The X architecture

Abstract

The X Architecture is an integrated-circuit wiring architecture based on the pervasive use of diagonal wires. Compared with the traditional, currently ubiquitous, Manhattan architecture, the X Architecture demonstrates a wire length reduction of more than 20% and a via reduction of more 30%. Because of the rapidly increasing percentage of delay due to interconnect and the manufacturing challenges due to vias in the nanometer realm, these length and via reductions result simultaneously in a chip performance improvement of 10%, a power reduction of 20%, and a die cost reduction of 30%. Furthermore, the reduction in both wire length and parallel runs on different layers often both reduces die size and improves signal integrity. Remarkably, on virtually every important measure of chip quality, the X Architecture is superior to the Manhattan architecture.While diagonal wiring has been discussed for years, and short diagonal jogs have even been used for years, pervasive diagonal wiring has not been used on an IC before 2002 (to our knowledge). The fundamental reasons for this are not manufacturing limitations, as might be suspected, but EDA limitations, and the changes required to take full advantage of the X Architecture are significant and numerous. In particular, routing must be not only octilinear, but also gridless and non-preferred direction. In addition, significant changes are required at least in floorplanning, placement, global routing, extraction, power routing, clock routing, wire length estimation (e.g., in synthesis), database, graphics, and even data interchange formats. The folklore that 45-degree wiring might not be worth the trouble because it can provide only a 10% reduction in wire length is rooted in the incorrect assumptions that (a) only the router must change, (b) the router must resemble contemporary, preferred-direction, net-at-a-time maze routers, and (c) that wire length is the only major contributor to interconnect delay.In this short paper, we present some of the challenges and opportunities afforded by the X Architecture and show some early results that demonstrate the promise of pervasive, diagonal wiring, reflecting our belief that five years from now, virtually all, high-performance, integrated circuits will use the X Architecture.