsystems run in user-mode server processes, rather than in kernel mode as on a monolithic kernel, and further because of the large number of design goals which resemble design goals of Mach (in particular the separation of OS personalities from a general kernel design). Conversely, the reason NT is not a microkernel system is because most of the system components run in the same [[address space]] as the kernel, as would be the case with a monolithic design (in a traditional monolithic design, there would not be a microkernel per se, but the kernel would implement broadly similar functionality to NT's microkernel and kernel-mode subsystems).

The primary operating system personality on Windows is the [[Windows API]], which is always present. The emulation subsystem which implements the Windows personality is called the [[Client/Server Runtime Subsystem]] (csrss.exe). On versions of NT prior to 4.0, this subsystem process also contained the window manager, graphics device interface and graphics device drivers. For performance reasons, however, in version 4.0 and later, these modules (which are often implemented in user mode even on monolithic systems, especially those designed without internal graphics support) run as a kernel-mode subsystem.<ref name="Microsoft_2007"/>

Applications that run on NT are written to one of the OS personalities (usually the Windows API), and not to the native NT API for which documentation is not publicly available (with the exception of routines used in device driver development). An OS personality is implemented via a set of user-mode DLLs (see [[Dynamic-link library]]), which are mapped into application processes' address spaces as required, together with an emulation subsystem server process (as described previously). Applications access system services by calling into the OS personality DLLs mapped into their address spaces, which in turn call into the NT run-time library (ntdll.dll), also mapped into the process address space. The NT run-time library services these requests by trapping into kernel mode to either call kernel-mode Executive routines or make [[Local Inter-Process Communication|Local Procedure Calls]] (LPCs) to the appropriate user-mode subsystem server processes, which in turn use the NT API to communicate with application processes, the kernel-mode subsystems and each other.<ref name="Probert2005">((cite web|url=http://research.microsoft.com/ur/asia/curriculum/download/BeijingPresentation.ppt|title=Overview of Windows Architecture|last=Probert|first=Dave|year=2005|work=Using Projects Based on Internal NT APIs to Teach OS Principles|publisher=Microsoft Research/Asia - Beijing|access-date=2007-03-01|archive-url=https://web.archive.org/web/20071128062331/http://research.microsoft.com/ur/asia/curriculum/download/BeijingPresentation.ppt|archive-date=2007-11-28|url-status=dead))</ref>

[[File:The_XNU_Kernel_Graphic.svg|275px|thumb|right|The XNU Kernel]]

===XNU kernel===
((Main|XNU))
'''XNU''' is the [[kernel (operating system)|kernel]] that [[Apple Inc.]] acquired and developed for use in the [[macOS]], [[iOS]], [[watchOS]], and [[tvOS]] operating systems and released as [[free and open source software]] as part of the [[Darwin (operating system)|Darwin operating system]]. ''XNU'' is an [[acronym]] for ''X is Not [[Unix]]''.<ref>((cite web | year=2005 | url=https://developer.apple.com/library/content/documentation/Porting/Conceptual/PortingUnix/glossary/glossary.html#//apple_ref/doc/uid/TP40002859-TPXREF101 | title=Porting UNIX/Linux Applications to OS X: Glossary | publisher=Apple Computer | access-date=2017-06-16))</ref><!-- Please read the footnote before making changes to the last line. XNU is NOT a recursive acronym, and it does NOT stand for "XNU is not Unix."-->

Originally developed by [[NeXT]] for the [[NeXTSTEP]] operating system, XNU was a hybrid kernel combining version 2.5 of the [[Mach (kernel)|Mach kernel]] with components from [[BSD#4.3BSD|4.3BSD]] and an object-oriented API for writing drivers called Driver Kit.

After Apple acquired NeXT, the Mach component was upgraded to OSFMK 7.3,<ref name="wwdc-2000-session-106">((cite AV media|url=https://www.youtube.com/watch?v=ggnFoDqzGMU|author=Jim Magee|title=WWDC 2000 Session 106 - Mac OS X: Kernel|minutes=14))</ref> which is a microkernel.<ref>((cite conference |url=https://pdfs.semanticscholar.org/03ac/1296f530719497b49d7580b55a2d9b8353ab.pdf|archive-url=https://web.archive.org/web/20170822053715/https://pdfs.semanticscholar.org/03ac/1296f530719497b49d7580b55a2d9b8353ab.pdf|archive-date=2017-08-22|url-status=dead|title=A Trusted, Scalable, Real-Time Operating System Environment |conference=1994 IEEE Dual-Use Technologies and Applications Conference |author=Douglas M. Wells|year=1994 |s2cid=5205380))</ref> Apple uses a heavily modified OSFMK 7.3 functioning as a hybrid kernel with parts of FreeBSD included.<ref name="wwdc-2000-session-106"/> (OSFMK 7.3 includes applicable code from the University of Utah Mach 4 kernel and applicable code from the many Mach 3.0 variants that [[Fork (software development)|forked]] off from the original Carnegie Mellon University Mach 3.0 kernel.) The BSD components were upgraded with code from the [[FreeBSD]] project and the Driver Kit was replaced with a [[C++]] [[API]] for writing drivers called I/O Kit((citation needed|date=February 2023)).

====Description====
Like some other modern [[Kernel (operating system)|kernels]], XNU is a hybrid, containing features of both [[monolithic kernel|monolithic]] and [[microkernel]]s, attempting to make the best use of both technologies, such as the [[message passing]] capability of microkernels enabling greater modularity((citation needed|date=October 2015)) and larger portions of the OS to benefit from [[Memory protection|protected memory]],((citation needed|date=June 2012)) as well as retaining the speed of monolithic kernels for certain critical tasks.

=== Others ===
* [[BeOS]]
** [[Haiku (operating system)|Haiku]]
* [[Syllable Desktop|Syllable]]
* [[Berkeley Software Distribution|BSD]]-based
** [[DragonFly BSD]] (first non-[[Mach (kernel)|Mach]] BSD OS to use a hybrid kernel)
* [[NetWare]]<ref>((cite web|url=http://www.usenix.org/publications/library/proceedings/sf94/full_papers/minshall.a|title=An Overview of the NetWare Operating System|author1=Drew Major|author2=Greg Minshall|author3=Kyle Powell))</ref>
* [[Plan 9 from Bell Labs]]
** [[Inferno (operating system)|Inferno]]
* [[OS/2]]<ref>((cite web|url=http://www.edm2.com/index.php/OS/2_Kernel|title=OS/2 Kernel|access-date=2020-09-04))</ref>
** [[eComStation]]
** [[ArcaOS]]
** OS/4
* [[ReactOS]]

== See also ==
*[[Microkernel]]
*[[Exokernel]]
*[[Nanokernel]]
*[[Monolithic kernel]]
*[[Single address space operating system]]
*[[Architecture of Windows NT]]
*[[XNU]]

==Notes==
((Reflist|30em))

==References==
*((cite web|url=http://www.sysinternals.com/Information/NativeApi.html |title=Inside the Native API |website=Sysinternals |author=Mark Russinovich |date=November 23, 2004 |access-date=July 24, 2006 |archive-url=https://web.archive.org/web/20060315213024/http://www.sysinternals.com/Information/NativeApi.html |archive-date=March 15, 2006|url-status=dead))

((Operating System))

((DEFAULTSORT:Hybrid Kernel))
[[Category:Operating system kernels]]
[[Category:Windows NT kernel]]

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