Nitro FS in a nutshell

This document explains how the file system works in DS games.

All games come in the nds file format. This file format has a leading header
with fixed positioned fields. The various pointed to parts of the file come
afterwards. The file system data is two of those parts. More specifically,
the file system uses the file allocation table (FAT for short) and the
file directory data, also known as the file name table (FNT for short).

You can find pointers to these sections in the NDS file header (offset 0x48 for
the FAT and offset 0x40 for the FNT), along with the sizes of the sections.

The FAT is a traditional start pointer (4 byte) + length table (4 byte) for each of the files
in the file system. The pointers are relative to the start of the NDS file.

The FNT is composed by one main list of all the directories followed by
file name lists for each of the directories. The main list is composed of records
with three fields, the offset to the name list (4 byte pointer, relative to
the start of the FNT), the id of the first file in the directory (2 byte int)
and the id of the parent folder (2 byte int).

The first record is for the root folder of the file system and uses the
parent folder id field as the number of directories instead.

Each pointed to file name list contains variable length records for
each item in the directory. First in each record comes a byte where the
low 7 bits is the file name length and the high 8 bit is a flag if it is
a sub directory or a file. After the flag/length byte comes just that many
characters of text that forms the file name. There is no terminating character;
the length is the only way to know where the file name ends. For directory
records follows a two byte directory id. File records have no such id field
and instead get incremental ids based on the first id field from the main
directory list. The record ends after the possible sub dir id.

The entry in the directory list corresponding to a given subdirectory id
can be found by clearing the uppermost nibble in the 2 byte directory id
to zeros and using that as the index in the directory list.
Remember that each such entry is 8 bytes long.

Sources:
GBATEK – The definitive guide to all things DS related

Written by henke37 2011.