associative memory.ppt.pptx

7/27/2019 Associative memory.ppt.pptx

http://slidepdf.com/reader/full/associative-memorypptpptx 1/9



Conventionally memory is accessed by an address wired in for each word.

Associative memory or CAM(Content AddressableMemory) is accessed by content of a word.

Data stored in memory words is divided into variousfields

One of these fields can be used as the key and thememory can be accessed by a specified key value(say v)



The input data v is simultaneously compared with key field of each of the words in the CAM.

If there is a compare equal signal corresponding to a word then tat word can be read out.

By providing content of only one field of storedinformation as the address complete information can

be accessed.



Tag field of MAR of the page address.

When a page is brought to cache its tag bits are stored

in the tag field of a CAM word. The cache page frame address in which it is stored ,is

noted in 2nd field(address in cache)

To find whether a page is currently there in cache or

not,its necessary to access the CAM by its 1st fieldcalled key field.



Tag value of page address to be searched is given asinput key value that is simultaneously compared with

the tag of each CAM word. If it compares equal with any match signal will be

generated telling that specified page is currently resident in the cache.

To find in which location of cache it resides, CAM word for which the match signal has been generatedcan be read out.

Now with this address the cache can be accessed.



Working principle of CAM Mask register specifies the key field

Input data is simultaneously compared with key field

of each word. Select ckt implements two functions:

-> it stores word locations for which matchhas occurred.

-> it reads out the words in apredetermined order for the matchpositions.



A word stored in the CAM is a pair(Key,Data)

Any subfield of the word can be specified as the key.

The read or write instruction is preceded by the matchinstruction having the format:

Match key, Input data

The read or write operation can next be performed on

each of the words for which match signal is generated.



Associative memory cell and array To realize the behaviour each cell needs to have a

compare ckt in addition to the storage element.

As per the key field specified by mask register, inputdata bit is compared with the storage content.

The select ckt has a F/F (marked as T) for each word.



With search at logic 1 level , T F/F is set by match=1signal.

The o/p of match=1 signal of different cells of key fieldof a word is ANDed to set T F/F corresponding to the word

If the logic is reversed(cells o/p mismatch=1 signal)

then output of each cell should be ORed to reset the TF/F preset to 1 initially.

associative memory.ppt.pptx

Documents