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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 1023
992
OPERATING SYSTEM DESIGN
CHAP. 12
system call keeps the operating system simple, yet the programmer gets the con-
venience of various ways to call
exec
.
Of course, trying to have one call to handle every possible case can easily get
out of hand.
In UNIX creating a process requires two calls:
fork
followed by
exec
.
The former has no parameters; the latter has three.
In contrast, the WinAPI call for
creating a process,
CreateProcess
, has 10 parameters, one of which is a pointer to
a structure with an additional 18 parameters.
A long time ago, someone should have asked whether something awful would
happen if some of these had been omitted.
The truthful answer would have been in
some cases programmers might have to do more work to achieve a particular ef-
fect, but the net result would have been a simpler, smaller, and more reliable oper-
ating system.
Of course, the person proposing the 10 + 18 parameter version might
have added: ‘‘But users like all these features.’’ The rejoinder might have been they
like systems that use little memory and never crash even more. Trade-offs between
more functionality at the cost of more memory are at least visible and can be given
a price tag (since the price of memory is known). However, it is hard to estimate
the additional crashes per year some feature will add and whether the users would
make the same choice if they knew the hidden price. This effect can be summa-
rized in Tanenbaum’s first law of software:
Adding more code adds more bugs.
Adding more features adds more code and thus adds more bugs. Programmers who
believe adding new features does not add new bugs either are new to computers or
believe the tooth fairy is out there watching over them.
Simplicity is not the only issue that comes out when designing system calls.
An important consideration is Lampson’s (1984) slogan:
Don’t hide power.
If the hardware has an extremely efficient way of doing something, it should be
exposed to the programmers in a simple way and not buried inside some other
abstraction. The purpose of abstractions is to hide undesirable properties, not hide
desirable ones. For example, suppose the hardware has a special way to move large
bitmaps around the screen (i.e., the video RAM) at high speed.
It would be justi-
fied to have a new system call to get at this mechanism, rather than just provide
ways to read video RAM into main memory and write it back again. The new call
should just move bits and nothing else.
If a system call is fast, users can always
build more convenient interfaces on top of it.
If it is slow, nobody will use it.
Another design issue is connection-oriented vs. connectionless calls. The Win-
dows and UNIX system calls for reading a file are connection-oriented, like using
the telephone. First you open a file, then you read it, finally you close it.
Some re-
mote file-access protocols are also connection-oriented. For example, to use FTP,
the user first logs in to the remote machine, reads the files, and then logs out.
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