In this paper, we present a review of the state of the art in memristor technologies. Along with ionic conducting devices [i.e., conductive bridging random access memory (CBRAM)], we include phase change, and organic/organo-metallic technologies, and we review the most recent advances in oxide-based memristor technologies. We present progress on 3-D integration techniques, and we discuss the behavior of more mature memristive technologies in extreme environments.

The articles in this special issues are the first of two special issues that will provide an in-depth treatment of the subject of reconfigurable systems as enabled through advancements in the electrical engineering field, as well as the approaches we use to manage complexity in systems having millions of fixed and programmable elements. It is our view that the frenetic evolution of the field programmable gate array (FPGA) as a preeminent digital platform will not only continue for the foreseeable future, but will serve as an inspiration/template for other functional domains to include analog, radio-frequency, photonic, and even those not strictly electrical in nature to include thermal, fluidic, and mechanical. This issue will serve as an immersion into the subject of reconfigurable systems, both for the purposes of gaining familiarity with essential theoretical elements, the key developments in the field, and the potential future directions.

Reconfigurability can be thought of as software-defined functionality, where flexibility is controlled predominately through the specification of bit patterns. Reconfigurable systems can be as simple as a single switch, or as abstract and powerful as programmable matter. This paper considers the generalization of reconfigurable systems as an important evolving discipline, bolstered by real-world archetypes such as field programmable gate arrays and software-definable radio (platform and application, respectively). It considers what reconfigurable systems actually are, their motivation, their taxonomy, the fundamental mechanisms and architectural considerations underlying them, designing them and using them in applications. With well-known real-world instances, such as the field programmable gate array, the paper attempts to motivate an understanding of the many possible directions and implications of a new class of system which is fundamentally based on the ability to change.

The articles in this special issue focus on reconfigurable systems. Reconfigurability is about "change," specifically soft-defined change, whereby through the manipulation of bit sequences we can customize the properties of components, and in some cases define systems themselves. One can design systems for reconfigurability (the art of engineering degrees of freedom into embedded systems), so that users expecting this reconfigurability can exploit it. This issue focuses on important reconfigurable platforms [field-programmable gate arrays (FPGAs)], development tools, and applications (reconfigurable computing and software radios). As we discussed, reconfigurability can be both dynamic and adaptive, but one should not confuse the two concepts. Reconfigurability refers to specific features that can be changed (to include dynamic and partial reconfigurability), whereas adaptiveness addresses the behavioral constructs that inform how reconfigurable features might best be exploited in live use conditions.