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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 1027
CHAP. 12
User mode
Kernel mode
Client obtains
service by
sending messages
to server processes
Figure 12-3.
Client-server computing based on a microkernel.
process could have the page for its device mapped in, but no other device pages.
the I/O port space can be partially protected, the correct portion of it could be made
available to each driver.
Even if no hardware assistance is available, the idea can still be made to work.
What is then needed is a new system call, available only to device-driver processes,
supplying a list of (port, value) pairs. What the kernel does is first check to see if
the process owns all the ports in the list.
If so, it then copies the corresponding val-
ues to the ports to initiate device I/O.
A similar call can be used to read I/O ports.
This approach keeps device drivers from examining (and damaging) kernel
data structures, which is (for the most part) a good thing.
An analogous set of calls
could be made available to allow driver processes to read and write kernel tables,
but only in a controlled way and with the approval of the kernel.
The main problem with this approach, and with microkernels in general, is the
performance hit all the extra context switches cause. However, virtually all work
on microkernels was done many years ago when CPUs were much slower. Now-
adays, applications that use every drop of CPU power and cannot tolerate a small
loss of performance are few and far between. After all, when running a word proc-
essor or Web browser, the CPU is probably idle 95% of the time. If a microkernel-
based operating system turned an unreliable 3.5-GHz system into a reliable
3.0-GHz system, probably few users would complain. Or even notice. After all,
most of them were quite happy only a few years ago when they got their previous
computer at the then-stupendous speed of 1 GHz. Also, it is not clear whether the
cost of interprocess communication is still as much of an issue if cores are no long-
er a scarce resource. If each device driver and each component of the operating
system has its own dedicated core, there is no context switching during interproc-
ess communication. In addition, the caches, branch predictors and TLBs will be all
warmed up and ready to run at full speed. Some experimental work on a high-per-
formance operating system based on a microkernel was presented by Hruby et al.
It is noteworthy that while microkernels are not popular on the desktop, they
are very widely used in cell phones, industrial systems, embedded systems, and

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