An Operating Systems Vade Mecum

318 Co-operating Processes Chapter 9a multicomputer and distribute the problem.One result of these pressures has been a trend toward operating systems whose processescommunicate with each other by standard protocols. Local-area networks of machineswith such operating systems allow sharing of services. A version of this trend is forfairly small and inexpensive individual machines, possibly even lacking backing store, tobe used as high-performance workstations. File and backing-store transfers are conductedthrough the communication medium to file-server machines.Personal computers are already showing the results of this trend. They have only afew standard operating systems, such as CP/M, and most can connect to other computerseither by direct connections that obey a universal protocol at the host-to-line level (calledRS-232) or by phone connections that translate to RS-232. Workstation-class computersare more expensive (as of 1987, they cost about $10,000), but they still cost much lessthan the mainframe computers of the previous generation.A second trend has been to connect local-area networks into continental networksof computers to link researchers with common interests. These networks are used mostlyfor correspondence between people, but also to support remote file transfer, remotelogon, user-information services, and distributed bulletin boards. The gateway betweenthe local-area network and the long-haul networks is any one machine in the local-areanetwork.A third trend has been to design operating systems especially for multicomputers.These operating systems provide only communication and scheduling services to theirprocesses. Files, other devices, and other services are provided through utility processes.Policy decisions are also made by utility processes. These multicomputers are particularlyappropriate for large computations that will not fit on single machines because ofeither space or time limitations. Multicomputer operating systems are the subject ofmajor research efforts both in industry and at universities. The communication-kernelapproach seems to be viable not only for multicomputers but also for a wide range ofother computer organizations.6 FURTHER READINGThere are several good introductions to inter-process communication methods. An excellentsurvey of the facilities proposed in high-level languages for distributed computinghas been written by Scott (1984). The distinction among input, output, and free ports wasinvented by Cashin (1980), who also surveyed various kinds of communication. Anothersurvey, by Andrews and Schneider (1983), discusses mechanisms for concurrency controlas well as inter-process communication.Tanenbaum’s excellent text (1981) is a standard reference for details on communicationdevices and protocols at various levels. A shorter form of the same treatment canbe found in his survey article (1981).A number of distributed operating systems have been designed and built for avariety of multiprocessors and multicomputers. Hydra was designed for C.mmp, a multiprocessorwith 16 cpu’s with shared access to 16 main store modules (Wulf et al.,