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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 360
SEC. 4.5
EXAMPLE FILE SYSTEMS
329
extension, a semicolon, and a binary version number (1 or 2 bytes).
The base
name and extension may use uppercase letters, the digits 0–9, and the underscore
character. All other characters are forbidden to make sure that every computer can
handle every file name. The base name can be up to eight characters; the extension
can be up to three characters. These choices were dictated by the need to be MS-
DOS compatible.
A file name may be present in a directory multiple times, as
long as each one has a different version number.
The last two fields are not always present. The
Padding
field is used to force
every directory entry to be an even number of bytes, to align the numeric fields of
subsequent entries on 2-byte boundaries.
If padding is needed, a 0 byte is used.
Finally, we have the
System use
field. Its function and size are undefined, except
that it must be an even number of bytes. Different systems use it in different ways.
The Macintosh keeps Finder flags here, for example.
Entries within a directory are listed in alphabetical order except for the first
two entries. The first entry is for the directory itself.
The second one is for its par-
ent. In this respect, these entries are similar to the UNIX . and .. directory entries.
The files themselves need not be in directory order.
There is no explicit limit to the number of entries in a directory. However,
there is a limit to the depth of nesting.
The maximum depth of directory nesting is
eight. This limit was arbitrarily set to make some implementations simpler.
ISO 9660 defines what are called three levels. Level 1 is the most restrictive
and specifies that file names are limited to 8 + 3 characters as we have described,
and also requires all files to be contiguous as we have described. Furthermore, it
specifies that directory names be limited to eight characters with no extensions.
Use of this level maximizes the chances that a CD-ROM can be read on every
computer.
Level 2 relaxes the length restriction.
It allows files and directories to have
names of up to 31 characters, but still from the same set of characters.
Level 3 uses the same name limits as level 2, but partially relaxes the assump-
tion that files have to be contiguous. With this level, a file may consist of several
sections (extents), each of which is a contiguous run of blocks. The same run may
occur multiple times in a file and may also occur in two or more files. If large
chunks of data are repeated in several files, level 3 provides some space optimiza-
tion by not requiring the data to be present multiple times.
Rock Ridge Extensions
As we have seen, ISO 9660 is highly restrictive in several ways. Shortly after it
came out, people in the UNIX community began working on an extension to make
it possible to represent UNIX file systems on a CD-ROM. These extensions were
named
Rock Ridge
, after a town in the Mel Brooks movie
Blazing Saddles
, proba-
bly because one of the committee members liked the film.
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