I don't understand the "stack frames are exempt from GC" thing -- do you mean the stack frames do not contribute reachability roots to the garbage collection or that the stack frames themselves are not garbage-collected?

Nothing directly involved in the call participates in GC. In VisualWorks Smalltalk, these calls are special. (These are only calls out through the DLLCC facility. I believe they are marked by pragmas.) They might contribute reachability roots, though that might be problematic. I seem to remember that objects created in that stack frame are not garbage collected. Since these methods are marked, the compiler can treat these calls specially. I'm pretty sure that the objects are spawned in a different space. I think this would mean that those stack frames can't mutate objects created before the call. (This would make these calls functional in flavor.) They should be able to refer to such objects, as this will only cause an exception if things go wrong.

The only thing that happens in these methods in VisualWorks Smalltalk is marshalling to call out to DLLs implemented in C.