IRHG - GC and threads

OK Guys,

  When GC gets to RB_GC_MARK_THREADS()
it only processes the thread for the CURRENT THREAD?

  1) Where does that leave objects associated with the
       processing of other threads. 

   2) I assume that the objects (Classes, modules, NODES)
        associated with the various thread code will be marked
        anyway by the standard routes.

    3) If no threads have been created, is the main processing
         considered the main thread?  In other words since threads
         are part of Ruby, is there always a thread running (the
          main processing sequence?)

   Thanks.
        Chuck T.