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4. Low-level input/output..   /* Write the data just received to the destination file. */. 1. The protocol is already designed, saving considerable work.. If a lease has expired and the file (still cached) is needed again shortly thereafter, the client can ask the server if the copy it has (identified by a time-stamp) is still the current one. If so, a new lease is generated, but the file need not be retransmitted.. The basic idea that makes this scheme work is that when the kernel knows that a thread has blocked (e.g., by its having executed a blocking system call or caused a page fault), the kernel notifies the process’ runtime system, passing as parameters on the stack the number of the thread in question and a description of the event that occurred. The notification happens by having the kernel activate the runtime system at a known starting address, roughly analogous to a signal in UNIX. This mechanism is called anupcall.. In Fig. 5-13(b) and (c), we see two more examples. The latter is especially interesting because it setsNr to 1, making it possible to read a replicated file by finding any copy and using it. The price paid, however, is that write updates need to acquire all copies.. Continuing along the spectrum, we come to the machines running a page-based distributed shared memory system such as IVY (Li, 1986) and Mirage (Fleisch and Popek, 1989). Each of the CPUs in such a system has its own private memory and, unlike the NUMA machines and UMA multiprocessors, cannot reference remote memory directly. When a CPU addresses a word in the address space that is backed by a page currently located on a different machine, a trap to the operating system occurs and the required page must be fetched by software. The operating system acquires the necessary page by sending a message to the machine where the page is currently residing and asking for it. Thus both placement and access are done in software here.. [Картинка: any2fbimgloader182]. The first version of Mach was released in 1986 for the VAX 11/784, a four-CPU multiprocessor. Shortly thereafter, ports to the IBM PC/RT and Sun 3 were done. By 1987, Mach was also running on the Encore and Sequent multiprocessors. Although Mach had networking facilities, at this time it was conceived of primarily as a single machine or multiprocessor system rather than as a transparent distributed operating system for a collection of machines on a LAN.. When the message arrives at the receiver, the kernel chooses an unallocated piece of virtual address space the same size as the out-of-line data, and maps the sender’s pages into the receiver’s address space, marking them copy-on-write. The address word following the descriptor is changed to reflect the address at which the region is located in the receiver’s address space. This mechanism provides a way to move blocks of data at extremely high speed, because no copying is required except for the message header and the two-word body (the descriptor and the address). Depending on a bit in the descriptor, the region is either removed from the sender’s address space or kept there.. The streams manager also handles Berkeley sockets and networking. In this way, a process on Chorus can communicate using TCP/IP as well as other network protocols. Pipes and named pipes are also handled here.The Interprocess Communication Manager. DCE supports the three thread scheduling algorithms illustrated in Fig. 10-4. The first, FIFO, searches the priority queues from highest to lowest, to locate the highest priority queue with one or more threads on it. The first thread on this queue is then run until it finishes, either by blocking or exiting. In principle, the selected thread can run as long as it needs to. When it has finished, it is removed from the queue of runnable threads. Then the scheduler once again searches the queues from high to low and takes the first thread it finds.. The Attribute Table tells how many values each attribute may have, how much memory they may occupy, and what their types are (e.g., integers, Boole-ans, or reals). Attributes are described in the OSI ASN.1 notation. DCE provides a compiler from ASN.1 to C (MAVROS), which is analogous to its IDL compiler for RPC stubs. This compiler is really for use in building DCE; normally, users do not encounter it.. Ciphertext = {Plaintext} Key. DFS is a DCE application, and as such can use all the other facilities of DCE. In particular, it uses threads to allow multiple file access requests to be served simultaneously, RPC for communication between clients and servers, DTS to synchronize server clocks, the directory system to allow file servers to be located, and the authentication and privilege servers to protect files..