Low Energy Asynchronous CAM Based On Reordered Overlapped Search Mechanism

Abstract

This paper introduces a reordered overlapped search mechanism for high-throughput low-energy content-addressable memories (CAMs). Most mismatches can be found by searching a few bits of a search word. To lower power dissipation, a word circuit is often divided into two sections that are sequentially searched or even pipelined. Because of this process, most of match lines in the second section are unused. Since searching the last few bits is very fast compared to searching the rest of the bits, we propose to increase throughput by asynchronously initiating second-stage searches on the unused match lines as soon as a ï¬rst-stage search is complete. This allows the circuits to be in the required phase for their own local operation: evaluate or precharge, instead of having to synchronize their phase to the rest of the word circuits, which greatly reduces the cycle time. The proposed asynchronous CAM operates faster than a synchronous CAM with smaller energy dissipation. An internet protocol (IP) filter is used to unblock particular applications which determine the next hop for a packet by ï¬nding the longest preï¬x match. This lookup often occurs in content addressable memory (CAM), which allows bit masking of the IP address.