A device, such as a constrained device that includes a processing device and memory, schedules user-defined independently executable functions to execute from a single stack common to all user-defined independently executable functions according to availability and priority of the user-defined independently executable functions relative to other user-defined independently executable functions and preempts currently running user-defined independently executable function by placing the particular user-defined independently executable function on a single stack that has register values for the currently running user-defined independently executable function.