View source for Inner FAT

This page describes a common FAT-like file system used in [[Savegames]], [[Extdata]] and [[Title Database]]. This file system format has several variants depending on which kind of data it stores. All the three kinds of data that use this file system structure also happen to use the [[DISA and DIFF]] container as well, but there is no direct relationship between the file system and the DISA/DIFF container. All data formats described here is in the inner data of the DISA/DIFF container (i.e. IVFC level 4). Please refer to the DISA/DIFF page for how to unwrap it first before trying to extract the file system.

== Overview ==

The file system consists of the following components:
* header
* directory hash table
* file hash table
* file allocation table
* directory entry table
* file entry table
* data region

The file allocation table (FAT) forms several linked lists inside, each of which represents a "file" allocated in the data region. Please refer to the File Allocation Table section below for more detail. In some variants, the directory entry table and the file entry table are also allocated as two special "files" in the data region, managed by the FAT, while in others they are stand-alone tables located outside the data region.

== Layout Variants ==
Four variants of the file system layout has been identified. A summary diagram:

[[File:Inner-fat.png]]

=== Savegame, <code>duplicate data = true</code> ===

Such savegame is a single DISA container that only has one partition which is always configured as external IVFC level 4 disabled (see [[DISA and DIFF|DISA]] format for details). All components are stored in this partition as

* filesystem header at the beginning
* directory hash table
* file hash table
* file allocation table
* data region
** directory entry table is allocated inside data region
** file entry table as well
** all file data is also allocated here

In this layout, all data is duplicated by DISA's DPFS tree, which is what the parameter <code>duplicate data</code> implies.

=== Savegame, <code>duplicate data = false</code> === 
Such savegame is a single DISA container that has two partitions. Partition A is always configured as external IVFC level 4 disabled, and partition B is configured as it enabled. Components are stored among the two partitions as

* Partition A
** filesystem header at the beginning.
** directory hash table
** file hash table
** (stand-alone) file allocation table
** (stand-alone) directory entry table
** file entry table
* Partition B
** used as data region entirely, and only has file data allocated.

In this layout, all file system metadata is duplicated by partition A DPFS tree, but file data is not as partition B has external IVFC level 4.

=== Extdata ===
An extdata consists of several DIFF containers (device files), among which the special device file <code>00000000/00000001</code> contains the inner FAT system, while other devices contains normal subfiles of the extdata. Please refer to [[Extdata]] for detail. The special file <code>00000000/00000001</code> contists of the following components

* filesystem header at the beginning
* directory hash table
* file hash table
* file allocation table (degenerate, because the data region only has two "files": the directory entry table and the file entry table)
* data region
** directory entry table allocated inside data region
** file entry table as well
** normal subfiles are NOT in the data region. They are in their DIFF containers instead.

The special file <code>00000000/00000001</code> is configured as external IVFC level 4 disabled, and all other device files are configured as it enabled.

=== Title database ===
All [[Title Database]] files are DIFF containers. Except for <code>certs.db</code>, all of them uses this filesystem in the DIFF inner data, which consists of

* database-specific pre-header at the beginning (See [[Title Database]])
* filesystem header
* directory Hash Table (degenerate and always has only one bucket, as there is only one directory for "root")
* file Hash Table
* file allocation table
* data region
** directory entry table allocated inside data region (degenerate, as there is only one directory for "root")
** file entry table as well
** title entries (title info or ticket) are allocated as normal files in the data region as well.

== Filesystem Header ==
Offsets listed in the table below are all relative to the beginning of the header, while all "starting block index" are relative to the beginning of data region. This is especially important for title database, as the offsets doesn't count the pre header.
{| class="wikitable" border="1"
! Offset
! Length
! Description
|-
| 0x00
| 4
| Magic ("SAVE" for savegame; "BDRI" for title database; "VSXE" for extdata)
|-
| 0x04
| 4
| Version (0x40000 for savegame; 0x30000 for title database and extdata)
|-
| 0x08
| 8
| Filesystem Information offset (Y, =0x20 for savegame and title database, =0x138 for extdata)
|-
| 0x10
| 8
| Filesystem image size in blocks (including pre header for title database)
|-
| 0x18
| 4
| Filesystem Image block size
|-
| 0x1C
| 4
| Padding
|-
| 0x20 
| 0x118 in total
| Below is additional data for extdata. Not present in savegame or title database
|-
| 0x20
| 8
| Unknown
|-
| 0x28
| 4
| 'Action' made on most recently mounted Extdata image
|-
| 0x2C
| 4
| Unknown
|-
| 0x30
| 4
| ID of most recently mounted Extdata image
|-
| 0x34
| 4
| Unknown
|-
| 0x38
| 0x100
| Mount path, from most recently mounted Extdata image
|-
| Y 
| 0x68 in total
| Below is Filesystem Information
|-
| Y + 0x00
| 4
| Unknown
|-
| Y + 0x04
| 4
| Data region block size
|-
| Y + 0x08
| 8
| Directory hash table offset
|-
| Y + 0x10
| 4
| Directory hash table bucket count (=1 for title database)
|-
| Y + 0x14
| 4
| Padding
|-
| Y + 0x18
| 8
| File hash table offset
|-
| Y + 0x20
| 4
| File hash table bucket count
|-
| Y + 0x24
| 4
| Padding
|-
| Y + 0x28
| 8
| File allocation table offset
|-
| Y + 0x30
| 4
| File allocation table entry count 
(excluding the leading 0th entry. See below)
|-
| Y + 0x34
| 4
| Padding
|-
| Y + 0x38
| 8
| Data region offset 
(0 for savegame <code>duplicate data = false</code> layout, as the data region is in partition B for that layout)
|-
| Y + 0x40
| 4
| Data region block count 
(= File allocation table entry count)
|-
| Y + 0x44
| 4
| Padding
|-
| Y + 0x48
| 8
| for savegame <code>duplicate data = false</code> layout: directory entry table offset;
otherwise: u32 directory entry table starting block index + u32 directory entry table block count
|-
| Y + 0x50
| 4
| Maximum directory count, excluding the mandatory "root" directory 
(=1 for title database, but that 1 free directory slot is never used)
|-
| Y + 0x54
| 4
| Padding
|-
| Y + 0x58
| 8
| for savegame <code>duplicate data = false</code> layout: file entry table offset;
otherwise: u32 file entry table starting block index + u32 file entry table block count
|-
| Y + 0x60
| 4
| Maximum file count
|-
| Y + 0x64
| 4
| Padding
|}

* For savegames, the file/directory bucket count & maximum count are specified by the parameters of [[FS:FormatSaveData]] or [[FS:CreateSystemSaveData]].
* For extdata, the maximum file/directory count are specified by the parameters of [[FS:CreateExtSaveData]]. The bucket count is likely calculated by the system.
* Directory & file entry tables are allocated in the data region as if they are two normal files (except for savegame <code>duplicate data = false</code> layout). However, only continuous allocation has been observed, so directly reading block_count * block_size bytes from data_region + starting_block_index * block_size should be safe.
* For title database (except for ticket), the range specified for data region seems overflow the file end by 0x80 bytes, which is exactly the size of the pre header. This makes it as if the data region offset should be relative to the pre header instead of the BDRI header. However, further investigation on the directory/file table allocated inside the data region shows that the data region offset is indeed relative to the BDRI header. It might be a bug in 3DS that the title database files miss 0x80-byte space at the end.

== Directory Entry Table ==

The directory entry table is an array of the entry type shown below. It describes the directory hierarchy of the file system. There are two variants of the directory entry type, and a dummy entry type.

=== Savegame/Extdata Variant===
{| class="wikitable" border="1"
! Offset
! Length
! Description
|-
| 0x00
| 4
| Parent directory index. 0 for root
|-
| 0x04
| 16
| ASCII directory name in. All zero for root
|-
| 0x14
| 4
| Next sibling directory index. 0 if this is the last one
|-
| 0x18
| 4
| First subdirectory index. 0 if not exists
|-
| 0x1C
| 4
| First file index in file entry table. 0 for empty directory
|-
| 0x20
| 4
| Padding / zero?
|-
| 0x24
| 4
| Index of the next directory in the same hash table bucket. 0 if this is the last one
|}

=== Title Database Variant===
Because title database only has one directory for "root", its directory entry table degenerates into many zeros whose structure is not recognizable. The size of one entry here is guessed.
{| class="wikitable" border="1"
! Offset
! Length
! Description
|-
| 0x00
| 4
| Parent directory index = 0 for root
|-
| 0x04
| 4
| Next sibling directory index = 0 because this is the last one
|-
| 0x08
| 4
| First subdirectory index = 0 because there is no subdirectory
|-
| 0x0C
| 4
| First file index in file entry table. 0 for empty directory
|-
| 0x10
| 12
| Padding / zero?
|-
| 0x1C
| 4
| Index of the next directory in the same hash table bucket = 0 because there is no other directory
|}

=== Dummy Entry===
There are also some dummy entries in the array
{| class="wikitable" border="1"
! Offset
! Length
! Description
|-
| 0x00
| 4
| Current Total entry count
|-
| 0x04
| 4
| Maximum entry count = maximum directory count + 2
|-
| 0x08
| 28/20
| Padding / All zero
|-
| 0x24/0x1C
| 4
| Index of the next dummy entry. 0 if this is the last one
|}

The 0-th entry of the array is always a dummy entry, which functions as the head of the dummy entry linked list. The 1-st entry of the array is always the root. Therefore maximum entry count is two more than maximum directory count. Dummy entries are left there when deleting directories, and reserved for future use.

== File Entry Table ==

The file entry table is an array of the entry type shown below. It contains information for each file. There are three variants of the file entry type, and a dummy entry type.

=== Savegame Variant ===
{| class="wikitable" border="1"
! Offset
! Length
! Description
|-
| 0x00
| 4
| Parent directory index in directory entry table
|-
| 0x04
| 16
| ASCII file name
|-
| 0x14
| 4
| Next sibling file index. 0 if this is the last one
|-
| 0x18
| 4
| Padding
|-
| 0x1C
| 4
| First block index in data region. 0x80000000 if the file is just created and has no data.
|-
| 0x20
| 8
| File Size
|-
| 0x28
| 4
| Padding?
|-
| 0x2C
| 4
| Index of the next file in the same hash table bucket. 0 if this is the last one
|}

=== Extdata Variant ===
{| class="wikitable" border="1"
! Offset
! Length
! Description
|-
| 0x00
| 4
| Parent directory index in directory entry table
|-
| 0x04
| 16
| ASCII file name
|-
| 0x14
| 4
| Next sibling file index. 0 if this is the last one
|-
| 0x18
| 4
| Padding
|-
| 0x1C
| 4
| Always 0x80000000
|-
| 0x20
| 8
| Unique identifier. See [[Extdata]]
|-
| 0x28
| 4
| Padding?
|-
| 0x2C
| 4
| Index of the next file in the same hash table bucket. 0 if this is the last one
|}

=== Title database Variant ===

{| class="wikitable" border="1"
! Offset
! Length
! Description
|-
| 0x00
| 4
| Parent directory index in directory entry table
|-
| 0x04
| 8
| Title ID
|-
| 0x0C
| 4
| Next sibling file index. 0 if this is the last one
|-
| 0x10
| 4
| Padding
|-
| 0x14
| 4
| First block index in data region.
|-
| 0x18
| 8
| File size
|-
| 0x20
| 8
| Padding?
|-
| 0x28
| 4
| Index of the next file in the same hash table bucket. 0 if this is the last one
|}

=== Dummy Entry ===
Like directory entry table, file entry table also has some dummy entries:

{| class="wikitable" border="1"
! Offset
! Length
! Description
|-
| 0x00
| 4
| Current total entry count
|-
| 0x04
| 4
| Maximum entry count = maximum file count + 1
|-
| 0x08
| 36/32
| Padding / All zero
|-
| 0x2C/0x28
| 4
| Index of the next dummy entry. 0 if this is the last one
|}

The 0-th entry of the array is always a dummy entry, which functions as the head of the dummy entry linked list. Therefore maximum entry count is one more than maximum file count. Dummy entries are left there when deleting files, and reserved for future use.

== Directory Hash Table &amp; File Hash Table ==

This is a u32 array of size = bucket count, each of which is an index to the directory / file entry table. The directory / file name is hashed and its entry index is put to the corresponding bucket. If there is already a directory/file entry in the bucket, then it appends to the linked list formed by <code>Index of the next directory/file in the same hash table bucket</code> field in the directory/file entry table. i.e. this is a hash table using separate chaining with linked lists

The hash function takes the parent index and the ASCII name (or title ID for title database) as key. The function is equivalent to

<pre>uint32_t GetBucket(
    uint8_t name[16 or 8], // For savegame/extdata, this takes all 16 bytes including trailing zeros; For title database, this is the 8-byte title ID
    uint32_t parent_dir_index,
    uint32_t bucket_count
) {
    uint32_t hash = parent_dir_index ^ 0x091A2B3C;
    for (int i = 0; i &lt; 4 or 2; ++i) {
        hash = (hash &gt;&gt; 1) | (hash &lt;&lt; 31);
        hash ^= (uint32_t)name[i * 4]
        hash ^= (uint32_t)name[i * 4 + 1] &lt;&lt; 8
        hash ^= (uint32_t)name[i * 4 + 2] &lt;&lt; 16
        hash ^= (uint32_t)name[i * 4 + 3] &lt;&lt; 24
    }
    return hash % bucket_count;
}
</pre>

== File Allocation Table ==

The file allocation table is an array of a 8-byte entry shown below. The array size is actually ''one larger than'' the size recorded in the filesystem header. Each entry corresponds to a block in the data region (the block size is defined in filesystem header). However, the 0th entry corresponds to nothing, so the corresponding block index is off by one. e.g. entry 31 in this table corresponds to block 30 in the data region.

{| class="wikitable" border="1"
! Offset
! Length
! Description
|-
| 0x00
| 4
| bit[0:30]: Index U; bit[31]: Flag U
|-
| 0x04
| 4
| bit[0:30]: Index V; bit[31]: Flag V
|}

Entries in this table forms several chains, representing how blocks in the data region should be linked together. However, unlike normal FAT systems, which uses chains of entries, 3DS savegames use chain of ''nodes''. Each node spans one or multiple entries.

One node spanning <code>n</code> entries starting from <code>FAT[k]</code> is in the following format:

<pre>FAT[k + 0]:
    Index_U = index of the first entry of the previous node. 0 if this is the first node.
    Index_V = index of the first entry of the next node. 0 if this is the last node.
    Flag_U set if this is the first node.
    Flag_V set if this node has multiple entries.

FAT[k + 1]:
    Index_U = k (the first entry index of this node)
    Index_V = k + n - 1 (the last entry index of this node)
    Flag_U always set
    Flag_V always clear

FAT[k + 2] ~ FAT[k + n - 2]:
    All these entries are uninitialized

FAT[k + n - 1]:
    Index_U = k
    Index_V = k + n - 1
    Flag_U always set
    Flag_V always clear
    (Same values as FAT[k + 1])
</pre>
* Note: all indices above are entry indices (block index + 1)

All free blocks that are not allocated to any files also form a node chain in the allocation table. The head index of this &quot;free chain&quot; is recorded in <code>FAT[0].Index_V</code>. Other fields of <code>FAT[0]</code> are all zero

Here is an example: 

[[File:Disa-fat.png]]

For extdata, because only two "files" (directory and file entry tables) are allocated in the data region, and their size never changes once the extdata is created, they are guaranteed continuous in the data region, and the FAT degenerates to two big nodes. Therefore, instead of going through FAT, the offset and size of directory / file entry table can be found directly by offset = entry_table_starting block * data_region_block_size + data_region_offset and size = entry_table_block_count * data_region_block_size.

==Tools==
https://github.com/wwylele/3ds-save-tool