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<!doctype html public "-//w3c//DTD HTML 3.2//EN">
<html>
<!-- http://home.teleport.com/~brainy/lfn.htm -->
<!-- http://members.aol.com/vindaci/pub/doc/lfn/lfn.html -->
<!-- Copyright (c)1996-1998 vinDaci -->
<head>
<title>Long Filename Specification</title>
</head>
<body>
<h2 align="center">Long Filename Specification</h2>
<font size="-1">
<p align="center">by vinDaci</font> <font size="-2"><br>
fourth release</font> <font size="-1"><br>
First Release: November 18th, 1996</font> <font size="-1"><br>
Last Update: January 6th, 1998 (Document readability update)</font> </p>
<hr>
<h3>Compatibility</h3>
<p>Long filename (here on forth referred to as &quot;LFN&quot;) design for Windows95 is
designed to be 99% compatible with the old DOS 8.3 format. The 1% discripency comes in
when old DOS programs can detect the <em>presence</em> of LFN (but unfortunately not the
LFN itself), which in no way interferes with regular program operations except for perhaps
low-level disk utility programs (such as disk error fixing programs, disk optimization
programs, anti-virus program, etc.) <a name="background"></p>
<hr>
<h3>DOS 8.3 Filename Background</h3>
<p>I trust that anyone who wish to know the details of LFN has at least a small knowledge
in DOS 8.3 filename specification. In this document, however, I'll assume you know very
little about the 8.3 filename specs, however. What you need to know in order to understand
this documentation is that 8.3 filenames are stored in a place on the disk called the <dfn>directory
table</dfn>. This place contains the list of filenames and other information associated
with each file, such as the file date, time, size, attributes, etc. (Note: Contrary to
some belief, the directory table is <em>not</em> the same as the FAT -- e-mail me if you
wish to know what FAT is.) </p>
<p>The file attributes, mentioned above, play some big roles in LFN. It is important to
note that a file's attributes are may consist of one or more of the following: </p>
<div align="center"><center>
<table width="50%" border="1">
<tr>
<td><code>Archive</code> </td>
</tr>
<tr>
<td><code>Read-Only</code> </td>
</tr>
<tr>
<td><code>System</code> </td>
</tr>
<tr>
<td><code>Hidden</code> </td>
</tr>
<tr>
<td><code>Directory</code> </td>
</tr>
<tr>
<td><code>Volume</code> </td>
</tr>
</table>
</center></div>
<p>Most programmers are aware of the Archive, Read-Only, System, and Hidden attrbiutes,
but for those of you who don't know, please allow me to explain what each of these
attributes is/does:
<ul>
<li>The <dfn>Archive attribute</dfn> tells that a file has been backed up (though most
programs just ignore this). </li>
<li>The <dfn>Read-Only attribute</dfn> keeps a file from accidentally getting overwritten;
note that any program can unset this attribute should it know how to detect this attribute
and unset it -- that's why it is used just to keep a file from <em>accidentally</em>
getting overwritten. </li>
<li>The <dfn>System attribute</dfn> tells that the file is used for some important operation
so it should not be messed with except by the program that created it; any file with this
attribute cannot be seen with the <kbd>DIR</kbd> command except with the <kbd>/a</kbd> or <kbd>/as</kbd>
arguments in DOS 5 and above. </li>
<li>The <dfn>Hidden</dfn> attribute tells that a file should normally be hidden from the
user's view. Any file with this attribute cannot be seen with <kbd>DIR</kbd> command
either, except with the <kbd>/a</kbd> or <kbd>/ah</kbd> arguments in DOS 5 and above. </li>
</ul>
<p>And the explanation of the other attributes (the <em>really</em> important ones):
<ul>
<li>The <dfn>Directory attribute</dfn> is used to tell that a file is not actually a file
but a directory. This type of file contains a pointer to a part of the disk that contains
another directory table; this directory table that's pointed to is the subdirectory of the
directory that has the pointer. In another words, when you &quot;CD&quot; to that file,
you go into the directory table the file points to, making it look as though you are
&quot;inside&quot; that directory. In reality, you only switch the directory tables. </li>
<li>The <dfn>volume attribute</dfn> too is used to tell that a file is not actually a file.
This attribute is used to indicate the volume label of the drive (the name of the disk). A
file with this attribute can never be displayed with the <kbd>DIR</kbd> command.
Furthermore, there can be only one file with this attribute on the entire disk, and this
file must be in the root directory of the disk. <br>
&nbsp;&nbsp;&nbsp;&nbsp; The volume attribute has a funny story attached to it -- There
not only exists a file with the volume attribute, but a copy of the volume label is also
located in the boot sector (the very beginning of the disk that has weird code and disk
info on it) as a readable text. But when you view a directory with the <kbd>DIR</kbd>
command, the one that actually gets displayed is the volume attributed file's name, not
the volume label in the boot sector. Furthermore, even though files with volume attribute
is hidden from the <kbd>DIR</kbd> command, programs, when retrieving filenames, can
retrieve volume labels.</a> All these factors about volume attributes come into a big
factor when we look at Long Filenames. </li>
</ul>
<p>As an addendum, it might be interesting to note that each 8.3 file entry is 32 bytes
long but that not all 32 bytes are used to store data -- some of them are plainly left as
blank bytes. In Windows95 version of the directory table, however, all 32 bytes are used. </p>
<hr>
<h3>General Specification</h3>
<p>Just like DOS 8.3 filenames, Windows95 LFNs are also stored on directory tables,
side-by-side with DOS 8.3 filenames. On top of that, to achieve compatibility with old DOS
programs Microsoft designed LFN in a way so it resembles the old DOS 8.3 format.
Furthermore, an 8.3 format version of LFN (<code>tttttt~n.xxx</code>) is also present next
to each LFN entry for compatibility with non-LFN supporting programs. <a
name="organization"></p>
<hr>
<h3>Organization</h3>
<p>From a low-level point-of-view, a normal directory table that only contains 8.3
filenames look like this: </p>
<div align="center"><center>
<table width="50%" border="1">
<tr>
<td><code>...</code> </td>
</tr>
<tr>
<td><code>8.3 entry</code> </td>
</tr>
<tr>
<td><code>8.3 entry</code> </td>
</tr>
<tr>
<td><code>8.3 entry</code> </td>
</tr>
<tr>
<td><code>8.3 entry</code> </td>
</tr>
</table>
</center></div>
<p>If you look at a directory table that contains LFN entries, however, this is what you
will see: </p>
<div align="center"><center>
<table width="50%" border="1">
<tr>
<td><code>...</code> </td>
</tr>
<tr>
<td><code>LFN entry 3</code> </td>
</tr>
<tr>
<td><code>LFN entry 2</code> </td>
</tr>
<tr>
<td><code>LFN entry 1</code> </td>
</tr>
<tr>
<td><code>8.3 entry (tttttt~n.xxx)</code> </td>
</tr>
</table>
</center></div>
<p>Notice that Long Filenames can be pretty long, so LFN entries in a 8.3 directory
structure can take up several 8.3 directory entry spaces. This is why the above file entry
has several LFN entries for a single 8.3 file entry. </p>
<p>Despite taking up 8.3 filename spaces, Long Filenames do not show up with the <kbd>DIR</kbd>
command or with any other program, even the ones that do not recognize the LFN. How, then,
do LFN entries get hidden from DOS? The answer is quite simple: By giving LFN entries
&quot;Read-only, System, Hidden, and Volume&quot; attributes. (If you do not know details
about file attributes, read the above text about </a><a href="#background">DOS 8.3
Filename Background</a>.) </p>
<p>A special attention should be given to the fact that this combination of attributes --
Read-only, System, Hidden, Volume -- is not possible to make under normal circumstances
using common utilities found in the market place (special disk-editing programs, such as
Norton Disk Editor, is an exception.) </p>
<p>This technique of setting Read-only, System, Hidden, Volume attributes works because
most programs ignore files with volume attributes altogether, and even if they don't, they
won't display any program that has system or hidden attributes set. And since the
Read-only attribute is set, programs won't write anything over it. However, you can view
&quot;parts&quot; of the LFN entries if any program is designed to show Volume attributed
files. </p>
<a name="storage">
<hr>
<h3>Storage Organization</h3>
<p>To understand the LFN storage organization within a directory table, it is important to
understand the 8.3 storage organization. This is mainly due to the fact that LFN entries
are stored similar to 8.3 filenames to avoid conflicts with DOS applications. </p>
<p>The format of the traditional DOS 8.3 is as follows: </p>
<div align="center"><center>
<table width="80%" border="1">
<tr>
<th width="15%" align="center">Offset </th>
<th width="15%" align="center">Length </th>
<th width="*" align="left">&nbsp;&nbsp;Value </th>
</tr>
<tr>
<td align="center">0 </td>
<td align="center">8 bytes </td>
<td align="left">&nbsp;&nbsp;Name </td>
</tr>
<tr>
<td align="center">8 </td>
<td align="center">3 bytes </td>
<td align="left">&nbsp;&nbsp;Extension </td>
</tr>
<tr>
<td align="center">11 </td>
<td align="center">byte </td>
<td align="left">&nbsp;&nbsp;Attribute (<code>00ARSHDV</code>) <br>
&nbsp;&nbsp;<code>0</code>: unused bit <br>
&nbsp;&nbsp;<code>A</code>: archive bit, <br>
&nbsp;&nbsp;<code>R</code>: read-only bit <br>
&nbsp;&nbsp;<code>S</code>: system bit <br>
&nbsp;&nbsp;<code>D</code>: directory bit <br>
&nbsp;&nbsp;<code>V</code>: volume bit </td>
</tr>
<tr>
<td align="center">22 </td>
<td align="center">word </td>
<td align="left">&nbsp;&nbsp;Time </td>
</tr>
<tr>
<td align="center">24 </td>
<td align="center">word </td>
<td align="left">&nbsp;&nbsp;Date </td>
</tr>
<tr>
<td align="center">26 </td>
<td align="center">word </td>
<td align="left">&nbsp;&nbsp;Cluster (desc. below) </td>
</tr>
<tr>
<td align="center">28 </td>
<td align="center">dword </td>
<td align="left">&nbsp;&nbsp;File Size </td>
</tr>
</table>
</center></div>
<p align="center">Note: <dfn>WORD</dfn> = 2 bytes, <dfn>DWORD</dfn> = 4 bytes </p>
<p>Everything above you should know what they are except perhaps for the cluster. The
cluster is a value representing another part of the disk, normally used to tell where the
beginning of a file is. In case of a directory, it is the cluster that tells where the
subdirectory's directory table starts. </p>
<p>You may not know this, but LFN specification not only added the capability of having
longer filenames, but it also improved the capability of 8.3 filenames as well. This new
8.3 filename improvements are accomplished by using the unused directory table spaces
(Remember how I told you that 8.3 filenames take up 32 bytes but not all 32 bytes are
used? Now it's all used up!) This new format is as follows -- try comparing it with the
traditional format shown above!: </p>
<div align="center"><center>
<table width="80%" border="1">
<tr>
<th width="15%" align="center">Offset </th>
<th width="15%" align="center">Length </th>
<th width="*" align="left">&nbsp;&nbsp;Value </th>
</tr>
<tr>
<td align="center">0 </td>
<td align="center">8 bytes </td>
<td align="left">&nbsp;&nbsp;Name </td>
</tr>
<tr>
<td align="center">8 </td>
<td align="center">3 bytes </td>
<td align="left">&nbsp;&nbsp;Extension </td>
</tr>
<tr>
<td align="center">11 </td>
<td align="center">byte </td>
<td align="left">&nbsp;&nbsp;Attribute (<code>00ARSHDV</code>) </td>
</tr>
<tr>
<td align="center">12 </td>
<td align="center">byte </td>
<td align="left" nowrap>&nbsp;&nbsp;NT (Reserved for WindowsNT; <br>
&nbsp;&nbsp;always 0) </td>
</tr>
<tr>
<td align="center">13 </td>
<td align="center">byte </td>
<td align="left">&nbsp;&nbsp;Created time; millisecond portion </td>
</tr>
<tr>
<td align="center">14 </td>
<td align="center">word </td>
<td align="left">&nbsp;&nbsp;Created time; hour and minute </td>
</tr>
<tr>
<td align="center">16 </td>
<td align="center">word </td>
<td align="left">&nbsp;&nbsp;Created date </td>
</tr>
<tr>
<td align="center">18 </td>
<td align="center">word </td>
<td align="left">&nbsp;&nbsp;Last accessed date </td>
</tr>
<tr>
<td align="center">20 </td>
<td align="center">word </td>
<td align="left" nowrap>&nbsp;&nbsp;Extended Attribute <br>
&nbsp;&nbsp;(reserved for OS/2; always 0) </td>
</tr>
<tr>
<td align="center">22 </td>
<td align="center">word </td>
<td align="left">&nbsp;&nbsp;Time </td>
</tr>
<tr>
<td align="center">24 </td>
<td align="center">word </td>
<td align="left">&nbsp;&nbsp;Date </td>
</tr>
<tr>
<td align="center">26 </td>
<td align="center">word </td>
<td align="left">&nbsp;&nbsp;Cluster </td>
</tr>
<tr>
<td align="center">28 </td>
<td align="center">dword </td>
<td align="left">&nbsp;&nbsp;File Size </td>
</tr>
</table>
</center></div>
<p>In any case, this new 8.3 filename format is the format used with the LFN. As for the
LFN format itself (seen </a><a href="#organization">previously</a>) is stored
&quot;backwards&quot;, with the first entry toward the bottom and the last entry at the
top, right above the new 8.3 filename entry. </p>
<p>Each LFN entry is stored as follows: </p>
<div align="center"><center>
<table width="80%" border="1">
<tr>
<th width="15%" align="center">Offset </th>
<th width="15%" align="center">Length </th>
<th width="*" align="left">&nbsp;&nbsp;Value </th>
</tr>
<tr>
<td align="center">0 </td>
<td align="center">byte </td>
<td align="left">&nbsp;&nbsp;Ordinal field (desc. below) </td>
</tr>
<tr>
<td align="center">1 </td>
<td align="center">word </td>
<td align="left">&nbsp;&nbsp;Unicode character 1 (desc. below) </td>
</tr>
<tr>
<td align="center">3 </td>
<td align="center">word </td>
<td align="left">&nbsp;&nbsp;Unicode character 2 </td>
</tr>
<tr>
<td align="center">5 </td>
<td align="center">word </td>
<td align="left">&nbsp;&nbsp;Unicode character 3 </td>
</tr>
<tr>
<td align="center">7 </td>
<td align="center">word </td>
<td align="left">&nbsp;&nbsp;Unicode character 4 </td>
</tr>
<tr>
<td align="center">9 </td>
<td align="center">word </td>
<td align="left">&nbsp;&nbsp;Unicode character 5 </td>
</tr>
<tr>
<td align="center">11 </td>
<td align="center">byte </td>
<td align="left">&nbsp;&nbsp;Attribute </td>
</tr>
<tr>
<td align="center">12 </td>
<td align="center">byte </td>
<td align="left">&nbsp;&nbsp;Type (reserved; always 0) </td>
</tr>
<tr>
<td align="center">13 </td>
<td align="center">byte </td>
<td align="left">&nbsp;&nbsp;Checksum (desc. below) </td>
</tr>
<tr>
<td align="center">14 </td>
<td align="center">word </td>
<td align="left">&nbsp;&nbsp;Unicode character 6 </td>
</tr>
<tr>
<td align="center">16 </td>
<td align="center">word </td>
<td align="left">&nbsp;&nbsp;Unicode character 7 </td>
</tr>
<tr>
<td align="center">18 </td>
<td align="center">word </td>
<td align="left">&nbsp;&nbsp;Unicode character 8 </td>
</tr>
<tr>
<td align="center">20 </td>
<td align="center">word </td>
<td align="left">&nbsp;&nbsp;Unicode character 9 </td>
</tr>
<tr>
<td align="center">22 </td>
<td align="center">word </td>
<td align="left">&nbsp;&nbsp;Unicode character 10 </td>
</tr>
<tr>
<td align="center">24 </td>
<td align="center">word </td>
<td align="left">&nbsp;&nbsp;Unicode character 11 </td>
</tr>
<tr>
<td align="center">26 </td>
<td align="center">word </td>
<td align="left">&nbsp;&nbsp;Cluster (unused; always 0) </td>
</tr>
<tr>
<td align="center">28 </td>
<td align="center">word </td>
<td align="left">&nbsp;&nbsp;Unicode character 12 </td>
</tr>
<tr>
<td align="center">30 </td>
<td align="center">word </td>
<td align="left">&nbsp;&nbsp;Unicode character 13 </td>
</tr>
</table>
</center></div>
<p>Throughout this entry, you see &quot;unicode characters&quot;. Unicode characters are
2-byte long characters (as opposed to ASCII characters that are 1-byte long each) that
support not only traditional latin alphabet characters and arabic numbers but they also
support the languages of the rest of the world, including the CJK trio (Chinese, Japanese,
Korean), Arabic, Hebrew, etc. Plus you have some space left over for more math and science
symbols. Unicode characters are still going through revisions (on their second revision as
I am writing, I believe) but Windows95 has left spaces to more fully support unicodes in
the future. You can keep up with the Unicode development by visiting the Unicode webpage
at <a href="http://www.unicode.org/">www.unicode.org</a>. Note that, in the 2-byte unicode
character, the first byte is always the character and the second byte is always the blank,
as opposed to having the first byte blank and the second byte blank. There is a perfectly
logical explanation for this but it's kind of long to get into at the moment so e-mail me
if you are curious. (If you have a computer dictionary, look up &quot;little endian&quot;
and it'll explain everything.) For our purposes, though, just treat every other word as an
ASCII character as long as the following byte is a blank character. Anyways, notice that,
in order to maintain the compatibility with older programs, the attribute byte and the
cluster word had to be kept. Because of this, each unicode character had to be scattered
throughout the entry. </p>
<p>By now you probably noticed that there is no file information (size, date, etc.) stored
in the LFN entry. Any information about the file itself is stored in the 8.3 filename,
located below all the LFN entries (as <a href="#organization">mentioned before</a>). </p>
<p>The checksum is created from the shortname data. The steps/equation used to calculate
this checksum is as follows: </p>
<div align="center"><center>
<table width="80%" border="1">
<tr>
<th>Step # </th>
<th>Task </th>
</tr>
<tr>
<td align="center">1 </td>
<td align="left">&nbsp;&nbsp; Take the ASCII value of the first character. This is your
first sum. </td>
</tr>
<tr>
<td align="center">2 </td>
<td align="left">&nbsp;&nbsp; Rotate all the bits of the sum rightward by one bit. </td>
</tr>
<tr>
<td align="center">3 </td>
<td align="left">&nbsp;&nbsp; Add the ASCII value of the next character with the value
from above. This is your next sum. </td>
</tr>
<tr>
<td align="center">4 </td>
<td align="left">&nbsp;&nbsp; Repeat steps 2 through 3 until you are all through with the
11 characters in the 8.3 filename. </td>
</tr>
</table>
</center></div>
<p align="center">In C/C++, the above steps look like this: </p>
<div align="center"><center>
<table width="80%" border="0">
<tr>
<td><br>
<code>for (sum = i = 0; i &lt; 11; i++) {</code> <br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;sum = (((sum &amp; 1) &lt;&lt; 7) | ((sum &amp; 0xfe)
&gt;&gt; 1)) + name[i];</code> <br>
<code>}</code> </td>
</tr>
</table>
</center></div>
<p>This resulting checksum value is stored in each of the LFN entry to ensure that the
short filename it points to indeed is the currently 8.3 entry it should be pointing to. </p>
<p>Also, note that any file with a name that does not fill up the 8.3 spaces completely
leaves a trace of space characters (ASCII value 32) in the blank spaces. These blank
spaces do go into the calculation of the checksum and does not get left out of the
calculation. </p>
<p>I'm sure you're dying to know what the ordinal field is. This byte of information tells
the order of the LFN entries (1st LFN entry, 2nd LFN entry, etc.) If it's out of order,
something is wrong! How Windows95 would deal with LFN when such a thing happen is a
mystery to me. <br>
&nbsp;&nbsp; An example of how ordinal field work: The first LFN entry, located at the
very bottom as we have <a href="#organization">seen before</a>, has an ordinal field value
1; the second entry (if any -- remember that a LFN doesn't always have to be tens of
characters long), located just before the first entry, has an ordinal field value of 2;
etc. As an added precaution, the last entry has a marking on the ordinal field that tells
that it is the last entry. This marking is done by setting the 6th bit of the ordinal
field. </p>
<p>That is basically all there is to long filenames. But before we end this conversation
(while we're on the subject of LFN,) I think it would be interesting to note that, since
any long filename can be up to 255 bytes long and each entry can hold up to 13 characters,
there can only be up to 20 entries of LFN per file. That means it only needs 5 bits (0th
bit to 4th bit) of the ordinal field. And with the 6th bit used to mark the last entry,
two bits are left for open usage -- the 5th and the 7th bit. Whether or not Microsoft is
going to do anything with these bits -- or why Microsoft used the 6th bit to indicate the
last entry instead of 7th or 5th bit and limited the file length to 255 bits -- remains to
be a mystery only Microsoft will keep to itself. </p>
<hr>
<h3>Credit</h3>
<p><font size="-1">Much the information in this documentation has been gathered from
Norton Utilities for Windows95 user's manual. Detailed researches were done using Norton
Utilities Disk Edit. The checksum calculation was graciously donated by Jacob Verhoeks to <code>comp.os.msdos.programmer</code>
Newsgroup as a reply to my request. Apparently the file Mr. Verhoeks used to get me the
checksum code, <code>vfat.txt</code>, that comes with newer Linux operating systems have
some good information on Windows95 LFN. BTW, I just (like, right now) found out that
checksum algorithm is also in Ralf Brown's Interrupt List. </font></p>
<hr>
<p>Copyright <EFBFBD>1996-1998 vinDaci </p>
</body>
</html>