block, preventing the PCLSR from succeeding. To prevent this, ITS doesn't allow memory pages in User space to be paged out after they're first accessed during a system call, and system calls typically start by touching pages in User space they know they will need to access. Unix-solution: restart on request Contrast this with the approach taken in the UNIX operating system, where there is restartability, but it is not transparent. Instead, an I/O operation returns the number of bytes actually transferred (or the EINTR error if the operation was interrupted before any bytes were actually transferred), and it is up to the application to check this and manage its own resumption of the operation until all the bytes have been transferred. In the philosophy of UNIX, this was given by Richard P. Gabriel as an example of the "worse is better" principle. Asynchronous approaches A different approach is possible. It is apparent in the above that the system call has to be synchronous—that is, the calling process has to wait for the operation to complete. This is not inevitable: in the OpenVMS operating system, all I/O and other time-consuming operations are inherently asynchronous, which means the semantics of the system call is "start the operation, and perform one or more of these notifications when it completes" after which it returns immediately to the caller. There is a standard set of available notifications (such as set an event flag, or deliver an asynchronous system trap), as well as a set of system calls for explicitly suspending the process while waiting for these, which are a) fully restartable in the ITS sense, and b) much smaller in number than the set of actual time-consuming system calls. OpenVMS provides alternative "start operation and wait for completion" synchronous versions of all time-consuming system calls. These are implemented as "perform the actual asynchronous operation" followed by "wait until the operation sets the event flag". Any access to the process context during this time will see it about to (re)enter the wait-for-event-flag call. Notes References Concurrent computing