NDS Format

This is a currently in-progress understanding of how NDS files are formatted to help guide me along the process of making this program functional. A lot of information regarding the header and the basics of the NitroROM File System where thanks to this guide.

Overview

This is an overview of how the NDS format looks on a very basic level. Values that are stored in the header will be noted as Header (offset). They are 4 byte little endian integers unless otherwise noted.

Name	Purpose	Start	Size
Header	Stores information about the file	0h	200h
Padding	Zero padding until 4000h	200h	3E00h
ARM9 ROM	ARM9 boot code (?)	Header 20h	Header 2Ch
ARM9 Overlay	ARM9 overlay table	Header 50h	Header 54h
Overlay Files (?)	Overlay entries placed here (?)	?	?
ARM7 ROM	ARM7 boot code (?)	Header 30h	Header 3Ch
ARM7 Overlay	ARM7 overlay table	Header 58h	Header 5Ch
File Name Table	Directory and file name information	Header 40h	Header 44h
File Allocation Table	File offset information	Header 48h	Header 4Ch

Notes

ARM code and padding

After the header is zero padding up to the 4000h byte mark. This is because ARM9 code must start at at least 4000h into the file and must be on a 1000h byte mark. ARM7 code should be at least at 8000h and also be on a 1000h byte mark, although there are working examples where it is on a 100h byte mark instead.

Padding after the header is usually filled with zeroes, but in many other places in the file FFh is used for padding instead. For example, when padding the file to the nearest power of two size FFh is used instead of 00h.

Overlays

Overlay files can be found in a ROM by finding the starting file id in the root node of the FNT. Any entry in the FAT that falls under the starting file id is an overlay file. These files vary in size and as of now their purpose is unknown. From my testing it seems it is best to put the overlay files exactly where they used to be in the file by reading the old FAT.

Here is a pseudo code example:

int start_file_id = FNT.root().file_id();

for (int i = 0; i < start_file_id; i++)
{
  //  Get FAT entry at index i and use it to read the overlay from the ROM
}

Header

Address	Bytes	Explanation
000h	12h	Game Title
00Ch	4h	Gamecode
010h	2h	Makercode
012h	1h	Unitcode
013h	1h	Encryption Seed Select
014h	1h	Devicecapacity
015h	9h	Reserved
01Eh	1h	ROM Version
01Fh	1h	Autostart
020h	4h	ARM9 rom_offset
024h	4h	ARM9 entry_address
028h	4h	ARM9 ram_address
02Ch	4h	ARM9 size
030h	4h	ARM7 rom_offset
034h	4h	ARM7 entry_address
038h	4h	ARM7 ram_address
03Ch	4h	ARM7 size
040h	4h	File Name Table
044h	4h	File Name Table
048h	4h	File Allocation Table
04Ch	4h	File Allocation Table
050h	4h	File ARM9 overlay_offset
054h	4h	File ARM9 overlay_size
058h	4h	File ARM7 overlay_offset
05Ch	4h	File ARM7 overlay_size
060h	4h	Port 40001A4h setting for normal commands
064h	4h	Port 40001A4h setting for KEY1 commands
068h	4h	Icon_title_offset
06Ch	2h	Secure Area Checksum, CRC-16 of [ [20h]..7FFFh]
06Eh	2h	Secure Area Loading Timeout
070h	4h	ARM9 Auto Load List RAM Address
074h	4h	ARM7 Auto Load List RAM Address
078h	8h	Secure Area Disable
080h	4h	Total Used ROM size
084h	4h	ROM Header Size
088h	38h	Reserved
0C0h	9Ch	Nintendo Logo
15Ch	2h	Nintendo Logo Checksum (CF56h)
15Eh	2h	Header Checksum, CRC-16 of [000h-15Dh]
160h	4h	Debug rom_offset
164h	4h	Debug size
168h	4h	Debug ram_address
16Ch	4h	Reserved
170h	90h	Reserved