Editing XNU (section)

===Mach===
{{main|Mach (kernel)}}
The basis of the XNU kernel is a heavily modified (hybrid) [[Open Software Foundation]] Mach kernel ([[OSFMK]]) 7.3.<ref name="wwdc-2000-session-106"/> OSFMK 7.3 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 |last=Wells |first=Douglas M. |year=1994 |s2cid=5205380}}</ref> that includes applicable code from the [[University of Utah]] Mach 4 kernel and from the many Mach 3.0 variants [[Fork (software development)|forked]] from the original [[Carnegie Mellon University]] Mach 3.0 microkernel.

OSFMK 7.3 is able to run the core of an operating system as separated processes, which allows a great flexibility (it could run several operating systems in parallel above the Mach core), but this often reduces performance{{citation needed|date=September 2024}} because of time-consuming kernel/user mode context switches and overhead stemming from mapping or copying messages between the address spaces of the kernel and that of the service daemons.

Apple licensed OSFMK 7.3 from the OSF, and attempted to streamline some tasks by building [[BSD]] functions into the kernel along with the Mach code. The result is a heavily modified (hybrid) OSFMK 7.3 kernel.