Lesson 23 · System exclusive - Build a MIDI Parser

System exclusive

System-exclusive messages carry manufacturer data of any length, framed in a file with a variable-length byte count. Today you read that frame and step over the payload, so a SysEx never derails the event loop.

The goal

Parse a system-exclusive event, reading its variable-length payload.

Start here - the target

TO DO

Scenario: A SysEx event reads a length-prefixed payload

Giventhe bytes 0xF0 0x03 0x43 0x10 0xF7

Whenthey are parsed as a system-exclusive event

Thenit is a SysEx event whose 3-byte payload is 0x43 0x10 0xF7

Andthe length 0x03 was read as a VLQ, so the reader advances exactly past the payload to the next event

Background

A system-exclusive (SysEx) message is a manufacturer’s private data - a synth patch, a device query - and it can be any length. Inside a Standard MIDI File it is framed like a meta event: the byte 0xF0, a variable-length byte count, then that many payload bytes (the payload normally ends in the terminator 0xF7). Read the VLQ length and step the reader exactly past the payload.

You do not need to interpret the bytes to handle them correctly - the length tells you precisely how far to jump, so an unknown SysEx never confuses the event loop. This is the last event kind. With channel messages, meta events, and SysEx all readable, the event loop can now walk any real track from its first delta-time to its end-of-track, which is exactly what the next chapter builds on.

Make it work

// 0xF0 begins SysEx; a VLQ length follows, then that many payload bytes
r.NextByte()          // consume 0xF0
n := r.ReadVLQ()      // payload length (3)
data := r.buf[r.pos : r.pos+int(n)]
r.pos += int(n)       // the payload usually ends in 0xF7

CheckpointDONE

You can parse a system-exclusive event. Commit and stop here.