The MS documentation states: "All methods that are members of real-time interfaces must be implemented as non-blocking. They should not block, use paged memory, or call any blocking system routines. Note that most system routines are blocking" This is most probably means that any kind of WaitXXX function should NOT be called explicitly or implicitly; so DeviceIoControl should be fine to use providing that a called driver completes a request synchronously Do I miss / misinterpret something? TIA Andrew _________________________________________________________________ Hotmail has tools for the New Busy. Search, chat and e-mail from your inbox. http://www.windowslive.com/campaign/thenewbusy?ocid=PID28326::T:WLMTAGL:ON:WL:en-US:WM_HMP:042010_1