RomFS

Overview

RomFS (or Read-Only Filesystem) is part of the NCCH format, and is used as external file storage.

RomFS can be used:

• in conjunction with the ExeFS of a NCCH

• to contain the game manual accessible from the Home Menu

• to contain the DLP Child CIA)

• or to contain game cartridge update data

(There may be more implementations in the future)

Format

The RomFS is wrapped inside a IVFC hash-tree container, and the actual data is structured like a node-based tree, starting at the root level directory node, moving down to other directory and file nodes. Each directory node has pointers to child directory nodes and siblings, together with a pointer to the first file node for that directory. Each file node has pointers to their next file node, together with information about the actual file data.

The RomFS IVFC hash-tree header is 0x5C bytes long and is structured as follows:

START	SIZE	DESCRIPTION
0x00	0x4	Magic "IVFC"
0x04	0x4	Magic number 0x10000
0x08	0x4	Master hash size
0x0C	0x8	Level 1 logical offset
0x14	0x8	Level 1 hashdata size
0x1C	0x4	Level 1 block size, in log2
0x20	0x4	Reserved
0x24	0x8	Level 2 logical offset
0x2C	0x8	Level 2 hashdata size
0x34	0x4	Level 2 block size, in log2.
0x38	0x4	Reserved
0x3C	0x8	Level 3 logical offset
0x44	0x8	Level 3 hashdata size
0x4C	0x4	Level 3 block size, in log2.
0x50	0x4	Reserved
0x54	0x4	Reserved
0x58	0x4	Optional info size.

Level 3 Format

The Level 3 partition of a RomFS consists of a header specifying offsets to four tables, followed by filedata (aligned to 16-bytes). Though their size varies by RomFS contents, the tables are always sequential and in the same order, as follows:

START	SIZE	DESCRIPTION
0x0	0x28	Header
0x28	Varies	Directory HashKey Table
Varies	Varies	Directory Metadata Table
Varies	Varies	File HashKey Table
Varies	Varies	File Metadata Table
Varies	Varies	File Data

Level 3 Header Format

RomFS Header data is always 0x28 bytes long, and follows this layout (offsets are from the start of the header):

START	SIZE	DESCRIPTION
0x0	0x4	Header Length
0x4	0x4	Directory Hash Table Offset
0x8	0x4	Directory Hash Table Length
0xC	0x4	Directory Metadata Table Offset
0x10	0x4	Directory Metadata Table Length
0x14	0x4	File Hash Table Offset
0x18	0x4	File Hash Table Length
0x1C	0x4	File Metadata Table Offset
0x20	0x4	File Metadata Table Length
0x24	0x4	File Data Offset

Directory Metadata Structure

When a RomFS is built, directories are added recursively starting with the root. When a directory is added, all of its files are added to the File Metadata Table, then all of its subdirectories (if any), are added to the table. If any of the directory's subdirectories have their own subdirectories, the current directory's subdirectories are all added before the subdirectories' subdirectories are added.

A Metadata entry for a directory has the following structure (all values are initialized to 0xFFFFFFFF, and remain that way when unused):

START	SIZE	DESCRIPTION
0x0	0x4	Offset of Parent Directory (self if Root)
0x4	0x4	Offset of next Sibling Directory
0x8	0x4	Offset of first Child Directory (Subdirectory)
0xC	0x4	Offset of first File (in File Metadata Table)
0x10	0x4	Offset of next Directory in the same Hash Table bucket
0x14	0x4	Name Length
0x18	Name Length (rounded up to multiple of 4)	Directory Name (Unicode)

File Metadata Structure

A Metadata entry for a file has the following structure (all values are initialized to 0xFFFFFFFF, and remain that way when unused):

START	SIZE	DESCRIPTION
0x0	0x4	Offset of Containing Directory (within Directory Metadata Table)
0x4	0x4	Offset of next Sibling File
0x8	0x8	Offset of File's Data
0x10	0x8	Length of File's Data
0x18	0x4	Offset of next File in the same Hash Table bucket
0x1C	0x4	Name Length
0x20	Name Length (rounded up to multiple of 4)	File Name (Unicode)

Hash Table Structure

For both files and directories, a separate chaining hash table is created for quick lookup.

A hash table consists of a number of buckets, all initialized to 0xFFFFFFFF. The size of the table is dependent on the number of entries in the relevant MetaData table (it's probably intended to always be the smallest prime number greater than or equal to the number of entries, but the implementation was lazy), illustrated by the following code (C#):

		public static byte[] GetHashTableLength(uint numEntries)
		{
			uint count = numEntries;
			if (numEntries < 3)
				count = 3;
			else if (numEntries < 19)
				count |= 1;
			else
			{
				while (count % 2 == 0 
					|| count % 3 == 0 
					|| count % 5 == 0 
					|| count % 7 == 0 
					|| count % 11 == 0 
					|| count % 13 == 0 
					|| count % 17 == 0)
				{
					count++;
				}
			}
			return count;
		}

The hash function is based off directory/file name (byte array taken from Metadata entry) and Parent Directory's offset (C#):

		public static uint CalcPathHash(byte[] Name, uint ParentOffset)
		{
			uint hash = ParentOffset ^ 123456789;
			for (int i = 0; i < Name.Length; i += 2)
			{
				hash = (hash >> 5) | (hash << 27);
				hash ^= (ushort)((Name[i]) | (Name[i + 1] << 8));
			}
			return hash;
		}

Each directory/file is put into the ith bucket, where i is the hash taken modulus of bucket count. The directories/files in the same bucket form a linked list, with the value in hash table as the offset to the head element. When creating the hash table, a latter added directory/file is always added as the head element of the linked list.