Let's examine the 6 data bytes (which follow the MThd header) in an MThd chunk.

The first two data bytes tell the Format (which I prefer to call "type"). There are actually 3 different types (ie, formats) of MIDI files. A type of 0 means that the file contains one single track containing midi data on possibly all 16 midi channels. If your sequencer sorts/stores all of its midi data in one single block of memory with the data in the order that it's "played", then it should read/write this type. A type of 1 means that the file contains one or more simultaneous (ie, all start from an assumed time of 0) tracks, perhaps each on a single midi channel. Together, all of these tracks are considered one sequence or pattern. If your sequencer separates its midi data (i.e. tracks) into different blocks of memory but plays them back simultaneously (ie, as one "pattern"), it will read/write this type. A type of 2 means that the file contains one or more sequentially independant single-track patterns. If your sequencer separates its midi data into different blocks of memory, but plays only one block at a time (ie, each block is considered a different "excerpt" or "song"), then it will read/write this type.

The next 2 bytes tell how many tracks are stored in the file, NumTracks. Of course, for format type 0, this is always 1. For the other 2 types, there can be numerous tracks.

The last two bytes indicate how many Pulses (i.e. clocks) Per Quarter Note (abbreviated as PPQN) resolution the time-stamps are based upon, Division. For example, if your sequencer has 96 ppqn, this field would be (in hex):

00 60

NOTE: The 2 bytes that make up the Division are stored in (Motorola) "Big Endian" byte order, not (Intel) "Little Endian" reverse byte order. The same is true for the NumTracks and Format.

Alternately, if the first byte of Division is negative, then this represents the division of a second that the time-stamps are based upon. The first byte will be -24, -25, -29, or -30, corresponding to the 4 SMPTE standards representing frames per second. The second byte (a positive number) is the resolution within a frame (ie, subframe). Typical values may be 4 (MIDI Time Code), 8, 10, 80 (SMPTE bit resolution), or 100.

You can specify millisecond-based timing by the data bytes of -25 and 40 subframes.

Here's what an MThd chunk looks like if you defined it in C:

struct MTHD_CHUNK
{
   /* Here's the 8 byte header that all chunks must have */
   char           ID[4];  /* This will be 'M','T','h','d' */
   unsigned long  Length; /* This will be 6 */

   /* Here are the 6 bytes */
   unsigned short Format;
   unsigned short NumTracks;
   unsigned short Division;
};

And here's an example of a complete MThd chunk (with header) if you examined it in a hex editor:

4D 54 68 64     MThd ID
00 00 00 06     Length of the MThd chunk is always 6.
00 01           The Format type is 1.
00 02           There are 2 MTrk chunks in this file.
E7 28           Each increment of delta-time represents a millisecond.