Low-Area TCAM Using A Don’t Care Reduction Scheme

Abstract

This paper proposes a low-area ternary content-addressable memory using a don’t care reduction (DCR) scheme. In Internet Protocol (IP) address, the prefix bits store “0” or “1,” and the remaining bits store “X” (don’t care). The conventional ternary content-addressable memory (TCAM) needs a 2N-bit memory for an N-bit IP address, because its TCAM cell uses 2-bit memory to store a data bit (“0” or “1”) and a don’t care (“X”) bit. However, the proposed DCR TCAM (DCR-TCAM) needs a ( $N + \log _{2}N$ )-bit memory for an N-bit IP address. It also stores N-bit data, but it encodes N-bit “X”s into a $\log_{2}N$ -bit code, storing the first “X” position. The proposed DCR-TCAM performs the function of “X” in the TCAM from the $\log_{2}N$ -bit code by using additional decoders and bypass transistors. A $256 \times 128$ -bit DCR-TCAM chip was fabricated using a 1.2-V, 65-nm CMOS process. Its area is 0.22 mm2, which is only 72% of the conventional TCAM. It expands an effective memory size by applying the data-relocation TCAM scheme. Its energy/bit/search is 0.41 fJ at a clock frequency of 330 MHz.