Search
Join ACM

Computing Profession

Advertisement

Research and Advances

Secure communications over insecure channels

According to traditional conceptions of cryptographic security, it is necessary to transmit a key, by secret means, before encrypted massages can be sent securely. This paper shows that it is possible to select a key over open communications channels in such a fashion that communications security can be maintained. A method is described which forces any enemy to expend an amount of work which increases as the square of the work required of the two communicants to select the key. The method provides a logically new kind of protection against the passive eavesdropper. It suggests that further research on this topic will be highly rewarding, both in a theoretical and a practical sense.
Research and Advances

Assembling code for machines with span-dependent instructions

Many modern computers contain instructions whose lengths depend on the distance from a given instance of such an instruction to the operand of that instruction. This paper considers the problem of minimizing the lengths of programs for such machines. An efficient solution is presented for the case in which the operand of every such “span-dependent” instruction is either a label or an assembly-time expression of a certain restricted form. If this restriction is relaxed by allowing these operands to be more general assembly-time expressions, then the problem is shown to be NP-complete.
Research and Advances

Implications of structured programming for machine architecture

Based on an empirical study of more than 10,000 lines of program text written in a GOTO-less language, a machine architecture specifically designed for structured programs is proposed. Since assignment, CALL, RETURN, and IF statements together account for 93 percent of all executable statements, special care is given to ensure that these statements can be implemented efficiently. A highly compact instruction encoding scheme is presented, which can reduce program size by a factor of 3. Unlike a Huffman code, which utilizes variable length fields, this method uses only fixed length (1-byte) opcode and address fields. The most frequent instructions consist of a single 1-byte field. As a consequence, instruction decoding time is minimized, and the machine is efficient with respect to both space and time.
Research and Advances

Value orientation of computer science students

Technological and nontechnological value orientations are investigated with special attention to the complexity of value structures. Computer science students, who are closely associated with technology, contrast with social science students, who are often technologically aloof. This is confirmed by the value ratings of 313 students at the University of Minnesota in 1972. Computer science majors were found to have a more complex value structure than social science majors.
Research and Advances

Management utilization of computers in American local governments

Traditional concepts of management information systems (MIS) bear little relation to the information systems currently in use by top management in most US local governments. What exists is management-oriented computing, involving the use of relatively unsophisticated applications. Despite the unsophisticated nature of these systems, management use of computing is surprisingly common, but also varied in its extent among local governments. Management computing is most prevalent in those governments with professional management practices where top management is supportive of computing and tends to control computing decisions and where department users have less control over design and implementation activities. Finally, management computing clearly has impacts for top managers, mostly involving improvements in decision information.
and
Research and Advances

Anomalies with variable partition paging algorithms

Five types of anomalous behaviour which may occur in paged virtual memory operating systems are defined. One type of anomaly, for example, concerns the fact that, with certain reference strings and paging algorithms, an increase in mean memory allocation may result in an increase in fault rate. Two paging algorithms, the page fault frequency and working set algorithms, are examined in terms of their anomaly potential, and reference string examples of various anomalies are presented. Two paging algorithm properties, the inclusion property and the generalized inclusion property, are discussed and the anomaly implications of these properties presented.
, , and
Research and Advances

Is “sometime” sometimes better than “always”?: intermittent assertions in proving program correctness

This paper explores a technique for proving the correctness and termination of programs simultaneously. This approach, the intermittent-assertion method, involves documenting the program with assertions that must be true at some time when control passes through the corresponding point, but that need not be true every time. The method, introduced by Burstall, promises to provide a valuable complement to the more conventional methods. The intermittent-assertion method is presented with a number of examples of correctness and termination proofs. Some of these proofs are markedly simpler than their conventional counterparts. On the other hand, it is shown that a proof of correctness or termination by any of the conventional techniques can be rephrased directly as a proof using intermittent assertions. Finally, it is shown how the intermittent-assertion method can be applied to prove the validity of program transformations and the correctness of continuously operating programs.
and
Research and Advances

Computer science faculties: the current status of minorities and women

The results of a survey conducted in the fall of 1975 to determine the status of women and minority faculty members in academic computer science are presented. Faculty members were compared with respect to professional background, salaries, teaching load, publication records, and research grants. Analysis of the data indicated that the over-all verdict is one of general equality among women, minorities, and men.
and
Research and Advances

A method for obtaining digital signatures and public-key cryptosystems

An encryption method is presented with the novel property that publicly revealing an encryption key does not thereby reveal the corresponding decryption key. This has two important consequences: (1) Couriers or other secure means are not needed to transmit keys, since a message can be enciphered using an encryption key publicly revealed by the intented recipient. Only he can decipher the message, since only he knows the corresponding decryption key. (2) A message can be “signed” using a privately held decryption key. Anyone can verify this signature using the corresponding publicly revealed encryption key. Signatures cannot be forged, and a signer cannot later deny the validity of his signature. This has obvious applications in “electronic mail” and “electronic funds transfer” systems. A message is encrypted by representing it as a number M, raising M to a publicly specified power e, and then taking the remainder when the result is divided by the publicly specified product, n, of two large secret primer numbers p and q. Decryption is similar; only a different, secret, power d is used, where e * d ≡ 1(mod (p - 1) * (q - 1)). The security of the system rests in part on the difficulty of factoring the published divisor, n.
, , and
Research and Advances

Systematic recursion removal

The recursion removal algorithm presented by Strong and Walker is amplified and applied to a relatively complex PL/I program. The aim is to demonstrate systematic recursion-removal techniques on something more complex than Knuth's “sturdy toddler” and to obtain measurements of the cost of procedure linkage in PL/I and the savings achievable via procedure integration in the presence of recursion. First, the paper describes the recursion-removal process and the example on which it will be illustrated. Recursion removal is then applied to the two major parts of this example and the final result of the process is displayed. Our performance comparison results are presented, and our conclusions are briefly discussed
and
Research and Advances

The evolution of the DECsystem 10

The DECsystem 10, also known as the PDP-10, evolved from the PDP-6 (circa 1963) over five generations of implementations to presently include systems covering a price range of five to one. The origin and evolution of the hardware, operating system, and languages are described in terms of technological change, user requirements, and user developments. The PDP-10's contributions to computing technology include: accelerating the transition from batch oriented to time sharing computing systems; transferring hardware technology within DEC (and elsewhere) to minicomputer design and manufacturing; supporting minicomputer hardware and software development; and serving as a model for single user and timeshared interactive minicomputer/microcomputer systems.
, , , and
Research and Advances

The development of the MU5 computer system

Following a brief outline of the background of the MU5 project, the aims and ideas for MU5 are discussed. A description is then given of the instruction set, which includes a number of features conducive to the production of efficient compiled code from high-level language source programs. The design of the processor is then traced from the initial ideas for an associatively addressed “name store” to the final multistage pipeline structure involving a prediction mechanism for instruction prefetching and a function queue for array element accessing. An overall view of the complete MU5 complex is presented together with a brief indication of its performance.
and
Research and Advances

The Manchester Mark I and atlas: a historical perspective

In 30 years of computer design at Manchester University two systems stand out: the Mark I (developed over the period 1946-49) and the Atlas (1956-62). This paper places each computer in its historical context and then describes the architecture and system software in present-day terminology. Several design concepts such as address-generation and store management have evolved in the progression from Mark I to Atlas. The wider impact of Manchester innovations in these and other areas is discussed, and the contemporary performance of the Mark I and Atlas is evaluated.
Research and Advances

The CRAY-1 computer system

This paper describes the CRAY-1, discusses the evolution of its architecture, and gives an account of some of the problems that were overcome during its manufacture. The CRAY-1 is the only computer to have been built to date that satisfies ERDA's Class VI requirement (a computer capable of processing from 20 to 60 million floating point operations per second) [1]. The CRAY-1's Fortran compiler (CFT) is designed to give the scientific user immediate access to the benefits of the CRAY-1's vector processing architecture. An optimizing compiler, CFT, “vectorizes” innermost DO loops. Compatible with the ANSI 1966 Fortran Standard and with many commonly supported Fortran extensions, CFT does not require any source program modifications or the use of additional nonstandard Fortran statements to achieve vectorization. Thus the user's investment of hundreds of man months of effort to develop Fortran programs for other contemporary computers is protected.
Research and Advances

Architecture of the IBM system/370

This paper discusses the design considerations for the architectural extensions that distinguish System/370 from System/360. It comments on some experiences with the original objectives for System/360 and on the efforts to achieve them, and it describes the reasons and objectives for extending the architecture. It covers virtual storage, program control, data-manipulation instructions, timing facilities, multiprocessing, debugging and monitoring, error handling, and input/output operations. A final section tabulates some of the important parameters of the various IBM machines which implement the architecture.
and
Research and Advances

The evolution of the Sperry Univac 1100 series: a history, analysis, and projection

The 1100 series systems are Sperry Univac's large-scale mainframe computer systems. Beginning with the 1107 in 1962, the 1100 series has progressed through a succession of eight compatible computer models to the latest system, the 1100/80, introduced in 1977. The 1100 series hardware architecture Is based on a 36-bit word, ones complement structure which obtains one operand from storage and one from a high-speed register, or two operands from high-speed registers. The 1100 Operating System is designed to support a symmetrical multiprocessor configuration simultaneously providing multiprogrammed batch, timesharing, and transaction environments.
, , and

The intrinsically exponential complexity of the circularity problem for attribute grammars

Attribute grammars are an extension of context-free grammars devised by Knuth as a mechanism for including the semantics of a context-free language with the syntax of the language. The circularity problem for a grammar is to determine whether the semantics for all possible sentences (programs) in fact will be well defined. It is proved that this problem is, in general, computationally intractable. Specifically, it is shown that any deterministic algorithm which solves the problem must for infinitely many cases use an exponential amount of time. An improved version of Knuth's circularity testing algorithm is also given, which actually solves the problem within exponential time.
, , and

Shape the Future of Computing

ACM encourages its members to take a direct hand in shaping the future of the association. There are more ways than ever to get involved.

Get Involved

Communications of the ACM (CACM) is now a fully Open Access publication.

By opening CACM to the world, we hope to increase engagement among the broader computer science community and encourage non-members to discover the rich resources ACM has to offer.

Learn More