The bulk of information for this article comes from public domain sources, in particular The MUS File Format by Vladimir Arnost. The rest was filled in through various sources and contributions. As such, even though most articles on this site are licensed under the Creative Commons Attribution license, this article is here by released into the public domain.

The MUS file format is similar to the MIDI file format. It consists of a header, a variable length list of instruments used followed by music events. Unlike the MIDI file, the MUS file has only a single track.

The header always starts with a magic value 'MUS' 0x1A. Following the magic value is a 2 byte length and 2 byte offset value. The offset specifies the offset from the beginning of the file where the actual midi commands begin. The length specifies the length of the score from the offset to the end of the file. Because the length is a 2 byte value the score is limited to 64KB.

The channels field specifies the number of channels that are used in the score not including channel 15. The sec_channels field specifies the number of secondary channels used. num_instruments indicates the number of instruments contained in the variable length instruments list.

4D 55 53 1A ll ll oo oo cc cc ss ss nn nn dd dd

struct mus_header {
    unsigned int   id;              // identifier "MUS" 0x1A
    unsigned short length;          // the length of the score
    unsigned short offset;          // score offset from beginning of file
    unsigned short channels;	    // count of primary channels
    unsigned short sec_channels;    // count of secondary channels
    unsigned short num_instruments; // number of instruments in variable length section
    unsigned short dummy;
 
    unsigned short instruments[];   // variable-length list of instruments
};

Because the length and offset specify the beginning of the score, extra space may be included between the last instrument in the instruments list and the beginning of the score to store other information such as author and copyright info. There is no standard format for this information and readers may or may not recognize it.

It is important to note that these 16 bit values are in little-endian format unlike MIDI which uses big-endian for integer values.

+--7----6----5----4----3----2----1----0-+
|Last|  Event type  |  Channel number   |
+---------------------------------------+

The MUS body is a single track of grouped sound events and time values. An event begins with a byte similar to a MIDI event. The left nibble (4 bits) contain the command in the three least significant bits. The most significant bit is 0 if more events follow in the group or 1 if the event is the last in the group and a time value follows. Events in a group are to be executed simultaneously. The time value specifies the number of ticks to wait before processing the next event (group).

The right nibble, 4 least significant bits of the event specifies the channel to which the event applies. Each instrument is assigned a channel which is used throughout the score. Channels 0 through 14 may be used for any instruments. Channel 15 is exclusively for percussion.

Following the event byte is one or more data bytes. The following table describes each event and the data that follows.

The following table lists the numbers corresponding to notes for use in note on and note off commands.

This is a list of 128 sounds, with corresponding MIDI program numbers. Most of these are imitative sounds, though the list includes synth sounds, ethnic instruments and a handful of sound effects.

The sounds fall roughly into sixteen families of eight variations each. Grouping sounds makes it easy to re-orchestrate a piece using similar sounds. The instrument map isn't the final word on musical instruments of the world, but it's pretty complete.

This mapping derives from the Roland/Sequential mapping used on early drum machines. As with the Instrument Map, it doesn't cover every percussive instrument in the world, but it's more than adequate as a basic set.

In channel #15, the note number does not affect the pitch but the instrument type. The default pitch for percussions is 60 (C-5).

• "The MUS File Format" - Vladimir Arnost

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