FM is frequency modulation, where the input of one or more waveforms is used to modify a second waveform. The Amiga has no hardware support for this, so I don't believe you'll find what you're looking for in the hardware documentation.
"Chip" tunes are just really small waveforms (a few hundred bytes at best) that are looped really fast. Fast arpegios give that traditional glimble effect. The source code for a number of old soundtracker players is available, though most are only in assembler.
http://www.pouet.net/prod.php?which=13419
