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Modern Operating Systems by Herbert Bos and Andrew S. Tanenb...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf
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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 1016
SEC. 12.1
THE NATURE OF THE DESIGN PROBLEM
985
regard to one another.
An example of where this diversity causes problems is the
need for an operating system to run on both little-endian and big-endian machines.
A second example was seen constantly under MS-DOS when users attempted to
install, say, a sound card and a modem that used the same I/O ports or interrupt re-
quest lines. Few programs other than operating systems have to deal with sorting
out problems caused by conflicting pieces of hardware.
Eighth, and last in our list, is the frequent need to be backward compatible
with some previous operating system. That system may have restrictions on word
lengths, file names, or other aspects that the designers now regard as obsolete, but
are stuck with.
It is like converting a factory to produce next year’s cars instead of
this year’s cars, but while continuing to produce this year’s cars at full capacity.
12.2 INTERFACE DESIGN
It should be clear by now that writing a modern operating system is not easy.
But where does one begin? Probably the best place to begin is to think about the
interfaces it provides. An operating system provides a set of abstractions, mostly
implemented by data types (e.g., files) and operations on them (e.g.,
read
). Toget-
her, these form the interface to its users. Note that in this context the users of the
operating system are programmers who write code that use system calls, not peo-
ple running application programs.
In addition to the main system-call interface, most operating systems have ad-
ditional interfaces. For example, some programmers need to write device drivers to
insert into the operating system. These drivers see certain features and can make
certain procedure calls.
These features and calls also define an interface, but a very
different one from one application programmers see. All of these interfaces must
be carefully designed if the system is to succeed.
12.2.1 Guiding Principles
Are there any principles that can guide interface design?
We believe there are.
Briefly summarized, they are simplicity, completeness, and the ability to be imple-
mented efficiently.
Principle 1: Simplicity
A simple interface is easier to understand and implement in a bug-free way. All
system designers should memorize this famous quote from the pioneer French avi-
ator and writer, Antoine de St. Exupe
´ry:
Perfection is reached not when there is no longer anything to add, but
when there is no longer anything to take away.
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