Modern Operating Systems by Herbert Bos ...
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Modern Operating Systems by Herbert Bos and Andrew...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Modern Operating Systems by Herbert...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Page 1028
SEC. 12.3
military systems, where very high reliability is absolutely essential.
Also, Apple’s
OS X, which runs on all Macs and Macbooks, consists of a modified version of
FreeBSD running on top of a modified version of the Mach microkernel.
Extensible Systems
With the client-server systems discussed above, the idea was to remove as
much out of the kernel as possible. The opposite approach is to put more modules
into the kernel, but in a protected way. The key word here is
, of course.
We studied some protection mechanisms in Sec. 9.5.6 that were initially intended
for importing applets over the Internet, but are equally applicable to inserting for-
eign code into the kernel. The most important ones are sandboxing and code sign-
ing, as interpretation is not really practical for kernel code.
Of course, an extensible system by itself is not a way to structure an operating
system. However, by starting with a minimal system consisting of little more than a
protection mechanism and then adding protected modules to the kernel one at a
time until reaching the functionality desired, a minimal system can be built for the
application at hand.
In this view, a new operating system can be tailored to each
application by including only the parts it requires. Paramecium is an example of
such a system (Van Doorn, 2001).
Kernel Threads
Another issue relevant here no matter which structuring model is chosen is that
of system threads.
It is sometimes convenient to allow kernel threads to exist, sep-
arate from any user process. These threads can run in the background, writing dirty
pages to disk, swapping processes between main memory and disk, and so forth.
In fact, the kernel itself can be structured entirely of such threads, so that when a
user does a system call, instead of the user’s thread executing in kernel mode, the
user’s thread blocks and passes control to a kernel thread that takes over to do the
In addition to kernel threads running in the background, most operating sys-
tems start up many daemon processes in the background. While these are not part
of the operating system, they often perform ‘‘system’’ type activities. These might
including getting and sending email and serving various kinds of requests for re-
mote users, such as FTP and Web pages.
12.3.2 Mechanism vs. Policy
Another principle that helps architectural coherence, along with keeping things
small and well structured, is that of separating mechanism from policy. By putting
the mechanism in the operating system and leaving the policy to user processes,
the system itself can be left unmodified, even if there is a need to change policy.

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