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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 296
SEC. 4.1
FILES
265
or bitmaps are used to keep track of free storage and how many sectors there are in
a logical disk block are of no interest, although they are of great importance to the
designers of the file system. For this reason, we have structured the chapter as sev-
eral sections.
The first two are concerned with the user interface to files and direc-
tories, respectively. Then comes a detailed discussion of how the file system is im-
plemented and managed.
Finally, we give some examples of real file systems.
4.1 FILES
In the following pages we will look at files from the user’s point of view, that
is, how they are used and what properties they have.
4.1.1 File Naming
A file is an abstraction mechanism. It provides a way to store information on
the disk and read it back later.
This must be done in such a way as to shield the
user from the details of how and where the information is stored, and how the disks
actually work.
Probably the most important characteristic of any abstraction mechanism is the
way the objects being managed are named, so we will start our examination of file
systems with the subject of file naming. When a process creates a file, it gives the
file a name.
When the process terminates, the file continues to exist and can be ac-
cessed by other processes using its name.
The exact rules for file naming vary somewhat from system to system, but all
current operating systems allow strings of one to eight letters as legal file names.
Thus
andrea
,
bruce
, and
cathy
are possible file names. Frequently digits and spe-
cial characters are also permitted, so names like
2
,
urgent!
, and
Fig.2-14
are often
valid as well. Many file systems support names as long as 255 characters.
Some file systems distinguish between upper- and lowercase letters, whereas
others do not.
UNIX falls in the first category; the old MS-DOS falls in the sec-
ond. (As an aside, while ancient, MS-DOS is still very widely used in embedded
systems, so it is by no means obsolete.)
Thus, a UNIX system can have all of the
following as three distinct files:
maria
,
Maria
, and
MARIA
.
In MS-DOS, all these
names refer to the same file.
An aside on file systems is probably in order here.
Windows 95 and Windows
98 both used the MS-DOS file system, called
FAT-16
, and thus inherit many of its
properties, such as how file names are constructed.
Windows 98 introduced some
extensions to FAT -16, leading to
FAT-32
, but these two are quite similar.
In addi-
tion, Windows NT, Windows 2000, Windows XP, Windows Vista, Windows 7, and
Windows 8 all still support both FAT file systems, which are really obsolete now.
However, these newer operating systems also have a much more advanced native
file system (
NTFS
) that has different properties (such as file names in Unicode). In
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