The use of monitors for describing concurrency has been much discussed in the literature. When monitors are used in real systems of any size, however, a number of problems arise which have not been adequately dealt with: the semantics of nested monitor calls; the various ways of defining the meaning of WAIT; priority scheduling; handling of timeouts, aborts and other exceptional conditions; interactions with process creation and destruction; monitoring large numbers of small objects. These problems are addressed by the facilities described here for concurrent programming in Mesa. Experience with several substantial applications gives us some confidence in the validity of our solutions.

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As experience with use of on-line operating systems has grown, the need to share information among system users has become increasingly apparent. Many contemporary systems permit some degree of sharing. Usually, sharing is accomplished by allowing several users to share data via input and output of information stored in files kept in secondary storage. Through the use of segmentation, however, Multics provides direct hardware addressing by user and system programs of all information, independent of its physical storage location. Information is stored in segments each of which is potentially sharable and carries its own independent attributes of size and access privilege. Here, the design and implementation considerations of segmentation and sharing in Multics are first discussed under the assumption that all information resides in a large, segmented main memory. Since the size of main memory on contemporary systems is rather limited, it is then shown how the Multics software achieves the effect of a large segmented main memory through the use of the Honeywell 645 segmentation and paging hardware.

Formal specifications of operating system interfaces can be a useful part of their documentation. We illustrate this by documenting the Threads synchronization primitives of the Taos operating system. We start with an informal description, present a way to formally specify interfaces in concurrent systems, and then give a formal specification of the synchronization primitives. We briefly discuss both the implementation and what we have learned from using the specification for more than a year. Our main conclusion is that programmers untrained in reading formal specifications have found this one helpful in getting their work done. iii Introduction The careful documentation of interfaces is an important step in the production of software upon which other software is to be built. If people are to use software without having to understand its implementation, documentation must convey semantic as well as syntactic information. When the software involves concurrency, adequate documentatio...

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The common features of third generation operating systems are surveyed from a general view, with emphasis on the common abstractions that constitute at least the basis for a "theory " of operating systems. Properties of specific systems are not discussed except where examples are useful. The technical aspects of issues and concepts are stressed, the nontechnical aspects mentioned only briefly. A perfunctory knowledge of third generation systems is presumed. Key words and phrases: multiprogramming systems, operating systems, supervisory systems, time-sharing systems, programming, storage allocation, memory allocation, processes, concurrency, parallelism, resource allocation, protection CR categories: 1.3, 4.0, 4.30, 6.20 It has been the custom to divide the era of electronic computing into "generations" whose approximate dates are:

This paper describes the design of the computer seen by a machine-language programmer in a timesharing system developed at the University of California at Berkeley. Some of the instructions in this machine are executed by the hardware, and some are implemented by software. The user, however, thinks of them all as part of his machine, a machine having extensive and unusual capabilities, many of which might be part of the hardware of a (considerably more expensive) computer. Among the important features of the machine are the arithmetic and string manipulation instructions, the very general memory allocation and configuration mechanism, and the multiple processes which can be created by the program. Facilities are provided for communication among these processes and for the control of exceptional conditions. The input-output system is capable of handling all of the peripheral equipment in a uniform and convenient manner through files having symbolic names. Programs can access files belonging to a number of people, but each person can protect his own files from unauthorized access by others. Some mention is made at various points of the techniques of implementation, but the main emphasis is on the appearance of the user's machine.

Control transfer is the fundamental activity in an operating system kernel. The resource management functionality and application programmer interfaces of an operating system may be delegated to other system components, but the kernel must manage control transfer. The current trend towards increased modularity in operating systems only increases the importance of control transfer. My thesis is that a programming language abstraction, continuations, can be adapted for use in operating system kernels to achieve increased flexibility and performance for control transfer. The flexibility that continuations provide allows the kernel designer when necessary to choose implementation performance over convenience, without affecting the design of the rest of the kernel. The continuation abstraction generalizes existing operating system control transfer optimizations. This dissertation also makes two practical contributions, an interface for machine-independent control transfer management inside ...

This thesis presents an orderly design approach for dynamically changing the configuration of constituent physical units in a modular computer system. Dynamic reconfiguration contributes to high system availability by allowing preventive maintenance, development of new operating systems, and changes in system capacity on a noninterference basis. The design presented includes the operating system primitives and hardware architecture for adding and removing any (primary or secondary) storage module and associated processing modules while the system is running. Reconfiguration is externally initiated by a simple request from a human operator and is accomplished automatically without disruption to users of the system. This design allows the modules in an installation to be partitioned into separate non- interfering systems. The viability of the design approach has been demonstrated by employing it for a practical implementation of processor and primary memory dynamic reconfiguration in the Multics system at M.I.T.

All of today’s popular systems for programming with multiple threads use designs based around three data types: • “Thread”, with operations Fork and Join • “Lock ” with operations Acquire and Release