Russ,

For smaller processors without a hardware stack the level of re-entrancy is restricted. The solution involves duplicating any function that may be called from both call graphs (mainline & interrupt) and does not allow for recursion within the a single call graph. Full recursion on such processors can be implemented but often comes at a cost which many people aren't willing to pay.

However, compilers for larger processors that warrant it and processors with hardware stacks do support full re-entrancy and recursion.

I hope that answers your question.

Ryan.