BMG file format

BMG (likely standing for "binary message"/"basic message") is a format used by several DS games (although only in first-party Nintendo games?).

Note: the format is also used in Wii games (and apparently GameCube as well), here only the format found in DS games is described. Check here for more Wii-related details of the format.

• INF1 section
  • Message entry
• DAT1 section
  • Message data
  • Escape sequences
• MID1 section

BMGs start like usual DS formats, containing a slightly different common header:

Offset	Size	Description
0x00	0x08	Magic (0x31676D624753454D = "MESGbmg1")
0x08	0x04	Total file size
0x0C	0x04	Section count
0x10	0x01	Text encoding (CP-1252 = 1, UTF-16 = 2, Shift JIS = 3, UTF-8 = 4)
0x11	0x1	Unknown (usually zero)
0x12	0x2	Unknown (usually zero)
0x14	0x4	Unknown (usually zero)
0x18	0x4	Unknown (usually zero)
0x1C	0x4	Unknown (usually zero)

All BMG files seem to have INF1 and DAT1 sections, optionally having a MID1 section (thus section count is 2 or higher).

The header is followed by the following sections (in order, in case they are present):

INF1 section

This section starts with a common header and has the following structure:

Offset	Size	Description
0x00	0x04	Block magic (0x31464E49 = "INF1")
0x04	0x04	Total block size
0x08	0x02	Message count
0x0A	0x02	Message entry size
0x0C	0x04	File ID
0x10	Message count * Message entry size	Message entries

Message entry

Message entries have the following structure:

Offset	Size	Description
0x00	0x04	Message data offset
0x04	Message entry size - 0x04	Additional attributes

The message data offset is an offset relative to past DAT1 section header (thus relative to the start of the section data).

In general games have no additional attributes in messages, hence message entries typically only contain the message data offset.

This section has (always?) end zero-byte padding to be 0x20-aligned.

DAT1 section

This section starts with a common header and has the following structure:

Offset	Size	Description
0x00	0x04	Block magic (0x31544144 = "DAT1")
0x04	0x04	Total block size
0x08	Variable	Message data

This section also has (always?) end zero-byte padding to be 0x20-aligned.

Message data

Message content has two kinds of possible data: plain text and escape sequences used to encode binary data.

Escape sequences (at least for UTF-8 and UTF-16, unknown for the others) have the following structure:

Offset	Size	Description
0x00	Character size (1 = UTF-8, 2 = UTF-16, etc)	Escape character '\u001A'
Character size	0x01	Total sequence size
Character size + 0x01	Total sequence size - Character size - 0x01	Encoded bytes

Every character read outside of escape sequences is treated as plain message text. These are read until a null character is found, which indicates the end of the message data.

For example, message data 41 00 41 00 1A 00 06 12 34 56 53 00 53 00 00 00 corresponds to an UTF-16 message with: plain text "AA", escape with bytes 12 34 56, plain text "BB" (finishing with the corresponding null character).

Escape sequences

Escape sequences are used for special text formatting. Common use cases are changing text color, formatting in-game strings...

Sequences differ between encodings: sequence formats in UTF-8 and UTF-16 BMG files

MID1 section

This section starts with a common header and has the following structure:

Offset	Size	Description
0x00	0x04	Block magic (0x3144494D = "MID1")
0x04	0x04	Total block size
0x08	0x02	Message ID count
0x0A	0x01	Unknown
0x0B	0x01	Unknown
0x0C	0x04	Unknown
0x10	0x04 * Message ID count	Message IDs

This section also has (always?) end zero-byte padding to be 0x20-aligned.

The message ID count is the same as the message count in the previous INF1 section. IDs are apparently used in games with multiple BMG files, where the same message is present in several BMG files, where the ID is probably used to access the message (since the message is probably in different indexes in the data section).