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Let us focus on SEAL, due to its simplicity. It uses only one bit in the ATM header, one of the bits in thePayload type field. This bit is normally 0, but is set to 1 in the last cell of a packet. The last cell contains a trailer in the final 8 bytes. In most cases there will be some padding (with zeros) between the end of the packet and the start of the trailer. With SEAL, the destination just assembles incoming cells for each virtual circuit until it finds one with the end-of-packet bit set. Then it extracts and processes the trailer.. A second method of group addressing is to require the sender to provide an explicit list of all destinations (e.g., IP addresses). When this method is used, the parameter in the call tosend that specifies the destination is a pointer to a list of addresses. This method has the serious drawback that it forces user processes (i.e., the group members) to be aware of precisely who is a member of which group. In other words, it is not transparent. Furthermore, whenever group membership changes, the user processes must update their membership lists. In Fig. 2-33, this administration can easily be done by the kernels to hide it from the user processes.. Fig. 3-1. When each machine has its own clock, an event that occurred after another event may nevertheless be assigned an earlier time.. Additional clock synchronization algorithms are discussed in the literature (e.g., Lundelius-Welch and Lynch, 1988; Ramanathan et al., 1990a; and Sri-kanth and Toueg, 1987).Multiple External Time Sources. 2. Deposit(amount, account2).. In Fig. 3-17(a)-(c) we have three transactions that are executed simultaneously by three separate processes. If they were to be run sequentially, the final value ofx would be 1,2, or 3, depending which one ran last(x could be a shared variable, a file, or some other kind of object). In Fig. 3-17(d) we see various orders, calledschedules,in which they might be interleaved. schedule 1 is actually serialized. In other words, the transactions run strictly sequentially, so it meets the serializability condition by definition. Schedule 2 is not serialized, but is still legal because it results in a value forx that could have been achieved by running the transactions strictly sequentially. The third one is illegal since it setsx to 5, something that no sequential order of the transactions could produce. It is up to the system to ensure that individual operations are interleaved correctly. By allowing the system the freedom to choose any ordering of the operations it wants to— provided that it gets the answer right — we eliminate the need for programmers to do their own mutual exclusion, thus simplifying the programming.. 4.6.3. Real-Time Communication. Our protocol manages cache blocks, each of which can be in one of the following three states:. On the left-hand side of Fig. 6-10 we have the single-bus multiprocessors that have hardware caches and keep them consistent by snooping on the bus. These are the simplest shared-memory machines and operate entirely in hardware. Various machines made by Sequent and other vendors and the experimental DEC Firefly workstation (five VAXes on a common bus) fall into this category. This design works fine for a small or medium number of CPUs, but degrades rapidly when the bus saturates.. 3. In the text it is stated that many minor variations of the cache ownership protocol of Fig. 6-6 are possible. Describe one such variation and give one advantage yours has over the one in the text. . Fig. 7-11. System structure for group communication in Amoeba.. Our third example of a modern, microkernel-based operating system is Chorus. The structure of this chapter is similar to that of the previous two: first a brief history, then an overview of the microkernel, followed by a more detailed look at process management, memory management, and communication. After that, we will study how Chorus tackles UNIX emulation. Next comes a section on distributed object-oriented programming in Chorus. We will conclude with a short comparison of Amoeba, Mach, and Chorus. More information about Chorus can be found in (Abrossimov et al., 1989, 1992; Armand and Dean, 1992; Batlivala, et al., 1992; Bricker et al., 1991; Gien and Grob, 1992; and Rozieret al., 1988).. 9.7.4. Memory Model. Before a client can call a server, it has to locate the server and bind to it. Naive users can ignore the binding process and let the stubs take care of it automatically, but binding happens nevertheless. Sophisticated users can control it in detail, for example, to select a specific server in a particular distant cell. In this section we will describe how binding works in DCE.. The DFS solution is to make each of the directoriesA, B, andC (and their subdirectories) a separate fileset. DFS allows filesets to be moved, so the system administrator can rebalance the disk space simply by moving directoryA to disk 2, as shown in Fig. 10-30(c). As long as both disks are in the same cell, no global names change, so everything continues to work after the move as it did before..