BYML

BYML is a simplified, binary form of YAML.

Only the version 2+ format is documented in this article.

Structure

The file is broken up into a node structure, with possible interlinking between the nodes. Each node has a one byte format identifier. The File begins with a header which points to three special nodes; the hash key table; the string table node and the root node.

Header

Offset	Size	Description
0x00	2	“BY” (big endian) or “YB” (little endian).
0x02	2	Version 2 in Breath of the Wild, Versions 4 to 7 in Tears of the Kingdom. 1 and 3 are also valid version numbers.
0x04	4	Offset to the hash key table, relative to start (usually 0x010). May be 0 if no hash nodes are used. Must be a string value node (0xc2).
0x08	4	Offset to the string table, relative to start. May be 0 if no strings are used. Must be a string value node (0xc2).
0x0c	4	Offset to the root node, relative to start. May be 0 if the document is totally empty. Must be either an array node (0xc0) or one of the hash nodes (0x20, 0x21 or 0xc1).

Nodes

Every node format has a unique one byte identifier as follows. Some nodes are considered value nodes as indicated below. The container nodes have a longer encoding in the file, which must be four byte aligned. The order of encoding full nodes within the file does not seem to matter.

Identifier	Type	Description	Version
0x20	Container	Plain hash. Node is a mapping from 32-bit hashes to other nodes.	7+
0x21	Container	Value hash. Node is a mapping from 32-bit hashes to other nodes and a second 4 byte value.	7+
0xA0	Value (Index)	String. Value is an index into the string table.	2+
0xA1	Value (Special)	Binary data.	4+
0xA2	Value (Special)	File data. Node contains the binary data of another file.	5+
0xC0	Container	Array. Node is an array of nodes, typically, though not necessarily, all of the same format.	2+
0xC1	Container	String hash. Node is a mapping from strings in the hash key table to other nodes.	2+
0xC2	Container (Special)	String table. Special purpose node type only seen in the hash key table and the string table.	2+
0xD0	Value	Bool. Node is 1 or 0 representing true or false respectively.	2+
0xD1	Value	Int. Node is a signed 32 bit integer value.	2+
0xD2	Value	Float. Node is a binary32 floating-point number.	2+
0xD3	Value	UInt. Node is an unsigned 32 bit integer value. The game uses this for some CRC32 hashes and for masks.	2+
0xD4	Value (Special)	Int64. Node is a 64 bit integer value.	3+
0xD5	Value (Special)	UInt64. Node is an unsigned 64 bit integer value.	3+
0xD6	Value (Special)	Double. Node is a binary64 floating-point number.	3+
0xFF	Value	Null. Value is always 0.	3+

Value Nodes

Value nodes can only be encoded as children of container nodes. Each value node has a direct four byte encoding.

For string nodes, this encoding is simply a four byte index into the string table respectively.

For special value nodes, the value is an offset relative to the start of the file.

0x20 - Hash Node

Hash / dictionary nodes are used to encode name value collections. Entries must sorted by their hash value.

Offset	Size	Description
0x00	1	0x20 node type.
0x01	3	Number of entries in dictionary.
0x04	8*N	Dictionary entries.
0x04+8*N	1*N	Array of N types, on for each entry in the dictionary. This is padded to 4 bytes.

Each entry in the dictionary has the following structure.

Offset	Size	Description
0x00	4	Hash. A 32-bit hash of the key.
0x04	4	Value. For regular value nodes, this is the 4 byte node value. For other nodes, this is a 4 byte offset to the node relative to the start of the file.

0x21 - Value Hash Node

This type is the similar to the hash node with the addition of a 4-byte value in each dictionary entry.

Offset	Size	Description
0x00	4	Value. For regular value nodes, this is the 4 byte node value. For other nodes, this is a 4 byte offset to the node relative to the start of the file.
0x04	4	Hash. A 32-bit hash of the key.
0x08	4	Unknown. Always 0 in Tears of the Kingdom.

0xA1 - Binary Data Node

Offset	Size	Description
0x00	4	Data length
0x04	variable	Data

0xA2 - File Node

Offset	Size	Description
0x00	4	File length
0x04	4	Unknown. Always 0x1000.
0x08	variable	File data

0xC0 - Array Node

Offset	Size	Description
0x00	1	0xC0 node type.
0x01	3	Number of entries in array.
0x04	N	Array of N node types; the type of each element in the array.
0x04 + N’	4*N	Array of N node values. For regular value nodes, this is a 4 byte node value. For other nodes, this is a 4 byte offset to the node relative to the start of the file.

N’ is N rounded up to the nearest multiple of 4.

0xC1 - String Hash Node

Hash / dictionary nodes are used to encode name value collections. Entries must be lexicographically sorted.

Offset	Size	Description
0x00	1	0xC1 node type.
0x01	3	Number of entries in dictionary.

What then follows is a variable length array of dictionary entries. Each entry has the following structure.

Offset	Size	Description
0x00	3	Name. Value is an index in the hash key table.
0x03	1	The node type.
0x04	4	Value. For regular value nodes, this is the 4 byte node value. For other nodes, this is a 4 byte offset to the node relative to the start of the file.

0xC2 - String Table Node

Offset	Size	Description
0x00	1	0xC2 node type.
0x01	3	Number of entries in the string table.
0x04	Y=4*(N+1)	Array of N+1 offsets to each string relative to the start of the node. The last entry is an offset to the end of the last string.
0x04+Y’	variable	Array of N null-terminated strings stored in alphabetical order.

Y’ is Y rounded up to the nearest multiple of 4.

Usage in Nintendo games

Game	Endianness	Version	Notes
Breath of the Wild (Wii U)	Big endian	2	Uses SMO's library (in al::)
Breath of the Wild (Switch)	Little endian	2	Uses SMO's library (in al::), but with unused code and support for big endian eliminated
Super Mario Odyssey (Switch)	Little endian (supports big endian as well)	3
Splatoon 2 (Switch)	Little endian (supports big endian as well)	3?	Uses SMO's library, but in the Lp::Utl:: namespace
Animal Crossing: New Horizons (Switch)	Little endian (supports big endian as well)	4

The v3 format seems to be identical to v2 but with the 64-bit node types added.

Extensions

Nintendo uses several file extensions for BYML files in BotW. The basic format is the same even though each kind has a different purpose and contains different parameters. Known extensions:

• .baischedule
• .baniminfo
• .bgdata
• .bgsvdata
• .bquestpack
• .byml
• .byaml
• .mubin

TotK has added the following extensions:

• .bgyml

Tools

Tool	Cross-platform	Setup	Known issues
byml-v2	Yes	Install the byml package. [CLI help]	None
ninten-file-tool	Yes	Install the Java runtime environment (JRE). Download the latest release from GitHub.	Wii U only: Only works with big endian files and unable to handle Switch (little endian) documents. Incomplete support for newer node types: Newer node types (0xd4, 0xd5, 0xd6, 0xff) are not supported. In practice, this is not an issue for Breath of the Wild as these are not known to be used in the game.
Byaml Editor	Yes
Wild Bits	Yes	Ensure you have a 64 bit version of Python 3.7+ before continuing. Windows users must also install the latest x64 Visual C++ redistributable. You will not be able to install or launch Wild Bits otherwise. See Help:Setting_up_tools. (Optional) For best results on Windows, install cefpython3. [CLI help] Install the wildbits package. [CLI help]	On some system configurations, launching with wildbits doesn't work and you must use python -m wildbits instead.
Switch Toolbox	No	Simply download the latest compiled release from GitHub, unzip, and launch the exe. Note that the latest release has an old date, but it is current, as it automatically rebuilds with each commit.

This article was adapted from documentation in the handsomematt/botw-modding repo.