Tahaxan

ARC files are Nintendo ARChive files. This is the same format used internally in every ROM to store files, but with a few differences, at least, no magic stamp nor section size.

Example file: /bg/grd_anm.arc from "Animal Crossing: Wild World".

NARC section

Field	Start	End	Size	Example	Description
Magic stamp	0x00000000	0x00000003	4 bytes	"NARC"	Magic stamp. "Nintendo ARChive"
Unknown constant	0x00000004	0x00000007	4 bytes	0x0100FFFE	This always appears with this value
Section size	0x00000008	0x0000000B	4 bytes	0x00002FA0	Size of this section always matches with file size (includes header)
Header size	0x0000000C	0x0000000D	2 bytes	0x0010	This is going to be always 0x0010, because size of the header is always the same
Number of sections	0x0000000E	0x0000000F	2 bytes	0x0003	Number of sections included inside "NARC" section

 

BTAF section

Field	Start	End	Size	Example	Description
Magic stamp	0x00000000	0x00000003	4 bytes	"BTAF"	Magic stamp. "File Allocation Table B?" reversed
Section size	0x00000004	0x00000007	4 bytes	0x0000003C	Section size, including header
Number of files	0x00000008	0x0000000B	4 bytes	0x00000006	Number of files in the archive
Start file offset	0x0000000C	0x0000000F	4 bytes	0x00000000	Starting offset RELATIVE to starting data offset in GMIF section
End file offset	0x00000010	0x00000013	4 bytes	0x0000022C	Ending offset RELATIVE to starting data offset in GMIF section
Two previous offsets repeats 'Number of files' times, so we get starting and end offset for each file in the archive

 

BTNF section

Field	Start	End	Size	Example	Description
Magic stamp	0x00000000	0x00000003	4 bytes	"BTNF"	Magic stamp. "File Name Table B?" reversed
Section size	0x00000004	0x00000007	4 bytes	0x0000006C	Section size, including header
Here starts directory info contents. Each piece of info is 8 bytes wide, and there are one for every directory in the file. We can know beforehand how much of them we have just looking at first offset.
Start directory contents offset	0x00000008	0x0000000B	4 bytes	0x00000010	Starting offset RELATIVE to this first offset. If you follow this offset, you'll find directories and filenames that come inside this one. This first offset also matches with size of directories info
Position of first file in FAT	0x0000000C	0x0000000D	2 bytes	0x0000	This number, multiplied by 4 bytes each offset multiplied by 2 offsets ('Start File Offset' and 'End Offset'), gives the offset to the first entry in the FAT table for first file in this directory
Number of directories / Parent directory	0x0000000E	0x0000000F	2 bytes	0x0002	If this is the first directory entry (root directory), this keeps the number of directories in archive. If's not, this number ANDed with 0x0FFF gives the parent directory number
This 8 bytes repeats for each directory, but we're going to put the next entry just to see differences between root directory and normal ones.
Start directory contents offset	0x00000010	0x00000013	4 bytes	0x00000015	Starting offset RELATIVE to the first offset.
Position of first file in FAT	0x00000014	0x00000015	2 bytes	0x0000	Same example data as previous, because in this example, root directory contents only directories, and FAT offsets are only needed for files
Number of directories / Parent directory	0x00000016	0x00000017	2 bytes	0xF000	In this case, parent directory is 0 (0xF000 & 0x0FFF = 0x0000)
This is the end of directory info contents. Now comes names of directories and files.
Size of name	0x00000018	0x00000019	1 byte	0x00000081	This size also gives type information. If first bit is set (just like in this case), the following name is a directory one, a filename otherwise. So, length in this example is 0x81 & 0x7F = 0x01
Name string	0x0000001A	0x0000001B	'Size of name' bytes	"a"	String containing the name
After a filename, comes another one in this fashion (size plus name) until you find a zero 'Size of name'. But when you find a directory, one more field comes into account.
Directory number	0x0000001C	0x0000001D	2 bytes	0xF001	Number of named directory. Just AND it with 0x0FFF. So, in this example, 0xF001 & 0x0FFF = 0x0001

  

GMIF section

Field	Start	End	Size	Example	Description
Magic stamp	0x00000000	0x00000003	4 bytes	"GMIF"	Magic stamp. Like others, it's likely to be reversed, but don't imagine what this stands for.
Section size	0x00000004	0x00000007	4 bytes	0x00002EE8	Section size, including header
After that, you'll find files data.

 

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