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We have now seen four kinds of distributed hardware and two kinds of distributed software. In theory, there should be eight combinations of hardware and software. In fact, only four are worth distinguishing, because to the user, the interconnection technology is not visible. For most purposes, a multiprocessor is a multiprocessor, whether it uses a bus with snoopy caches or uses an omega network. In the following sections we will look at some of the most common combinations of hardware and software.. The trailer has four fields. The first two are each 1 byte long and are not used. Then comes a 2-byte field giving the packet length, and a 4-byte checksum over the packet, padding, and trailer.. Many other switch designs have been proposed and tried. These include time division switches using shared memory, buses or rings, as well as space division switches with one or more paths between each input and each output..  (b)  Compiling a program.. In other systems, the file server and directory server are just user programs, so a system can be configured to run client and server software on the same machines or not, as it wishes. Finally, at the other extreme, are systems in which clients and servers are fundamentally different machines, in terms of either hardware or software. The servers may even run a different version of the operating system from the clients. While separation of function may seem a bit cleaner, there is no fundamental reason to prefer one approach over the others.. The real difference between the multiprocessors and the DSM systems is whether or not remote data can be accessed just by referring to their addresses. On all the multiprocessors the answer is yes. On the DSM systems it is no: software intervention is always needed. Similarly, unattached global memory, that is, memory not associated with any particular CPU, is possible on the multiprocessors but not on the DSM systems (because the latter are collections of separate computers connected by a network).. This strategy brings up the issue of how the system keeps track of what has been modified and when. To keep track of which shared variables have been changed, a special compiler can be used that generates code to maintain a runtime table with an entry in it for each shared variable in the program. Whenever a shared variable is updated, the change is noted in the table. If this special compiler is not available, the MMU hardware is used to detect writes to shared data, as in Munin.. When a program performs an operation on an object, the compiler calls a runtime system procedure,invoke_op, specifying the object, the operation, the parameters, and a flag telling whether the object will be modified (called a write) or not modified (called a read). The action taken byinvoke_op depends on whether the object is replicated, whether a copy is available locally, whether it is being read or written, and whether the underlying system supports reliable, totally-ordered broadcasting. Four cases have to be distinguished, as illustrated in Fig. 6-42.. Midway supports only one kind of shared data variable, whereas Munin has four kinds (read only, migratory, write-shared, and conventional). On the other hand, Midway supports three different consistency protocols (entry, release, and processor), whereas Munin only supports release consistency. Whether it is better to have multiple types of shared data or multiple protocols is open to discussion. More research will be needed before we understand this subject fully. . In this section we will give an introduction to Amoeba, starting with a brief history and its current research goals. Then we will look at the architecture of a typical Amoeba system. Finally, we will begin our study of the Amoeba software, both the kernel and the servers.. Instead of this personal multiprocessor approach, Amoeba is based on the model shown in Fig. 7-1. In this model, all the computing power is located in one or moreprocessor pools.A processor pool consists of a substantial number of CPUs, each with its own local memory and network connection. Shared memory is not required, or even expected, but if it is present it could be used to optimize message passing by doing memory-to-memory copying instead of sending messages over the network.. 7.1.4. The Amoeba Microkernel. For dedicated applications, such as an X-terminal, it is known in advance which system call the program may do and which it may not do. If no local file system and no System V IPC are needed, the object manager and interprocess communication manager can be omitted, as in Fig. 9-23(c).. Chorus provides full binary compatibility with System V UNIX. The emulation is done by a collection of processes (like Amoeba) rather than by running UNIX as an application program (like Mach). However, some of these processes contain actual UNIX code, like Mach and unlike Amoeba. Like Amoeba, the native servers were designed from scratch with distributed computing in mind, so the emulation translates the UNIX constructs onto the native ones. Just as in Amoeba, for example, when a file is opened, a capability for it is fetched and stored in the file descriptor table.. The DCE security system consists of several servers and programs, the most important of which are shown in Fig. 10-26. Theregistry servermanages the security data base, the registry, which contains the names of all the principals, groups, and organizations. For each principal, it gives account information, groups and organizations the principal belongs to, whether the principal is a client or a server, and other information. The registry also contains policy information per cell, including the length, format, and lifetime for passwords and related information. The registry can be thought of as the successor to the password file in UNIX(/etc/passwd). It can be edited by the system administrator using the registry editor. These can add and delete principals, change keys, and so on..