I'm not sure what you're asking.
The CD bios processes what Squirrel outputs, which is MML that's been compiled into the bytecode stuff that the BIOS expects. If you look at the file generated by Squirrel, you will see all of the stuff that is expected by the player.
You basically just load up the data, setup the call to the bios, and then call it, and it goes on its merry way doing what it should do.
Is that what you meant? I may have answered the wrong question here, lol
EDIT: Also, to clear it up more, maybe: If you check the squirrel.c test file, you will notice that the psgPlay(0) call, which is the one to play .TRACK 0 of any of the included files, is outside of the main loop.
ONE single call is all it takes to make the BIOS know what to do to sit and play the whole entire song.
It doesn't have to be track 0, but that is what the numbering starts at, so the first song of an MML file is always track 0 (see any of the examples).