📄 e_fat_intro.htm

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<p><font face="Verdana, Arial, Helvetica, sans-serif"><b><font color="#006666">Introduction 
  to the File Allocation Table (FAT)</font></b></font></p>
<p><font face="Verdana, Arial, Helvetica, sans-serif" size="-1">Every disk or 
  hard disk, that has been formatted with the FAT system is built as follows</font></p>
<table width="93%" border="0">
  <tr> 
    <td width="64%"><font face="Verdana, Arial, Helvetica, sans-serif" size="-1"><img src="fat_disk.jpg" width="400" height="405"></font></td>
    <td width="36%"> 
      <p><font face="Verdana, Arial, Helvetica, sans-serif" size="-2" color="#006666">The 
        write process</font><font face="Verdana, Arial, Helvetica, sans-serif" size="-2"><br>
        The operating system writes system information like file size and first 
        cluster number to the directory entry, position information to the FAT, 
        and data itself to the data area.<br>
        <br>
        <font color="#006666">The read process</font> <br>
        (1) System information, like file name, size and first cluster number 
        is determined from the directory entry.<br>
        (2) Further cluster numbers are found from the file allocation table (FAT) 
        and read from the corresponding clusters in the data area.</font></p>
      <p>&nbsp; </p>
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<p><font face="Verdana, Arial, Helvetica, sans-serif" size="-1">The file system 
  exists from a number of special areas of the disk set aside for organization 
  when the disk is formatted: the master boot record, the partition table, the 
  boot record, the file allocation table (from which the FAT system takes its 
  name), and the root directory. At a low level, disk are organized into 512 byte 
  groups called sectors. The FAT system allocates space for files using a unit 
  called a <b>cluster</b>, made up of an integral number of sectors.</font></p>
<p><font face="Verdana, Arial, Helvetica, sans-serif" size="-1">A <b>boot record</b> 
  is a sector which contains code that can is executed by the computer. The master 
  boot record is the first boot record that the computer executes when it accesses 
  the hard disk. Additonally a boot record contains important information about 
  the FAT file system, e.g. the cluster size and the positions of the file allocation 
  table, data area and the root directory. </font></p>
<p><font face="Verdana, Arial, Helvetica, sans-serif" size="-1">The file allocation 
  table (<b>FAT</b>), located behind the boot record is a database that associates 
  clusters of disk space with files. It has one entry (each 12,16 or 32 bits) 
  for each cluster. Because the first two entries are reserved for the file system, 
  the third entry and those following are assigned to clusters of disk space (<b>data 
  area</b>). Files saved in the data area are not necessarily stored successively 
  and therefore the operating system has to know where a complete file is located 
  in the data area. That is the task of the FAT. For any cluster that is used 
  by a file but is not the file's last cluster, the FAT entry contains the number 
  of the next cluster used by the file. When a program asks the operating system 
  (OS) to provide the content of a file, the OS has to read the first cluster 
  of a file. It then looks at the corresponding first cluster entry in the FAT 
  and knows the next cluster number where the file continues. Now it reads the 
  associated cluster in the data area. After this cluster is also totally read 
  the OS repeats this method until the whole file is read. This way of organizing 
  a file is called the FAT chain. </font></p>
<p><font face="Verdana, Arial, Helvetica, sans-serif" size="-1">FAT entries may 
  contain a few special values to indicate that</font></p>
<ul>
  <li><font face="Verdana, Arial, Helvetica, sans-serif" size="-1"> the cluster 
    is free-that is, not in use by a file (0000H for FAT16) </font></li>
  <li><font face="Verdana, Arial, Helvetica, sans-serif" size="-1">the cluster 
    contains one or more sectors that are physically damaged and should not be 
    used (FFF7H for FAT16) </font></li>
  <li><font face="Verdana, Arial, Helvetica, sans-serif" size="-1">and the cluster 
    is the final cluster in a file (FFF8-FFFFH for FAT16), also called End Of 
    File (EOF)</font></li>
</ul>
<p><font face="Verdana, Arial, Helvetica, sans-serif" size="-1"> But from where 
  does the OS know what files are on the disk and where to find the first cluster 
  of that files? That is the reason for the <b>directory entries</b> which are 
  also stored in the data area. Each directory entry has a size of 32 byte and 
  includes information about the file or directory name, size, first cluster number 
  and its attributes.</font></p>
<p>&nbsp;</p>
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