As the size of system software increases, the algorithms and data structures of the computation no longer constitute the major design problems. When systema are constructed from many components, the organization of the overall system - the software architecture - presents a new set of dsign problems. This level of design has been address a number of ways including informal diagrams and descriptive terms, module interconnection languages, templates and frmeworks for systems that serve the needs of specific domains, and formal models of component integration mechanisms.

This paper describes the fundamentals and some of the details of task commincation in DEMCS, the operating system for the CRAY-1 computer being developed at the Los Alamos Scientific Laboratory. The communication mech- anism is a message system with several novel features. Meesages are sent from one task to another over links. Links are the primary protected objects in the system; they provide messages paths and optional data sharing between tasks. They can be used to represent other objects with capability-like ac- cess controls. Links point to the tasks that created them. A task that creates a link determines its contents and possibly restricts its use. A link may be passed on from on task to another along with a message sent over some other link suject to the restrictions imposed by the creatorof the link be- ing passed. The link based message and sata sharing system is an attractive alternative to semaphore or monitor type shared variable based operating system on machines with only very simple memory protection mechanisms or on machines connected together in a network.

This paper discussesthe design of the file system for DEMCS, an operating system being developed for the CRAY-1 somputer at Los Alamos Scientific Laboratory. The goals to be met, in particular the performance and usability considerations are outlined. A descriptionis given of the user interface and the general structure of the file systemand the file system routines. A simple model of program behavior is used to demonstrate the effet of buffer- ing data by the file system routines. A disk space allocation strategy is descibed which will take advantage of this buffering. The last section out- lines how the performance mechanisms are itegrated into the file system routines.

Mach is a multiproceesor operating system kernel and environment under development at Carnegie-Mellon University. Mach provides a new foundation for UNIX development that spans networks of uniprocessors and multiprocessors . This paper describes Mach and the motivations that led to its design. Also described are some of the details of its implementation and current status.