Sig, Will Spock have restrictions on which thread invokes a callback? Suppose a library wants to spawn its own thread and invoke a callback from it; would that be possible? As I (partially) understand Eliot's suggestion for making calls on OS threads, the vm would be able to associate each Process with an OS thread; external calls made by a Process would be made on the specified OS thread. That Process, and only that Process, would be blocked during each external call. The first "level" of threaded callbacks that I can envision would be to allow callbacks that come from this associated thread. Does that seem reasonable? It would be great if restrictions are not required, but something tells me they will be necessary. I am being vague in part because I'm over my head here, and because I don't know if the supplier of an evolving library wants to speak yet. The source will be open when they release. Bill