Content Addressable Memory (CAM)

 

Content addressable memory (CAM) is a storage memory in which it can be accessed by using its contents instead of using memory locations [6]. When CAM receives the input data word to search it against table of data words stored in the CAM memory, it returns the address at which the search data word is stored.

Content Addressable Memory is a storage device that stores data in its memory cell like usual memory. But additionally it also has a comparison circuitry which is used to compare search data with the data contents stored in its memory simultaneously. This comparison circuitry in the CAM cell occupies extra area than usual memory cell. Hence there is more power dissipation but high speed due to parallel searching operation.

Hence, a basic CAM cell has two functions:

1. Bit Storage like usual memory RAM. So this bit storage uses simple SRAM cell which contains two cross-coupled inverters forming positive feedback working as a D-latch.
2. Bit Comparison which is equivalent to XNOR logic operation. It is unique in CAM.

So, it has three modes of operation: read, write and compare.

Design Concept of CAM

The conceptual view diagram of CAM is shown below. It shows that CAM contains m data words in which data is stored. The search word is the n bit input data which is broadcasted onto the search lines to compare it with the table of stored words simultaneously [6]. There is a matchline associated with each stored word which indicates whether the search data is matched with the stored data or not. If the search data is matched with stored data, it is a match case otherwise mismatch case. These matchlines are fed to an encoder. This encoder generates the binary location corresponding to matchline which indicates the match case. If there are more than one matchline that indicates the match case then the priority encoder can be used to generate the matched memory location. The priority encoder gives the matching address location corresponding to highest priority matchline.

Comparison b/w RAM and CAM