An Operating Systems Vade Mecum

312 Co-operating Processes Chapter 9and normal postal service), telephone numbers, and current projects.A new facility supported by software on many machines is a bulletin board,which accepts notices posted by individuals and makes them available to all interestedusers. Typically, notices are cataloged by subject, and users may subscribe to particularsubjects that interest them. Distributed bulletin boards on long-haul networks allow peoplewith common interests across the country to participate in continuing discussions.These facilities of long-haul networks have recently become standard withinlocal-area networks as well. For example, BSD4.2 and BSD4.3 Unix are available on anumber of different machines, including computers made by Digital Equipment Corporation,Gould Corporation, Sun Microsystems, and Pyramid Technology. This operatingsystem includes drivers for a number of communication devices. A local-area network ofmachines running this operating system automatically enjoys remote job entry, remotelogon, mail, remote file transfer, and user-information services.3.4 MulticomputersThe uses mentioned for local-area and long-haul networks omitted one important possibility— complex algorithms that require several machines in order to achieve acceptableperformance. Distributed algorithms have not been commonly run on such networks,mostly because the area of distributed algorithms is still quite young. Recently, however,several operating systems have been designed with the goal of unifying a local-area networkinto a single operating system.Multicomputers are different from the multiprocessors we saw earlier in that multicomputersdo not share main store but send messages to each other in order to cooperate.We distinguish multicomputers from networks in that multicomputers have asingle operating system that governs all the individual machines, whereas a computernetwork is built of autonomous machines often under separate management.These distinctions are not always simple. A recent trend toward less expensivecomputers has led to the emergence of the workstation, a powerful one-user computer,usually with a small local disk and a high-quality display. The workstations at an installationare usually linked by a network so that they can share expensive devices likeprinters and large secondary store. All the workstations use the same operating system,but each has its own copy and is mostly autonomous. However, each might provide servicesto the others on request, and processes on separate machines can communicate withone another through the network. Except for that facility, the operating systems on theworkstations are fairly conventional. Workstation networks occupy a role somewherebetween computer networks and true multicomputers.Operating systems designed especially for multicomputers often resemble thosedesigned especially for multiprocessors. Certain characteristics are fairly common inthese operating systems:Each machine has a copy of the code necessary for communication and primitiveservice to processes (such as setting up mapping registers and preempting at theend of a quantum). This code is the kernel of the operating system.