The MIDI protocol is made up of messages. A message consists of a string (ie, series) of 8-bit bytes. MIDI has many such defined messages. Some messages consist of only 1 byte. Other messages have 2 bytes. Still others have 3 bytes. One type of MIDI message can even have an unlimited number of bytes. The one thing that all messages have in common is that the first byte of the message is the Status byte. This is a special byte because it's the only byte that has bit #7 set. Any other following bytes in that message will not have bit #7 set. So, you can always detect the start a MIDI message because that's when you receive a byte with bit #7 set. This will be a Status byte in the range 0x80 to 0xFF. The remaining bytes of the message (ie, the data bytes, if any) will be in the range 0x00 to 0x7F. (Note that I'm using the C programming language convention of prefacing a value with 0x to indicate hexadecimal).

The Status bytes of 0x80 to 0xEF are for messages that can be broadcast on any one of the 16 MIDI channels. Because of this, these are called Voice messages. (My own preference is to say that these messages belong in the Voice Category). For these Status bytes, you break up the 8-bit byte into 2 4-bit nibbles. For example, a Status byte of 0x92 can be broken up into 2 nibbles with values of 9 (high nibble) and 2 (low nibble). The high nibble tells you what type of MIDI message this is. Here are the possible values for the high nibble, and what type of Voice Category message each represents: 


8 = Note Off
9 = Note On
A = AfterTouch (ie, key pressure)
B = Control Change
C = Program (patch) change
D = Channel Pressure
E = Pitch Wheel
So, for our example status of 0x92, we see that its message type is Note On (ie, the high nibble is 9). What's the low nibble of 2 mean? This means that the message is on MIDI channel 2. There are 16 possible (logical) MIDI channels, with 0 being the first. So, this message is a Note On on channel 2. What status byte would specify a Program Change on channel 0? The high nibble would need to be C for a Program Change type of message, and the low nibble would need to be 0 for channel 0. Thus, the status byte would be 0xC0. How about a Program Change on channel 15 (ie, the last MIDI channel). Again, the high nibble would be C, but the low nibble would be F (ie, the hexademical digit for 15).Thus, the status would be 0xCF.

NOTE: Although the MIDI Status byte counts the 16 MIDI channels as numbers 0 to F (ie, 15), all MIDI gear (including computer software) displays a channel number to the musician as 1 to 16. So, a Status byte sent on MIDI channel 0 is considered to be on "channel 1" as far as the musician is concerned. This discrepancy between the status byte's channel number, and what channel the musician "believes" that a MIDI message is on, is accepted because most humans start counting things from 1, rather than 0.

The Status bytes of 0xF0 to 0xFF are for messages that aren't on any particular channel (and therefore all daisy-chained MIDI devices always can "hear" and choose to act upon these messages. Contrast this with the Voice Category messages, where a MIDI device can be set to respond to those MIDI messages only on a specified channel). These status bytes are used for messages that carry information of interest to all MIDI devices, such as syncronizing all playback devices to a particular time. (By contrast, Voice Category messages deal with the individual musical parts that each instrument might play, so the channel nibble scheme allows a device to respond to its own MIDI channel while ignoring the Voice Category messages intended for another device on another channel).

These status bytes are further divided into two categories. Status bytes of 0xF0 to 0xF7 are called System Common messages. Status bytes of 0xF8 to 0xFF are called System Realtime messages. The implications of such will be discussed later.

Actually, certain Status bytes within this range are not defined by the MIDI spec to date, and are reserved for future use. For example, Status bytes of 0xF4, 0xF5, 0xF9, and 0xFD are not used. If a MIDI device ever receives such a Status, it should ignore that message. See Ignoring MIDI Messages.

What follows is a description of each message type. The description tells what the message does, what its status byte is, and whether it has any subsequent data bytes and what information those carry. Generally, these descriptions take the view of a device receiving such messages (ie, what the device would typically be expected to do when receiving particular messages). When applicable, remarks about a device that transmits such messages may be made.