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Fig. 1-4. A taxonomy of parallel and distributed computer systems.. [Картинка: any2fbimgloader5]. The physical layer just sends bits. As long as no errors occur, all is well. However, real communication networks are subject to errors, so some mechanism is needed to detect and correct them. This mechanism is the main task of the data link layer. What it does is to group the bits into units, sometimes calledframes,and see that each frame is correctly received..  position = 0;. A compromise between Fig. 2-13(a) and Fig. 2-13(b) that often works goes like this. When a request arrives at the server’s kernel, a timer is started. If the server sends the reply quickly enough (i.e., before the timer expires), the reply functions as the acknowledgement. If the timer goes off, a separate acknowledgement is sent. Thus in most cases, only two messages are needed, but when a complicated request is being carried out, a third one is used.. [Картинка: any2fbimgloader50]. A fourth way to deal with shared files in a distributed system is to use atomic transactions, as we discussed in detail in Chap. 3. To summarize briefly, to access a file or a group of files, a process first executes some type of BEGIN TRANSACTION primitive to signal that what follows must be executed indivisibly. Then come system calls to read and write one or more files. When the work has been completed, an END TRANSACTION primitive is executed. The key property of this method is that the system guarantees that all the calls contained within the transaction will be carried out in order, without any interference from other, concurrent transactions. If two or more transactions start up at the same time, the system ensures that the final result is the same as if they were all run in some (undefined) sequential order.. A simple but practical way to build a multiprocessor is to base it on a bus to which more than one CPU is connected. Fig. 6-2(a) illustrates a system with three CPUs and a memory shared among all of them. When any of the CPUs wants to read a word from the memory, it puts the address of the word it wants on the bus and asserts (puts a signal on) a bus control line indicating that it wants to do a read. When the memory has fetched the requested word, it puts the word on the bus and asserts another control line to announce that it is ready. The CPU then reads in the word. Writes work in an analogous way.. In an object-based distributed shared memory, processes on multiple machines share an abstract space filled with shared objects, as shown in Fig. 6-35. The location and management of the objects is handled automatically by the runtime system. This model is in contrast to page-based DSM systems such as IVY, which just provide a raw linear memory of bytes from 0 tosome maximum.. It is worth noting that when the kernel sends a message to a memory manager, it is effectively making an upcall. Although flexibility is gained this way, some system designers consider it inelegant for the kernel to call user programs to perform services for it. These people usually believe in hierarchical systems, with the lower layers providing services to the upper layers, not vice versa.. When a message is sent and successfully received, it is copied into the destination’s address space. It can happen, however, that the destination port is already full. What happens then depends on the various options and the rights associated with the destination port. One possibility is that the sender is blocked and simply waits until space becomes available in the port. Another is that the sender times out. In some cases, it can exceed the port limit and send anyway.. The following states are distinguished, but they are not mutually exclusive:. RgnInit is similar torgnAllocate except that after the region is allocated, it is also filled in from a segment whose capability is a parameter to the call. Several other calls that are similar torgnInit are also present, filling the region in different ways.. Fig. 10-8. Selected mutex calls.. 1. A university is installing DCE on all computers on campus. Suggest at least two ways of dividing the machines into cells..