SAT-Based Synthesis Methods for Safety Specs

Automatic synthesis of hardware components from declarative specifications is an ambitious endeavor in computer aided design. Existing synthesis algorithms are often implemented with Binary Decision Diagrams (BDDs), inheriting their scalability limitations. Instead of BDDs, we propose several new methods to synthesize finite-state systems from safety specifications using decision procedures for the satisfiability of quantified and unquantified Boolean formulas (SAT-, QBF- and EPR-solvers). The presented approaches are based on computational learning, templates, or reduction to first-order logic. We also present an efficient parallelization, and optimizations to utilize reachability information and incremental solving. Finally, we compare all methods in an extensive case study. Our new methods outperform BDDs and other existing work on some classes of benchmarks, and our parallelization achieves a super-linear speedup. This is an extended version of [5], featuring an additional appendix.Comment: Extended version of a paper at VMCAI'1

Emulation of scan paths in sequential circuit synthesis

Scan paths are generally added to a sequential circuit in a final design for testability step. We present an approach to incorporate the behavior of a scan path during circuit synthesis, thus avoiding to implement the scan path shift register as a separate structural entity. The shift transitions of the scan path are treated as a part of the system functionality. Depending on the minimization strategy for the system logic, either the delay or the area of the circuit can be reduced compared to a conventional scan path. which may be interpreted as a special case of realizing the combinational logic. The approach is also extended to partial scan paths. It is shown that the resulting structure is fully testable and test patterns can be efficiently produced by a combinational test generator. The advantages of the approach are illustrated with a collection of finite state machine examples

Software Model Checking with Explicit Scheduler and Symbolic Threads

In many practical application domains, the software is organized into a set of threads, whose activation is exclusive and controlled by a cooperative scheduling policy: threads execute, without any interruption, until they either terminate or yield the control explicitly to the scheduler. The formal verification of such software poses significant challenges. On the one side, each thread may have infinite state space, and might call for abstraction. On the other side, the scheduling policy is often important for correctness, and an approach based on abstracting the scheduler may result in loss of precision and false positives. Unfortunately, the translation of the problem into a purely sequential software model checking problem turns out to be highly inefficient for the available technologies. We propose a software model checking technique that exploits the intrinsic structure of these programs. Each thread is translated into a separate sequential program and explored symbolically with lazy abstraction, while the overall verification is orchestrated by the direct execution of the scheduler. The approach is optimized by filtering the exploration of the scheduler with the integration of partial-order reduction. The technique, called ESST (Explicit Scheduler, Symbolic Threads) has been implemented and experimentally evaluated on a significant set of benchmarks. The results demonstrate that ESST technique is way more effective than software model checking applied to the sequentialized programs, and that partial-order reduction can lead to further performance improvements.Comment: 40 pages, 10 figures, accepted for publication in journal of logical methods in computer scienc

Artificial Intelligence

Contains research objectives and reports on five research projects.Computation Center, M.I.T

A comparative modeling of supernova 1993J

The light curve of Supernova (SN) 1993J is calculated using two approaches to radiation transport as exemplified by the two computer codes, STELLA and EDDINGTON. Particular attention is paid to shock breakout and the photometry in the U, B, and V bands during the first 120 days. The hydrodynamical model, the explosion of a 13 Msun star which had lost most of its hydrogenic envelope to a companion, is the same in each calculation. The comparison elucidates differences between the approaches and also serves to validate the results of both. STELLA includes implicit hydrodynamics and is able to model supernova evolution at early times, before the expansion is homologous. STELLA also employs multi-group photonics and is able to follow the radiation as it decouples from the matter. EDDINGTON uses a different algorithm for integrating the transport equation, assumes homologous expansion, and uses a finer frequency resolution. Good agreement is achieved between the two codes only when compatible physical assumptions are made about the opacity. A new result for SN 1993J is a prediction of the continuum spectrum near the shock breakout (calculated by STELLA) which is superior to the results of other standard single energy group hydrocodes such as VISPHOT or TITAN. Based on the results of our independent codes, we discuss the uncertainties involved in the current time dependent models of supernova light curves.Comment: 43 pages with 22 eps figures, aaspp4.sty + epsf.sty, Accepted by ApJ, to appear in March 20, 1998 issue, Vol. 49

Variable Reuse for Efficient Image Computation

Image computation, that is, computing the set of states reachable from a given set in one step, is a crucial component in typical tools for BDD-based symbolic reachability analysis. It has been shown that the size of the intermediate BDDs during image computation can be dramatically reduced via conjunctive partitioning of the transition relation and ordering the conjuncts for facilitating early quantification. In this paper, we propose to enhance the effectiveness of these techniques by reusing the quantified variables. Given an ordered set of conjuncts, if the last conjunct that uses a variable u appears before the first conjunct that uses another variable v, then v can be renamed to u, assuming u will be quantified immediately after its last use. In general, multiple variables can share the same identifier so the BDD nodes that are inactive but not garbage collected may be activated. We give a polynomial-time algorithm for generating the optimum number of variables that are required for image computation and show how to modify the image computation accounting for variable reuse. The savings for image computation are demonstrated on ISCAS\u2789 and Texas\u2797 benchmark models

The alpha 7 nicotinic receptor agonist PHA-543613 hydrochloride inhibits <i>Porphyromonas gingivalis</i>-induced expression of interleukin-8 by oral keratinocytes

Objective: The alpha 7 nicotinic receptor (α7nAChR) is expressed by oral keratinocytes. α7nAChR activation mediates anti-inflammatory responses. The objective of this study was to determine if α7nAChR activation inhibited pathogen-induced interleukin-8 (IL-8) expression by oral keratinocytes.&lt;p&gt;&lt;/p&gt; Materials and methods: Periodontal tissue expression of α7nAChR was determined by real-time PCR. OKF6/TERT-2 oral keratinocytes were exposed to &lt;i&gt;Porphyromonas gingivalis&lt;/i&gt; in the presence and absence of a α7nAChR agonist (PHA-543613 hydrochloride) alone or after pre-exposure to a specific α7nAChR antagonist (α-bungarotoxin). Interleukin-8 (IL-8) expression was measured by ELISA and real-time PCR. Phosphorylation of the NF-κB p65 subunit was determined using an NF-κB p65 profiler assay and STAT-3 activation by STAT-3 in-cell ELISA. The release of ACh from oral keratinocytes in response to &lt;i&gt;P. gingivalis&lt;/i&gt; lipopolysaccharide was determined using a GeneBLAzer M3 CHO-K1-blacell reporter assay.&lt;p&gt;&lt;/p&gt; Results: Expression of α7nAChR mRNA was elevated in diseased periodontal tissue. PHA-543613 hydrochloride inhibited &lt;i&gt;P. Gingivalis&lt;/i&gt;-induced expression of IL-8 at the transcriptional level. This effect was abolished when cells were pre-exposed to a specific α7nAChR antagonist, α-bungarotoxin. PHA-543613 hydrochloride downregulated NF-κB signalling through reduced phosphorylation of the NF-κB p65-subunit. In addition, PHA-543613 hydrochloride promoted STAT-3 signalling by maintenance of phosphorylation. Furthermore, oral keratinocytes upregulated ACh release in response to &lt;i&gt;P. Gingivalis&lt;/i&gt; lipopolysaccharide.&lt;p&gt;&lt;/p&gt; Conclusion: These data suggest that α7nAChR plays a role in regulating the innate immune responses of oral keratinocytes.&lt;p&gt;&lt;/p&gt