Editing User space and kernel space (section)

== Implementation ==
The most common way of implementing a '''user mode''' separate from [[supervisor mode|kernel mode]] involves operating system [[protection ring]]s.
Protection rings, in turn, are implemented using [[CPU modes]].
Typically, kernel space programs run in [[kernel mode]], also called [[supervisor mode]];
standard applications in user space run in user mode.

Some operating systems are [[single address space operating system]]s—with a single address space for all user-mode code. (The kernel-mode code may be in the same address space, or it may be in a second address space).
Other operating systems have a per-process address space, with a separate address space for each user-mode process.

Another approach taken in experimental operating systems is to have a single [[address space]] for all software, and rely on a programming language's semantics to ensure that arbitrary memory cannot be accessed – applications cannot acquire any [[reference (computer science)|reference]]s to the objects that they are not allowed to access.<ref>{{cite web
| url=http://uuu.sourceforge.net/si.php#SEC6
| title=Unununium System Introduction
| archive-url=https://web.archive.org/web/20011215052223/http://uuu.sourceforge.net/si.php#SEC6
| archive-date=2001-12-15
| url-status=dead
| access-date = 2016-08-14}}</ref><ref>{{cite web
| url=http://uuu.cvs.sourceforge.net/viewvc/uuu/uuu/docs/system_introduction/uuu_intro.tex?view=markup
| title=uuu/docs/system_introduction/uuu_intro.tex
| work=UUU System Introduction Guide
| date=2001-06-01
| access-date = 2016-08-14}}</ref> This approach has been implemented in [[JX (operating system)|JXOS]], Unununium and Microsoft's [[Singularity (operating system)|Singularity]] research project.