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Modern Operating Systems by Herbert Bos and Andrew S. Tanenb...
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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 359
328
FILE SYSTEMS
CHAP. 4
then people from companies whose products were big endian would have felt like
second-class citizens and would not have accepted the standard. The emotional
content of a CD-ROM can thus be quantified and measured exactly in kilo-
bytes/hour of wasted space.
The format of an ISO 9660 directory entry is illustrated in Fig. 4-35. Since di-
rectory entries have variable lengths, the first field is a byte telling how long the
entry is. This byte is defined to have the high-order bit on the left to avoid any
ambiguity.
11
8
8
7
1
2
4
Location of file
Extended attribute record length
Directory entry length
File size
Date and time
CD #
L
File name
Sys
1
4-15
Padding
Flags
Interleave
Base name
Ext
Ver
•
;
Bytes
Figure 4-35.
The ISO 9660 directory enty.
Directory entries may optionally have extended attributes. If this feature is
used, the second byte tells how long the extended attributes are.
Next comes the starting block of the file itself. Files are stored as contiguous
runs of blocks, so a file’s location is completely specified by the starting block and
the size, which is contained in the next field.
The date and time that the CD-ROM was recorded is stored in the next field,
with separate bytes for the year, month, day, hour, minute, second, and time zone.
Years begin to count at 1900, which means that CD-ROMs will suffer from a
Y2156 problem because the year following 2155 will be 1900.
This problem could
have been delayed by defining the origin of time to be 1988 (the year the standard
was adopted). Had that been done, the problem would have been postponed until
2244. Every 88 extra years helps.
The
Flags
field contains a few miscellaneous bits, including one to hide the
entry in listings (a feature copied from MS-DOS), one to distinguish an entry that
is a file from an entry that is a directory, one to enable the use of the extended at-
tributes, and one to mark the last entry in a directory.
A few other bits are also
present in this field but they will not concern us here. The next field deals with
interleaving pieces of files in a way that is not used in the simplest version of ISO
9660, so we will not consider it further.
The next field tells which CD-ROM the file is located on.
It is permitted that a
directory entry on one CD-ROM refers to a file located on another CD-ROM in the
set. In this way, it is possible to build a master directory on the first CD-ROM that
lists all the files on all the CD-ROMs in the complete set.
The field marked
L
in Fig. 4-35 gives the size of the file name in bytes.
It is
followed by the file name itself.
A file name consists of a base name, a dot, an
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