A computing device which runs non-pageable real time and pageable non-real time processes is provided with non-pageable real time and pageable non-real time versions of operating system services where the necessity to page in memory would block a real-time thread of execution. In one embodiment, a real time operating system service has all its code and data locked, and only supports clients that similarly have their code and data locked. This ensures that such a service will not block due to a page fault caused by client memory being unavailable. A non-real time operating system service does not have its data locked and supports clients whose memory can be paged out. In a preferred embodiment servers which are required to provide real time behaviour are multithreaded and arrange for requests from real time and non-real time clients to be serviced in different threads.