V3/man/man5/dir.5

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NAME		format of directories
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DESCRIPTION	A directory
behaves exactly like an ordinary file, save that no
user may write into a directory.
The fact that a file is a directory is indicated by
a bit in the flag word of its i-node entry.

Directory entries are 10 bytes long.
The first word is the i-number of the file represented
by the entry, if non-zero; if zero, the entry is empty.

Bytes 2-9 represent the (8-character)
file name, null padded on the right.
These bytes are not cleared for empty slots.

By convention, the first two entries in each directory
are for "." and "..".  The first is an entry for the
directory itself.  The second is for the parent
directory.
The meaning of ".." is modified for the root directory
of the master file system and for the root directories of removable
file systems.
In the first case, there is no parent, and in the second,
the system does not permit off-device references.
Therefore in both cases ".." has the
same meaning as ".".
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SEE ALSO	file system (V)