OCCAM: An Error Oblivious CAM

Abstract

Content addressable memories (CAMs) are widely used in many applications in general purpose computer microarchitecture, networking and domain-specific hardware accelerators. In addition to storing and reading data, CAMs enable simultaneous compare of query datawords with the entire memory content. Similar to SRAM and DRAM, CAMs are prone to errors and faults. While error correcting codes (ECCs) are widely used in DRAM and SRAM, they are not directly applicable in CAM: if a dataword that is supposed to match a query altered due to an error, it will falsely mismatch even if it is ECC-encoded. We propose OCCAM, an error oblivious CAM, which combines ECC and approximate search (matching) to allow tolerating a large and dynamically configurable number of errors. We manufactured the OCCAM silicon prototype using 65-nm commercial process and verified its error tolerance capabilities through silicon measurements. OCCAM tolerates 11% error rate (7 bit errors in each 64-bit memory row) with 100% sensitivity and specificity.