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| <li class="toctree-l4 current"><a class="current reference internal" href="#">FAT</a><ul> |
| <li class="toctree-l5"><a class="reference internal" href="#fat-structure">FAT Structure</a></li> |
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| <section id="fat"> |
| <h1>FAT<a class="headerlink" href="#fat" title="Permalink to this heading"></a></h1> |
| <p>File Allocation Table (FAT) is a very simple file system designed by Microsoft |
| and, as the name suggests, it uses a table to track clusters on a storage |
| volume. There have been multiple versions of FAT, like <code class="docutils literal notranslate"><span class="pre">FAT12</span></code>, <code class="docutils literal notranslate"><span class="pre">FAT16</span></code>, |
| and lastly <code class="docutils literal notranslate"><span class="pre">FAT32</span></code> to consider for growing data volumes. Even though FAT |
| has been superseded by more recent file systems in most Windows computers, |
| current Windows editions, macOS, Linux, Unix as well as Apache NuttX support |
| FAT. It is still used for floppy disks, USB flash drives as well as for |
| storage media in smaller, portable devices like digital cameras.</p> |
| <p>FAT uses a magic number <code class="docutils literal notranslate"><span class="pre">0x4D44</span></code>, and has been named as <code class="docutils literal notranslate"><span class="pre">vfat</span></code> in the |
| codebase, due to the implementation being actually VFAT, which is an extension |
| that allows for long file names. This page contains information about the FAT |
| file system from the perspective of the implementation of FAT that exists in |
| NuttX.</p> |
| <section id="fat-structure"> |
| <h2>FAT Structure<a class="headerlink" href="#fat-structure" title="Permalink to this heading"></a></h2> |
| <p>The number beside <code class="docutils literal notranslate"><span class="pre">FAT</span></code> (eg. <code class="docutils literal notranslate"><span class="pre">16</span></code> in <code class="docutils literal notranslate"><span class="pre">FAT16</span></code>) represents the number of |
| bits used for each entry in the allocation table. A FAT volume has multiple |
| sections:</p> |
| <ul class="simple"> |
| <li><p><strong>Master Boot Record</strong>: Master Boot Record (MBR) or Boot Sector contains |
| information needed by the file system to access the volume, including the |
| volume’s layout and file system structure, akin to a superblock in Linux |
| file systems.</p></li> |
| <li><p><strong>Allocation Table Region</strong>: Stores the file allocation table, as well as |
| its copy which acts as a backup. The file allocation table keeps a track of |
| all the clusters in the volume, and thus has one entry for each cluster.</p></li> |
| <li><p><strong>Root Directory</strong>: It stores a directory table containing entries |
| describing the files and directories stored on the volume. Each entry |
| contains metadata about file system objects.</p></li> |
| </ul> |
| <div class="admonition note"> |
| <p class="admonition-title">Note</p> |
| <p>Root Directory exists as a separate section if the volume is for <code class="docutils literal notranslate"><span class="pre">FAT12</span></code> |
| or <code class="docutils literal notranslate"><span class="pre">FAT16</span></code>, right after the Allocation Table Region and has a fixed upper |
| limit to the number of entries. <code class="docutils literal notranslate"><span class="pre">FAT32</span></code> does not have a dedicated region |
| for the Root Directory, but has it incorporated into the following Data |
| Region, and thus <code class="docutils literal notranslate"><span class="pre">FAT32</span></code> does not have any hard coded upper limit to the |
| number of entries.</p> |
| </div> |
| <ul class="simple"> |
| <li><p><strong>Data Region</strong>: This region stores the actual data of the files as well as |
| directory data. Data Region is divided into multiple clusters, which are |
| numbered sequentially and have corresponding entries in the allocation |
| table. Cluster sizes can vary depending on the volume size as well as on |
| the type of the FAT file system.</p></li> |
| </ul> |
| <p>The first entry in the Allocation Table is for the volume’s FAT ID while the |
| second entry indicates that the cluster is reserved. Given that <code class="docutils literal notranslate"><span class="pre">FAT32</span></code> does |
| not have a dedicated Root Directory section, in <code class="docutils literal notranslate"><span class="pre">FAT32</span></code> volume, the third |
| entry points to the root directory.</p> |
| <p>File names can be either <a class="reference external" href="https://en.wikipedia.org/wiki/8.3_filename">short</a> |
| (also known as 8.3 filename or SFN), or long (LFN), till a maximum length |
| limit.</p> |
| </section> |
| <section id="master-boot-record-mbr"> |
| <h2>Master Boot Record (MBR)<a class="headerlink" href="#master-boot-record-mbr" title="Permalink to this heading"></a></h2> |
| <p>The Boot Record may be one of two types. One is a much older type which does |
| not contain partitions, and one more recent with them. One distinguishing |
| feature of FAT is that all MBRs contain <code class="docutils literal notranslate"><span class="pre">0x55AA</span></code> at an offset of 510 from |
| the start, which can be used to determine the type of FAT being used in the |
| block device.</p> |
| <p>For the older type, the MBR exists at the start of the drive. It does not |
| contain any partitions.</p> |
| <p>The newer type has a partition table at an offset of 446 from the start of |
| the drive. It allows for 4 primary partitions. It has FAT Boot Records (FBRs) |
| at the start of every partition, which, for most parts, are identical in |
| structure to the older MBRs.</p> |
| </section> |
| <section id="partition-table-entries"> |
| <h2>Partition Table Entries<a class="headerlink" href="#partition-table-entries" title="Permalink to this heading"></a></h2> |
| <p>The partition table contains information for 4 primary partitions, each |
| partition having entries in the table of 16 bytes each. These have various |
| information about the partition including, but not limited to, type of the |
| partition, starting sector, and partition size.</p> |
| </section> |
| <section id="allocation-table"> |
| <h2>Allocation Table<a class="headerlink" href="#allocation-table" title="Permalink to this heading"></a></h2> |
| <p>The allocation table follows the boot record, and has an entry for each |
| cluster available in the data region. Each entry has a size defined by the |
| file system type. <code class="docutils literal notranslate"><span class="pre">FAT12</span></code> has 12 byte long entries, <code class="docutils literal notranslate"><span class="pre">FAT16</span></code> has 16 byte |
| entries and <code class="docutils literal notranslate"><span class="pre">FAT32</span></code> has 32 byte entries.</p> |
| <p>File data is made up of a linked list (or chain) of clusters. Entries in the |
| allocation table, which correspond to actual clusters, contain the cluster |
| number of the next cluster that appears in the file. The last cluster of a |
| file has a value in their allocation table entry such that all of its bits |
| are set (<code class="docutils literal notranslate"><span class="pre">0xFFF</span></code> for FAT12, <code class="docutils literal notranslate"><span class="pre">0xFFFF</span></code> for FAT16, <code class="docutils literal notranslate"><span class="pre">0xFFFFFFFF</span></code> for FAT32).</p> |
| <p>The starting cluster (head of the file chain) is pointed to by the directory |
| entry of that file.</p> |
| </section> |
| <section id="directory-entires"> |
| <h2>Directory Entires<a class="headerlink" href="#directory-entires" title="Permalink to this heading"></a></h2> |
| <p>A directory is basically a file which has a table that contains directory |
| entries that contain the metadata about the files and subdirectories in it. |
| Each directory entry describes a file or a subdirectory inside the directory.</p> |
| <p>Root directory in FAT12/16 have a dedicated region, separate from the data |
| area. In FAT32, root directory is just like a regular directory in the data |
| region (ie. without any dedicated region) except for:</p> |
| <ul class="simple"> |
| <li><p>Its starting cluster is denoted in the boot record.</p></li> |
| <li><p>It is not pointed to by any other directory.</p></li> |
| <li><p>It has no entries corresponding to <code class="docutils literal notranslate"><span class="pre">.</span></code> and <code class="docutils literal notranslate"><span class="pre">..</span></code> (all non-root |
| directories have both of these entries).</p></li> |
| </ul> |
| <p>Directory entries are 32 bytes long, and start with an 11 bytes long SFN. The |
| rest of the directory entry contain information like the file attributes, |
| timestamps of creation as well as access and write, cluster number and |
| file size, to name a few.</p> |
| <section id="filename"> |
| <h3>8.3 filename<a class="headerlink" href="#filename" title="Permalink to this heading"></a></h3> |
| <p>The first byte of a directory’s 8.3 filename (and hence first byte of the |
| entire directory entry) has a special meaning. It has 3 possible values:</p> |
| <ul class="simple"> |
| <li><p><code class="docutils literal notranslate"><span class="pre">0xe5</span></code> : Denotes that the current directory entry is empty.</p></li> |
| <li><p><code class="docutils literal notranslate"><span class="pre">0x00</span></code> : Denotes that this entry, as well as all following entries, are |
| empty.</p></li> |
| <li><p><code class="docutils literal notranslate"><span class="pre">0x05</span></code> : Actual value is <code class="docutils literal notranslate"><span class="pre">0xe5</span></code>.</p></li> |
| </ul> |
| <p>The rest of the 7 + 3 byte of the directory entry are for the name (with |
| extension).</p> |
| </section> |
| </section> |
| <section id="files"> |
| <h2>Files<a class="headerlink" href="#files" title="Permalink to this heading"></a></h2> |
| <p>The starting cluster of data in a file is pointed by the directory entry of |
| the file.</p> |
| <p>File allocation is very primitive in FAT, and the first available location in |
| the volume is given to a file.</p> |
| <section id="file-attributes"> |
| <h3>File attributes<a class="headerlink" href="#file-attributes" title="Permalink to this heading"></a></h3> |
| <p>File attributes are denoted by a bit flag of the size of a single byte. |
| The file flags in FAT, with their bit representation, are as follows:</p> |
| <table class="docutils align-default" id="id1"> |
| <caption><span class="caption-text">File Attributes</span><a class="headerlink" href="#id1" title="Permalink to this table"></a></caption> |
| <thead> |
| <tr class="row-odd"><th class="head"><p>Attribute Macro</p></th> |
| <th class="head"><p>Bit representation</p></th> |
| <th class="head"><p>Hex value</p></th> |
| </tr> |
| </thead> |
| <tbody> |
| <tr class="row-even"><td><p><code class="docutils literal notranslate"><span class="pre">FATATTR_READONLY</span></code></p></td> |
| <td><p><code class="docutils literal notranslate"><span class="pre">00000001</span></code></p></td> |
| <td><p>0x1</p></td> |
| </tr> |
| <tr class="row-odd"><td><p><code class="docutils literal notranslate"><span class="pre">FATATTR_HIDDEN</span></code></p></td> |
| <td><p><code class="docutils literal notranslate"><span class="pre">00000010</span></code></p></td> |
| <td><p>0x2</p></td> |
| </tr> |
| <tr class="row-even"><td><p><code class="docutils literal notranslate"><span class="pre">FATATTR_SYSTEM</span></code></p></td> |
| <td><p><code class="docutils literal notranslate"><span class="pre">00000100</span></code></p></td> |
| <td><p>0x4</p></td> |
| </tr> |
| <tr class="row-odd"><td><p><code class="docutils literal notranslate"><span class="pre">FATATTR_VOLUMEID</span></code></p></td> |
| <td><p><code class="docutils literal notranslate"><span class="pre">00001000</span></code></p></td> |
| <td><p>0x8</p></td> |
| </tr> |
| <tr class="row-even"><td><p><code class="docutils literal notranslate"><span class="pre">FATATTR_DIRECTORY</span></code></p></td> |
| <td><p><code class="docutils literal notranslate"><span class="pre">00010000</span></code></p></td> |
| <td><p>0x10</p></td> |
| </tr> |
| <tr class="row-odd"><td><p><code class="docutils literal notranslate"><span class="pre">FATATTR_ARCHIVE</span></code></p></td> |
| <td><p><code class="docutils literal notranslate"><span class="pre">00100000</span></code></p></td> |
| <td><p>0x20</p></td> |
| </tr> |
| </tbody> |
| </table> |
| <p>Out of these, FAT exposes a user to <code class="docutils literal notranslate"><span class="pre">FATATTR_READONLY</span></code>, <code class="docutils literal notranslate"><span class="pre">FATATTR_HIDDEN</span></code>, |
| <code class="docutils literal notranslate"><span class="pre">FATATTR_SYSTEM</span></code> and <code class="docutils literal notranslate"><span class="pre">FATATTR_ARCHIVE</span></code> to the user.</p> |
| </section> |
| </section> |
| <section id="implementation"> |
| <h2>Implementation<a class="headerlink" href="#implementation" title="Permalink to this heading"></a></h2> |
| <p>The Apache NuttX implementation of VFAT can be found in:</p> |
| <ul class="simple"> |
| <li><p><code class="docutils literal notranslate"><span class="pre">fs/fat</span></code> directory.</p></li> |
| <li><p><code class="docutils literal notranslate"><span class="pre">include/nuttx/fs/fat.h</span></code> header file.</p></li> |
| </ul> |
| </section> |
| </section> |
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