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Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Page 107
recompile them, thus reducing the number of compilations to the bare minimum.
In large projects, creating the
is error prone, so there are tools that do it
Once all the
files are ready, they are passed to a program called the
combine all of them into a single executable binary file. Any library functions cal-
led are also included at this point, interfunction references are resolved, and ma-
chine addresses are relocated as need be. When the linker is finished, the result is
an executable program, traditionally called
on UNIX systems. The various
components of this process are illustrated in Fig. 1-30 for a program with three C
files and two header files. Although we have been discussing operating system de-
velopment here, all of this applies to developing any large program.
binary program
Figure 1-30.
The process of compiling C and header files to make an executable.
1.8.4 The Model of Run Time
Once the operating system binary has been linked, the computer can be
rebooted and the new operating system started. Once running, it may dynamically
load pieces that were not statically included in the binary such as device drivers

Page 108
SEC. 1.8
and file systems.
At run time the operating system may consist of multiple seg-
ments, for the text (the program code), the data, and the stack. The text segment is
normally immutable, not changing during execution. The data segment starts out
at a certain size and initialized with certain values, but it can change and grow as
need be. The stack is initially empty but grows and shrinks as functions are called
and returned from.
Often the text segment is placed near the bottom of memory,
the data segment just above it, with the ability to grow upward, and the stack seg-
ment at a high virtual address, with the ability to grow downward, but different
systems work differently.
In all cases, the operating system code is directly executed by the hardware,
with no interpreter and no just-in-time compilation, as is normal with Java.
Computer science is a rapidly advancing field and it is hard to predict where it
is going. Researchers at universities and industrial research labs are constantly
thinking up new ideas, some of which go nowhere but some of which become the
cornerstone of future products and have massive impact on the industry and users.
Telling which is which turns out to be easier to do in hindsight than in real time.
Separating the wheat from the chaff is especially difficult because it often takes 20
to 30 years from idea to impact.
For example, when President Eisenhower set up the Dept. of Defense’s Ad-
vanced Research Projects Agency (ARPA) in 1958, he was trying to keep the
Army from killing the Navy and the Air Force over the Pentagon’s research bud-
get. He was not trying to invent the Internet.
But one of the things ARPA did was
fund some university research on the then-obscure concept of packet switching,
which led to the first experimental packet-switched network, the ARPANET.
went live in 1969. Before long, other ARPA-funded research networks were con-
nected to the ARPANET, and the Internet was born. The Internet was then happily
used by academic researchers for sending email to each other for 20 years.
In the
early 1990s, Tim Berners-Lee invented the World Wide Web at the CERN research
lab in Geneva and Marc Andreesen wrote a graphical browser for it at the Univer-
sity of Illinois. All of a sudden the Internet was full of twittering teenagers.
dent Eisenhower is probably rolling over in his grave.
Research in operating systems has also led to dramatic changes in practical
systems. As we discussed earlier, the first commercial computer systems were all
batch systems, until M.I.T. invented general-purpose timesharing in the early
1960s. Computers were all text-based until Doug Engelbart invented the mouse
and the graphical user interface at Stanford Research Institute in the late 1960s.
Who knows what will come next?
In this section and in comparable sections throughout the book, we will take a
brief look at some of the research in operating systems that has taken place during

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