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Modern Operating Systems by Herbert Bos and Andrew...
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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 1015
CHAP. 12
Or even a few years. All current versions of UNIX contain millions of lines of
code; Linux has hit 15 million, for example. Windows 8 is probably in the range
of 50–100 million lines of code, depending on what you count (Vista was 70 mil-
lion, but changes since then have both added code and removed it). No one person
can understand a million lines of code, let alone 50 or 100 million. When you have
a product that none of the designers can hope to fully understand, it should be no
surprise that the results are often far from optimal.
Operating systems are not the most complex systems around. Aircraft carriers
are far more complicated, for example, but they partition into isolated subsystems
much better. The people designing the toilets on a aircraft carrier do not have to
worry about the radar system. The two subsystems do not interact much.
There are
no known cases of a clogged toilet on an aircraft carrier causing the ship to start
firing missiles.
In an operating system, the file system often interacts with the
memory system in unexpected and unforeseen ways.
Second, operating systems have to deal with concurrency. There are multiple
users and multiple I/O devices all active at once. Managing concurrency is inher-
ently much harder than managing a single sequential activity. Race conditions and
deadlocks are just two of the problems that come up.
Third, operating systems have to deal with potentially hostile users—users who
want to interfere with system operation or do things that they are forbidden from
doing, such as stealing another user’s files. The operating system needs to take
measures to prevent these users from behaving improperly. Word-processing pro-
grams and photo editors do not have this problem.
Fourth, despite the fact that not all users trust each other, many users do want
to share some of their information and resources with selected other users. The op-
erating system has to make this possible, but in such a way that malicious users
cannot interfere. Again, application programs do not face anything like this chal-
Fifth, operating systems live for a very long time.
UNIX has been around for
40 years.
Windows has been around for about 30 years and shows no signs of van-
ishing. Consequently, the designers have to think about how hardware and applica-
tions may change in the distant future and how they should prepare for it. Systems
that are locked too closely into one particular vision of the world usually die off.
Sixth, operating system designers really do not have a good idea of how their
systems will be used, so they need to provide for considerable generality. Neither
UNIX nor Windows was designed with a
Web browser or streaming HD video in
mind, yet many computers running these systems do little else. Nobody tells a ship
designer to build a ship without specifying whether they want a fishing vessel, a
cruise ship, or a battleship. And even fewer change their minds after the product
has arrived.
Seventh, modern operating systems are generally designed to be portable,
meaning they have to run on multiple hardware platforms. They also have to sup-
port thousands of I/O devices, all of which are independently designed with no

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