U8 Archive

U8 Archive is a hierarchical archive format developed by Nintendo during the GameCube era and was predominantly used on the Wii.

Titles using U8 Archive

• Sonic Heroes (PlayStation 2) - released 2004
• Sonic the Hedgehog (Xbox 360, PlayStation 3) - released 2006
• Sonic and the Secret Rings^† (Wii) - released 2007
• Sonic and the Black Knight^† (Wii) - released 2009
• Sonic Colours (Wii) - released 2010
• Sonic Colours: Ultimate (Windows, Xbox One, PlayStation 4, Switch) - released 2021

^† — used for operating system only.

Specification

Header

The header consists of the following data structure:

Length: 0x20

Offset	Type	Name	Description
0x00	UInt32	Signature	Always 0x55AA382D (big-endian) or 0x2D38AA55 (little-endian).
0x04	UInt32	FSTableOffset	The offset of the file system table.
0x08	UInt32	FSTableLength	The length of the file system table (including the string pool).
0x0C	UInt32	DataOffset	The offset of the file data.
0x10	UInt32[4]	Reserved	These fields appear to be populated in archives for Sonic the Hedgehog (2006), but this is bogus data from uninitialised memory and should be ignored. For the sake of tooling heuristics, writing 0xE4F91300 (big-endian) to index 0 and 0x78013800 (big-endian) to index 3 would be ideal for detecting whether an archive is from that game specifically.

File System

The file system consists of an array of the following data structure:

Length: 0x0C (or 0x10 for Sonic the Hedgehog (2006))

Offset	Type	Name	Description
0x00	Boolean (UInt8)	IsDirectory	Determines whether this node is a directory.
0x01	UInt24	NameOffset	The offset of this node's name in the string pool. If root, always zero.
0x04	Union (UInt32, UInt32)	DataOffset, ParentIndex	File The offset of this file's data. Directory The index of the parent node for this directory. If root, always zero.
0x08	Union (UInt32, UInt32)	Length, NodeCount	File The length of this file's data. Directory The total number of nodes in this directory. If root, the total number of nodes in this archive.
0x0C	UInt32	UncompressedLength	This field is unique to archives for Sonic the Hedgehog (2006). File The uncompressed length of this file's data. This field can be set to zero to disable compression for this file, allowing its data to be stored uncompressed. Directory Undefined. Much like the reserved bytes in the header, this field will appear to be populated, but again, this is bogus data from uninitialised memory.

The total number of nodes to read is determined by the root node's NodeCount field. As this is a hierarchical file system, nodes should be read recursively to build the file system correctly.

The first node must be a root node defined as a directory, with the NodeCount field set to the total number of nodes in the archive. This is necessary for determining the length of the file system table.

String Pool

The string pool offset can be calculated after reading the root node and using the following expression:

FSTableOffset + (Root.NodeCount * 16)

Each string in the pool is ASCII-encoded and null-terminated, the offsets to which are relative to the start of the pool.

The string pool must have a single null terminator at the beginning for the root node, as it does not have a name.

File Data

Files are aligned by 32 bytes per file after the string pool.

In Sonic the Hedgehog (2006), files may be compressed using zlib. This compression is optional, and files can be stored uncompressed so long as UncompressedLength is set to zero.