This page describes the format and encryption of extdata, “extra data” stored on SD card and NAND, at:

• nand/data/<ID>/extdata/<ExtdataID-High>
• sdmc/Nintendo 3DS/<ID0>/<ID1>/extdata/<ExtdataID-High>

ExtdataID-High is always 00000000 for SD, and always 00048000 for NAND. Regular apps can only mount SD extdata using the same extdataID which is stored in the CXI exheader. Therefore, regular apps which have the exheader extdataID set to zero can’t use extdata. This restriction doesn’t apply for shared extdata with extdataID high bitmask 0x48000 stored on NAND. System apps with a certain access right can mount arbitrary extdata. All NAND extdata is shared extdata, while all SD extdata is normal extdata.

All data in this page is little-endian. All “unused / padding” fields can contain uninitialized data unless otherwise specified.

To avoid confusion, the terms device directory / file and virtual directory / file are used with the following meanings:

• Device directory / file are the real directory / file stored on SD / NAND that can be seen under path nand/data/<ID>/extdata/ or sdmc/Nintendo 3DS/<ID0>/<ID1>/extdata/.
• Virtual directory / file are directory / file stored inside extdata virtual file system, which can be seen by applications in the mounted extdata archives.

An extdata consists of several device directories and files, which forms a file system consisting of multiple virtual directories and files.

An extdata with ID ExtdataId has the following device files:

• .../extdata/<ExtdataID-High>/<ExtdataId-Low>/Quota.dat (optional)
• .../extdata/<ExtdataID-High>/<ExtdataId-Low>/<SubDirID>/<SubFileID>

Note:

• All device files are DIFF containers. All format description below is about the inner content of the containers. Please unwrap these files first according to the DIFF format description before reading them using the extdata format description below.
• Quota.dat is only observed existing for NAND shared extdata.
• <SubDirID> and <SubFileID> are 8-digit hex strings.
• Device file with SubDirID = SubFileID = 00000000 doesn’t exist. Other ID combinations can exists.
• Device file with SubDirID = 00000000 and SubFileID = 00000001 is the VSXE metadata file and must exist.
• Other files, besides Quota.dat and 00000000/00000001, are normal sub files, are these device files one-to-one correspond to virtual files. They contain raw virtual file data in the DIFF inner content.
• SubDirID = 00000000 is usually the only one device directory that can be seen. See Device Directory Capacity for more information.

The inner data of Quota.dat is 0x48 bytes with the following format. The file seems to limit the extdata total size.

A device directory in an extdata (a <SubDirID> directory) seems to have a maximum number of device files it can contain. For SD extdata, this maximum number seems to be hard-coded as 126. For NAND extdata, the number is probably indicated by a field in Quota.dat, which is, again, always 126 as observed. 3DS FS tries to put all device files in the device directory 00000000 if possible, and only when more than 126 files needed to add, a second device directory 00000001 and so on are created. However, few extdata have such amount of files to store, so the behavior lacks of use cases to confirm.

The number 126 is probably from some kind of other capacity of 128 with "." and ".." entries reserved. It is theorized that this is to keep a FAT directory table, with 0x20 bytes for each entry, in one 0x1000 cluster. The motivation is unclear.

This is one variant of the FAT filesystem. Please refer to its page for the description of the filesystem. In general, device file 00000000/00000001 contains the metadata of the filesystem, while other device files (except for the Quota file) contains normal sub-files

Each non-dummy file entry corresponds to a device file. The path to the device file is generated by the following computation:

// See previous section about this capacity
const uint32_t device_dir_capacity = 126;

// entry index is the index in the file entry table, with the first dummy entry as
// index = 0, which is never used for a real file.
// file_index = 1 is reserved for the VSXE Filesystem Metadata itself, so real files
// started from file_index = 2.
uint32_t file_index = entry_index + 1;

uint32_t SubDirID = file_index / device_dir_capacity;
uint32_t SubFileID = file_index % pdevice_dir_capacity;

char extdata_path[...]; // ".../extdata/<ExtdataID-High>/<ExtdataId-Low>"
char device_path[...]; // output path
sprintf(device_path, "%s/%08x/%08x", extdata_path, SubDirID, SubFileID);

When mounting extdata, the unique identifier is used to match the ID stored in subfile’s DIFF header. If the ID doesn’t match, mounting will fail.

When extdata is created, these are always created regardless of whether the title actually uses them.

• /icon This virtual file contains the extdata icon displayed in data management. This icon can only be written to by titles when creating extdata, titles would have to recreate extdata to change the icon. This file can’t be read directly, instead it is read via FS:ReadExtSaveDataIcon.
• /user/ This virtual directory contains the title’s actual extdata files.
• /boss/ This virtual directory can contain SpotPass content. SpotPass content can only be downloaded to this /boss virtual directory.

User extdata and SpotPass extdata use separate mount points at /user and /boss. Therefore one mount can’t access the other virtual directory, and also can’t access /icon.(The title’s SpotPass extdata can be mounted by the title itself, if it uses SpotPass)

Other optional but notable directories include:

• /user/ExBanner This virtual directory can optionally store extended banners. When this is available, this banner is displayed instead of the CXI ExeFS banner. COMMON.bin stores the common exbanner, while <regionlang_code>.bin stores an optional separate region/language specific banner.(regionlang_code can be “JPN_JP”, “USA_EN”, etc)

Usually the ExtdataID low is in the format ‘00’

The above date stores the last time new Play Coin(s) were obtained. The contents of this file is updated by home-menu. PTM:GetTotalStepCount is not checked constantly, after home-menu boot this is only checked when waking from sleep-mode. Each time home-menu updates the contents of this file, home-menu will set the Play Coin total to 300 if it’s higher than the 300 Play Coin limit.

Home Menu loads this file / opens this archive during startup. When accessing this file fails, like when the file/archive is corrupted(or at least on older system-versions), the result is a brick due to Home Menu using svcBreak. Yellows8 bricked a 3DS this way due to corruption via invalid FSFile:Write flush flags. When opening this extdata archive(0xf000000b) fails, Home Menu executes svcBreak.

List of blocked users.

Empty space is filled with 0xC-long sequences of 00 00 … 07

• 🔗 3ds-save-tool - Extract/verifies extdata