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<concept id="GUID-AFFC4002-3A06-5458-A4D9-1E0AEDB8C534" xml:lang="en"><title>FAT

File System Guide</title><shortdesc>Describes the FAT (File Allocation Table) file system that was

originally devised for MS-DOS. </shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>

<section id="GUID-32E85FBB-B762-5BF1-9D13-90B3735629ED"><title>Introduction</title> <p>FAT

file system was originally used by IBM PCs and is an industrial standard for

file systems, supported by all operating systems that execute on PCs. Symbian

platform FAT file system implementation supports FAT12, FAT16

and FAT32. FAT file system is used for storing user data on NAND Flash, internal

RAM drives and removable media such as SD cards and MMC cards. </p> </section>

<section id="GUID-EC9D2DDC-EC74-43E7-A8A2-B4EE4B0F42C5"><title>Background

information</title> <p>On a FAT file system , the smallest size of data storage

that can be allocated to a file called a cluster. The FAT structure is used

to track how clusters are allocated to each file. The FAT table stores the

address of the clusters. The cluster size is different on different media. </p> <p>Over

the years this file system has been enhanced to keep track of increasing drive

sizes. The number after the FAT relates to the size of the entry in the FAT

data structure, for example FAT12 uses 12 bit entries. </p> </section>

<section id="GUID-E9826F50-A205-475D-995B-679412A2CA73"><title>FAT features</title> <p>FAT

provides the following features: </p> <ul>

<li id="GUID-8DAD60FA-8793-5B1A-A3EC-10108210CC67"><p>The industrial standard

for filing systems on PCs. </p> </li>

<li id="GUID-5254D046-DD0D-5157-80C6-B1CEA9927038"><p>Used for user storage. </p> </li>

<li id="GUID-272D6E2A-6441-5BE2-A1BF-196B0E992E2A"><p>There is a rugged FAT

system that can cope with the loss of power during a write operation. </p> </li>

<li id="GUID-E2004F06-9421-5E75-A969-210AD9430422"><p>Used on a wide variety

of media such as NAND Flash, internal RAM drives and removable media like

SD cards and MMC cards. </p> </li>

<li id="GUID-4D649750-FBAD-57E3-9D64-9DD0EAB26EF2"><p>There is a maximum upper

limit for the size of a drive, which has been increased in FAT32 </p> </li>

</ul> <p>When files are written or deleted from the media, the files can be

scattered over the media known as fragmentation. The fragmentation results

in slower access times of the files. </p> </section>

<section id="GUID-266ACC2C-DEE2-51E0-915B-DBB75AA9BEA8"><title>Leaf Directory

Name Cache</title> <p>Leaf directory name refers to the parent directory path

of a file or a folder. For example the leaf directory of a file <filepath>somefile.txt</filepath> in

the path <filepath>C:\1\2\3\4\5\6\somefile.txt</filepath> is <filepath>C:\1\2\3\4\5\6\</filepath>.

Each leaf directory name cache entry contains the following information: </p> <ul>

<li id="GUID-0242CD6B-88DC-5C0D-8425-3CC2867793E8"><p>Full path of the leaf

directory </p> </li>

<li id="GUID-F7DE41A7-0625-5124-8005-2A458782CCAA"><p>Starting cluster number </p> </li>

<li id="GUID-DE2C1348-9C0C-54FB-8683-FDB7FB061179"><p>Most recently used entry

position </p> </li>

</ul> <p>To perform any file operations such as read, write or update the

file must be first located. The leaf directory name cache improves the performance

of locating a file in deeply nested directories. The number of entries that

can be cached is configured in the <filepath>estart.txt</filepath> file. See <xref href="GUID-DC0BF2A3-613B-595C-BF47-07F29E2A7893.dita">Leaf Directory Name Cache

Configuration Tutorial</xref>, for more details. </p> <p> </p> </section>

<section id="GUID-86BEEEF5-F4F0-50AD-AF29-0A0277455BDB"><title> Directory

Cache</title> <p>A directory is a file that contains a list of files and directory

names. The list items are called directory entries. Each directory entry contains

the filename, size of the file, starting cluster of the file and other metadata

information. The maximum size of a directory entry is 32 bytes. Directory

cache is used to store the directory entries. In the older versions of Symbian

platform, the directory cache was implemented as a fixed array. The new directory

cache is variable and allows the device creators to customise with a dynamic

range. </p> <p>The configuration of directory cache specifies the size of

the directory cache dynamically. The size of a directory cache depends on

the available system memory. The dynamic range can be configured in the <filepath>estart.txt</filepath> file.

See <xref href="GUID-3D8B134E-3C2F-5BAF-82AB-0AB786A5D22D.dita">Dynamic Directory

Cache Configuration Tutorial</xref>, for more information. The global cache

memory manager allocates the memory to the different cache on each drive. </p> </section>

<section id="GUID-7675348A-9D74-54DB-A09A-0537AD4D6CA5"><title>Global Cache

Memory Manager</title> <p>The global cache memory manager is introduced to

dynamically manage the cache memory on different drives. The global cache

memory is configured to reserve the address space required by different cache

memories. The <filepath>estart.txt</filepath> file is used to configure the

global cache memory manager. There are two parameters: </p> <ul>

<li id="GUID-3E196DC8-238F-5CE1-AC5E-1B74BDAD09B9"><p> <codeph>GlobalCacheMemorySize</codeph> parameter

specifies the overall memory available to various cache. The parameter is

specified in kilobytes and the recommended value is 32768. The maximum value

depends on the sum of maximum size required by cache on each drive. </p> </li>

<li id="GUID-2B3475BE-8AAD-580D-A409-4CBD5C223E91"><p> <codeph>LowMemoryThreshold</codeph> specifies

the percentage of RAM that should be reserved for the system. When the memory

on RAM reaches the threshold value, the dynamic memory growth is stopped.

The default value is 10% of RAM. </p> </li>

</ul> <p>The text below is the sample <filepath>estart.txt </filepath> entry

for the global cache memory manager. </p> <codeblock id="GUID-F365FA22-4E35-5FC0-B791-67CFCFF7BC70" xml:space="preserve">

…

[CacheMemory]

GlobalCacheMemorySize    32768

LowMemoryThreshold         10

</codeblock> <p>The maximum or the global cache memory reserves address space

but the actual memory allocation depends on the available system memory. </p> </section>

</conbody><related-links>

<link href="GUID-9E92EE30-F2E2-5F28-BB2A-391C09EC69D2.dita"><linktext>Using   

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<link href="GUID-F3BD37EC-0CCB-5859-908F-215E22C9FC20.dita#GUID-F3BD37EC-0CCB-5859-908F-215E22C9FC20/GUID-143780ED-4A23-5E72-A923-F605172EC8B5">

<linktext>File Server</linktext></link>

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