This is really an operating system question not a CPU question. The CPU clearly persists this value indefinitely but upon task switching all of the context is dumped and restored. So upon process creation this has some default values, and is often set on startup by the C library itself e.g. fastmath will turn on DAZ/FTZ. Upon thread creation, it is unlikely the OS would inherit the various processor state registers. So you must also set them if you want them there. Within your app they should persist indefinitely as on each context switch the state will be reloaded on a per thread basis.