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Research and Advances

Improving locality by critical working sets

A new approach to program locality improvement via restructuring is described. The method is particularly suited to those systems where primary memory is managed according to a working set strategy. It is based on the concept of critical working set, a working set which does not contain the next memory reference. The data the method operates upon are extracted from a trace of the program to be restructured. It is shown that, except in some special cases, the method is not optimum. However, the experimental results obtained by using the method to restructure an interactive text editor and the file system module of an operating system have shown its substantial superiority over the other methods proposed in the literature.
Research and Advances

A locally-organized parser for spoken input

This paper describes LPARS, a locally-organized parsing system, designed for use in a continuous speech recognizer. LPARS processes a string of phonemes which contains ambiguity and error. The system is locally-organized in the sense that it builds local parse structures from reliable word candidates recognized anywhere in an input utterance. These local structures are used as “islands of reliability” to guide the search for more highly garbledwords which might complete the utterance.
Research and Advances

Register allocation via usage counts

This paper introduces the notion of usage counts, shows how usage counts can be developed by algorithms that eliminate redundant computations, and describes how usage counts can provide the basis for register allocation. The paper compares register allocation based on usage counts to other commonly used register allocation techniques, and presents evidence which shows that the usage count technique is significantly better than these other techniques.
Research and Advances

Enumerating full-time programmers

Data from the 1970 Census and the Department of Labor's Area Wage Surveys are used to derive estimates of the number of full-time programmers employed during the years 1969 through 1973. The 1973 figure of 180,000 is considerably less than suggested in earlier reports. It is recommended that educational administrators consider whether the many courses aimed at training programmers are justified on a vocational basis.
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Research and Advances

A note on the calculation of average working set size

Finite-length reference string of arbitrary structure are considered, and an exact expression for average working set size in terms of “corrected” interreference interval statistics is derived. An example is discussed; upper and lower bounds are obtained; and the average working set size function is shown to be efficiently obtained for a set of page sizes, in a single pass of the reference string. This work follows the developments of a paper by Denning and Schwartz, who consider infinite-length reference strings which satisfy certain statistical properties and who derive an expression relating the asymptotic average working set size to the asymptotic missing page rate function under working set replacement.
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Research and Advances

A back-end computer for data base management

It is proposed that the data base management function be placed on a dedicated back-end computer which accepts commands (in a relatively high level language such as the CODASYL Data Base Task Group, April 1971 Report) from a host computer, accesses the data base on secondary storage, and returns results. The advantages of such a configuration are discussed. An experimental implementation, called the eXperimental Data Management System, XDMS, is described and certain conclusions about the back-end approach are drawn from this implementation.
Research and Advances

A weighted buddy method for dynamic storage allocation

An extension of the buddy method, called the weighted buddy method, for dynamic storage allocation is presented. The weighted buddy method allows block sizes of 2k and 3·2k, whereas the original buddy method allowed only block sizes of 2k. This extension is achieved at an additional cost of only two bits per block. Simulation results are presented which compare this method with the buddy method. These results indicate that, for a uniform request distribution, the buddy system has less total memory fragmentation than the weighted buddy algorithm. However, the total fragmentation is smaller for the weighted buddy method when the requests are for exponentially distributed block sizes.
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Research and Advances

Monitors: an operating system structuring concept

This paper develops Brinch-Hansen's concept of a monitor as a method of structuring an operating system. It introduces a form of synchronization, describes a possible method of implementation in terms of semaphores and gives a suitable proof rule. Illustrative examples include a single resource scheduler, a bounded buffer, an alarm clock, a buffer pool, a disk head optimizer, and a version of the problem of readers and writers.
Research and Advances

Structured data structures

Programming systems which permit arbitrary linked list structures enable the user to create complicated structures without sufficient protection. Deletions can result in unreachable data elements, and there is no guarantee that additions will be performed properly. To remedy this situation, this paper proposes a Data Structure Description and Manipulation Language which provides for the creation of a restricted class of data structures but ensures the correctness of the program. This is accomplished by an explicit structure declaration facility, a restriction on the permissible operations, and execution-time checks.
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Research and Advances

A new technique for compression and storage of data

The widespread tendency toward storage of large programs and blocks of text has produced a need for efficient methods of compressing and storing data. This paper describes techniques that can, in most cases, decrease storage size by a factor of from two to four. The techniques involve special handling of leading and trailing blanks, and the encoding of other symbols in groups of fixed size as unique fixed point numbers. The efficiency of the system is considered and pertinent statistics are given and compared with statistics for other information coding techniques.
Research and Advances

A user authentication scheme not requiring secrecy in the computer

In many computer operating systems a user authenticates himself by entering a secret password known solely to himself and the system. The system compares this password with one recorded in a Password Table which is available to only the authentication program. The integrity of the system depends on keeping the table secret. In this paper a password scheme is presented which does not require secrecy in the computer. All aspects of the system, including all relevant code and data bases, may be known by anyone attempting to intrude. The scheme is based on using a function H which the would-be intruder is unable to invert. This function is applied to the user's password and the result compared to a table entry, a match being interpreted as authentication of the user. The intruder may know all about H and have access to the table, but he can penetrate the system only if he can invert H to determine an input that produces a given output. This paper discusses issues surrounding selection of a suitable H. Two different plausible arguments are given that penetration would be exceedingly difficult, and it is then argued that more rigorous results are unlikely. Finally, some human engineering problems relating to the scheme are discussed.
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Research and Advances

A high security log-in procedure

The protection of time sharing systems from unauthorized users is often achieved by the use of passwords. By using one-way ciphers to code the passwords, the risks involved with storing the passwords in the computer can be avoided. We discuss the selection of a suitable one-way cipher and suggest that for this purpose polynomials over a prime modulus are superior to one-way ciphers derived from Shannon codes.
Research and Advances

Execution time requirements for encipherment programs

Although encipherment has often been discussed as a means to protect computer data, its costs are not well established. Five experiments were conducted to measure the cpu time on a CDC 6400 required by additive ciphers programmed both in assembly language and in Fortran: a “null transformation” to measure the time to move data without encipherment; encipherment with a one-word key; encipherment with a 125-word key; double key encipherment; and encipherment using a pseudo random key. The results were analyzed for consistency over 100 runs, and the effects of constant and intermittent errors were considered. Timing rates for assembly language encipherment ranged from 498,800 characters per second for a pseudo random key cipher to 2,092,000 characters per second for a constant one-word key cipher. The latter is almost equivalent to the rate required simply to move data without encipherment. Fortran tests required over four times as much cpu time. This paper introduces the idea of enciphering time coefficient the ratio of enciphering time to the time taken to fetch and store data without encipherment.
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Research and Advances

Formal requirements for virtualizable third generation architectures

Virtual machine systems have been implemented on a limited number of third generation computer systems, e.g. CP-67 on the IBM 360/67. From previous empirical studies, it is known that certain third generation computer systems, e.g. the DEC PDP-10, cannot support a virtual machine system. In this paper, model of a third-generation-like computer system is developed. Formal techniques are used to derive precise sufficient conditions to test whether such an architecture can support virtual machines.
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Research and Advances

Capability-based addressing

Various addressing schemes making use of segment tables are examined. The inadequacies of these schemes when dealing with shared addresses are explained. These inadequacies are traced to the lack of an efficient absolute address for objects in these systems. The direct use of a capability as an address is shown to overcome these difficulties because it provides the needed absolute address. Implementation of capability-based addressing is discussed. It is predicted that the use of tags to identify capabilities will dominate. A hardware address translation scheme which never requires the modification of the representation of capabilities is suggested. The scheme uses a main memory hash table for obtaining a segment's location in main memory given its unique code. The hash table is avoided for recently accessed segments by means of a set of associative registers. A computer using capability-based addressing may be substantially superior to present systems on the basis of protection, simplicity of programming conventions, and efficient implementation.

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