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Research and Advances

Minimal-total-processing time drum and disk scheduling disciplines

This article investigates the application of minimal-total-processing-time (MTPT) scheduling disciplines to rotating storage units when random arrival of requests is allowed. Fixed-head drum and moving-head disk storage units are considered, and emphasis is placed on the relative merits of the MTPT scheduling discipline with respect to the shortest-latency-time-first (SLTF) scheduling discipline. The results of the simulation studies presented show that neither scheduling discipline is unconditionally superior to the other. For most fixed-head drum applications, the SLTF discipline is preferable to MTPT, but for intra-cylinder disk scheduling the MTPT discipline offers a distinct advantage over the SLTF discipline. The computational requirements of an algorithm that implements the MTPT scheduling discipline are shown to be comparable to SLTF algorithms. In both cases, the sorting procedure is the most time-consuming phase of the algorithm.
Research and Advances

Protection and the control of information sharing in multics

The design of mechanisms to control the sharing of information in the Multics system is described. Five design principles help provide insight into the tradeoffs among different possible designs. The key mechanisms described include access control lists, hierarchical control of access specifications, identification and authentication of users, and primary memory protection. The paper ends with a discussion of several known weaknesses in the current protection mechanism design.
Research and Advances

Scheduling independent tasks to reduce mean finishing time

Sequencing to minimize mean finishing time (or mean time in system) is not only desirable to the user, but it also tends to minimize at each point in time the storage required to hold incomplete tasks. In this paper a deterministic model of independent tasks is introduced and new results are derived which extend and generalize the algorithms known for minimizing mean finishing time. In addition to presenting and analyzing new algorithms it is shown that the most general mean-finishing-time problem for independent tasks is polynomial complete, and hence unlikely to admit of a non-enumerative solution.
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Research and Advances

A theorem-proving language for experimentation

Because of the large number of strategies and inference rules presently under consideration in automated theorem proving, there is a need for developing a language especially oriented toward automated theorem proving. This paper discusses some of the features and instructions of this language. The use of this language permits easy extension of automated theorem-proving programs to include new strategies and/or new inference rules. Such extendability will permit general experimentation with the various alternative systems.
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Research and Advances

HYDRA: the kernel of a multiprocessor operating system

This paper describes the design philosophy of HYDRA—the kernel of an operating system for C.mmp, the Carnegie-Mellon Multi-Mini-Processor. This philosophy is realized through the introduction of a generalized notion of “resource,” both physical and virtual, called an “object.” Mechanisms are presented for dealing with objects, including the creation of new types, specification of new operations applicable to a given type, sharing, and protection of any reference to a given object against improper application of any of the operations defined with respect to that type of object. The mechanisms provide a coherent basis for extension of the system in two directions: the introduction of new facilities, and the creation of highly secure systems.
Research and Advances

An information-theoretic approach to text searching in direct access systems

Using direct access computer files of bibliographic information, an attempt is made to overcome one of the problems often associated with information retrieval, namely, the maintenance and use of large dictionaries, the greater part of which is used only infrequently. A novel method is presented, which maps the hyperbolic frequency distribution of text characteristics onto a rectangular distribution. This is more suited to implementation on storage devices. This method treats text as a string of characters rather than words bounded by spaces, and chooses subsets of strings such that their frequencies of occurrence are more even than those of word types. The members of this subset are then used as index keys for retrieval. The rectangular distribution of key frequencies results in a much simplified file organization and promises considerable cost advantages.
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Research and Advances

Two languages for estimating program efficiency

Two languages enabling their users to estimate the efficiency of computer programs are presented. The program whose efficiency one wishes to estimate is written in the first language, a go-to-less programming language which includes most of the features of Algol 60. The second language consists of interactive commands enabling its users to provide additional information about the program written in the first language and to output results estimating its efficiency. Processors for the two languages are also described. The first processor is a syntax-directed translator which compiles a program into a symbolic formula representing the execution time for that program. The second processor is a set of procedures for algebraic manipulation which can be called by the user to operate on the formula produced by the first processor. Examples of the usage of the two languages are included. The limitations of the present system, its relation to Knuth's work on the analysis of algorithms, and some of the directions for further research are also discussed.
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Research and Advances

Order-n correction for regular languages

A method is presented for calculating a string B, belonging to a given regular language L, which is “nearest” (in number of edit operations) to a given input string &agr;. B is viewed as a reasonable “correction” for the possibly erroneous string &agr;, where &agr; was originally intended to be a string of L. The calculation of B by the method presented requires time proportional to |&agr;|, the number of characters in &agr;. The method should find applications in information retrieval, artificial intelligence, and spelling correction systems.
Research and Advances

The treatment of data types in EL1

In constructing a general purpose programming language, a key issue is providing a sufficient set of data types and associated operations in a manner that permits both natural problem-oriented notation and efficient implementation. The EL1 language contains a number of features specifically designed to simultaneously satisfy both requirements. The resulting treatment of data types includes provision for programmer-defined data types and generaic routines, programmer control over type conversion, and very flexible data type behavior, in a context that allows efficient compiled code and compact data representation.
Research and Advances

Reduction of compilation costs through language contraction

Programming languages tailored to particular groups of users can often be constructed by removing unwanted features from a general purpose language. This paper describes the use of simulation techniques to predict the savings in compilation cost achievable by such an approach. The results suggest a function which describes the effect of changes in the power of a language on the compilation cost of an algorithm expressed in that language: when features not actually used by the algorithm are removed from the language, the cost of compiling the algorithm decreases moderately, but when features that are needed are removed, the compilation cost increases sharply.
Research and Advances

A model for masking rotational latency by dynamic disk allocation

This paper presents the background and algorithms for masking the rotational latency of a disk or drum. It discusses the anticipatory input and output of blocks of data to buffer and primary memories for a mono-programmed computer system. A basic permutation algorithm and several variations are given. Because of the anticipatory nature of the I/O scheduling, these algorithms are restricted to classes of programs with predictable behavior. While the methods are not restricted to numerical computations, matrix and partial differential equation methods are typical examples of their use. It is shown that latency may be masked using a small amount of buffer memory. The methods discussed are independent of the overall size of the data base being considered.
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Research and Advances

More on algorithms that reveal properties of floating point arithmetic units

In the interests of producing portable mathematical software, it is highly desirable for a program to be able directly to obtain fundamental properties of the environment in which it is to run. The installer would then not be obliged to change appropriate magic constants in the source code, and the user would not have to provide information he may very well not understand. Until the standard definitions of programming languages are changed to require builtin functions that provide this information [1, 3], we will have to resort to writing routines that discover it.
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Research and Advances

Computation of page fault probability from program transition diagram

An algorithm is given for calculating page fault probability in a virtual memory system operating under demand paging with various memory sizes and replacement rules. A first order Markov model of program behavior is assumed, and a representation of the system based on memory states, control states, and memory substates is presented. The algorithm is general in the sense that the page fault probabilities can be calculated for nonpredictive replacement rules applied to any program represented by a one-step Markov chain. A detailed example is given to illustrate the algorithm for Random and Least Recently Used (LRU) replacement rules.
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