If you want call/cc (for repeated executions of the continuation) you need a first class stack. A CPS execution model can use green threads right up until you w

If you want call/cc (for repeated executions of the continuation) you need a first class stack. A CPS execution model can use green threads right up until you want that explicit continuation in hand and split the stack. In most implementations of CPS the continuation is not in fact available for the user to execute: stack-oriented programming is the mental model, until you pull out call/cc or something similar. Note that you don't get the continuation in hand for the async-everywhere advocated by GP, only blocking operations get the continuation, and most of those are implemented by the runtime.