hey gregfox, I am not sure if this is the answer to your question but I found something in the SEGGER embOS generic manual: Quote: “ Before embOS takes over control (before the call to OS_Start()), a program does use the so-called system stack. This is the same stack that a non-embOS program for this CPU would use. After transferring control to the embOS scheduler by calling OS_Start(), the system stack is used only when no task is executed for the follow- ing: - embOS scheduler - embOS software…