This work was supported by NSF Presidential Young Investigator grant CCR-8351942, and by NSF grant CCR-9102553. Partial support for this work was provided by the Directorate for Computer and Information Science and Engineering, NSF Grant No. CDA-8805910. We also used equipment at the Computer Science Department and Rensselær Design Research Center at RPI. Part of this work was conducted using the computational resources of the Northeast Parallel Architectures Center (NPAC) at Syracuse University, which is funded by and operates under contract to DARPA and the Air Force Systems Command, Rome Air Development Center (RADC), Griffiss Air Force Base, NY, under contract # F306002-88-C-0031. Part of the research reported here was made possible through the support of the New Jersey Commission on Science and Technology and the Rutgers University CAIP Center's Industrial Members. Finally, the referees' many detailed and constructive comments greatly improved this paper.