I´m not exactly sure what kind of impact these two things have on kernel design. Are servers more flexible because of running in user space, for instance, and does adding a module require a new build of a kernel?
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1$\begingroup$ Welcome! I am unsure as to the scope of your question. Are you sure it is a CS question, that is refers to operating system (theory) in general? Or would you rather discuss implementation details of specific OSs? $\endgroup$ – Raphael♦ Sep 30 '12 at 16:51
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1$\begingroup$ It might be more suitable for superuser.com. $\endgroup$ – Dave Clarke Sep 30 '12 at 17:15
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3$\begingroup$ @DaveClarke I'm pretty sure that Super User would close this question as too theoretical, they're rather focused on “how do I do X”-type questions. Unix & Linux is more open to “how does this work under the hood”, but even there I think the question would be considered borderline. I think this question is ok here as a general question on OS design. $\endgroup$ – Gilles♦ Sep 30 '12 at 21:51
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$\begingroup$ @Raphael I'm just looking into OS theory, not a specific OS. I'd say it falls within the field of computer science, since it's a general question Gilles stated. $\endgroup$ – 1000 Gibibit Oct 1 '12 at 7:07
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$\begingroup$ I'm certainly happy for it to stay here. $\endgroup$ – Dave Clarke Oct 1 '12 at 7:20
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Basically, a module runs at the same memory space than the kernel and a server runs in a different one.
Although a module can surely be added/removed on-the-fly (linux kernel does that), any misbehavior caused by it may affect the entire system, whether in the micro-kernel architecture, only the server gets compromised.
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1$\begingroup$ Rather than "memory space", one should probably say "protection domain". In addition, the protection is not absolute; a server's misbehavior can become a denial of service attack or exploit bugs in other code that would not manifest under normal provision of the server's functionality. $\endgroup$ – Paul A. Clayton Jan 30 at 14:05
