Associative memory, also known as content-addressable memory (CAM) or associative storage, is a type of computer memory that allows the retrieval of information based on content rather than a specific memory address.
• Unlike traditional memory systems where data is accessed using a specific address, associative memory enables searching for content, and the system returns the address or addresses where the matching content is stored.
Types of Associative Memory:
There are two primary types of associative memory:
- Content-Addressable Memory (CAM)
- Associative cache memory.
Hardware Organization of Associative Memory:
The diagram shown above is a block diagram of the hardware organization of associative memory. It includes input register, mask register, select register and output register, and an associative array with m words, each with n bits.
Here, the function of the input register is to hold the content that we want to write or search in the associative memory. At the moment, it can hold only one word having length let us say n.
• The function of the mask register is to provide a mask for selecting a specific key or field in the word of the input register. The input register can hold single word length data, where length is n so that the maximum value of the mask register’s length can be n.
• The function of the select register is to contain m-bits, one for each memory’s word. If the input data of the input register get compared with the key in the m registers, and we get a match, then that particular bit is set in the select register.
• The function of the output register is to contain the data word that got matched retrieved from the associative memory.
Following is the diagram showing the relationship between the memory array and external registers in associative memory:
Fig: Associative memory of m words and n cells per word
These are the cells marked by the letter C in the array with two subscripts. The first subscripts provide the word number, and the second one depicts the bit’s position in the world. So, cell Cij is the cell for bit j in word i. Bit Ai in the argument register got compared with every bit in column j of the array provided that Ki = 1. This has been done for every column j = 1, 2, . . . , n. If one match occurs between every unmasked bit of the argument and the bits in word i, the corresponding bit M1 in the match register is set to 1. If one or more unmasked bits of the argument and the word do not match, M1 is cleared to 0.
Read-Write operation in Associative Memory:
- Read Operation:
- In an associative memory, a read operation involves presenting a search key or a portion of data to the memory.
- The memory compares the presented key with all stored entries simultaneously, searching for a match.
- If a match is found, the memory returns the associated data or information corresponding to the matched entry.
- Associative memory allows for extremely fast retrieval of data since it can search the entire memory in parallel rather than sequentially.
- Write Operation:
- In associative memory, a write operation involves storing data along with its associated key.
- When writing data into associative memory, both the data and its corresponding key are presented to the memory.
- The memory then compares the presented key with all stored keys simultaneously to determine if there is a match.
- If a match is found, the memory updates the associated data with the newly provided data.
- If no match is found, the memory creates a new entry and stores the provided data along with its associated key.
- It’s worth noting that some associative memory systems may have limitations on write operations, such as allowing updates only to existing entries or requiring specific protocols for adding new entries.
