Associative memory 14208

Associative Memory
Computation
Ameer Mehmood 14208
Adeel Ahmad 13700

Introduction
To search particular data in memory, data is read from
certain address and compared if the match is not
found content of the next address is accessed and
compared.
This goes on until required data is found. The number
of access depend on the location of data and efficiency
of searching algorithm.
This searching time can be reduced if data is searched
on the basis of content.

Introduction
A memory unit accessed by content is called
associative memory or content addressable
memory(CAM) or associative storage or associative
array.
This type of memory is accessed simultaneously and
in parallel on the basis of data content.
Memory is capable of finding empty unused location
to store the word.

Associative Memory Organization

Associative Memory is organized in such a way.
Argument register(A): It contains the word to be
searched. It has n bits(one for each bit of the word).
Key Register(K):This specifies which part of the
argument word needs to be compared with words in
memory. If all bits in register are 1, The entire word
should be compared. Otherwise, only the bits having k-
bit set to 1 will be compared.

Associative memory array: It contains the words which
are to be compared with the argument word.
Match Register(M):
It has m bits, one bit corresponding to each word in
the memory array. After the matching process, the bits
corresponding to matching words in match register are
set to 1.

Key register provide the mask for choosing the
particular field in A register.
The entire content of A register is compared if key
register content all 1.
Otherwise only bit that have 1 in key register are
compared.
If the compared data is matched corresponding bits in
the match register are set.

Reading is accomplished by sequential access in
memory for those words whose bit are set.

Let us include key register. If Kj=0 then there is no
need to compare Aj and Fij.
Only when Kj=1, comparison is needed.
This achieved by ORing each term with Kj.

Write operation:
If the entire memory is loaded with new information at
once prior to search operation then writing can be done
by addressing each location in sequence.
Tag register contain as many bits as there are words in
memory.
It contain 1 for active word and 0 for inactive word.

If the word is to be inserted, tag register is scanned
until 0 is found and word is written at that position and
bit is change to 1.
Read Operation:
When a word is to be read from an associative
memory, the contents of the word, or a part of the
word is specified.

 If more than one word match with the content, all the
matched words will have 1 in the corresponding bit
position in match register.
Matched words are then read in sequence by applying
a read signal to each word line.
In most application, the associative memory stores a
table with no two identical items under a given key.

Associative memory Architecture
It is a hardware search engines, a special type of
computer memory used in certain very high searching
applications.
composed of conventional semiconductor memory
(usually SRAM) with added comparison circuitry that
enable a search operation to complete in a single clock
cycle.
SRAM is a type of semiconductor memory that
uses bistable latching circuitry to store each bit.

Types of Associative memory
There are two types of Associative memory, which both
are used in different conditions.
 Auto-associative
 Auto-associative memory takes back(retrieves) a
previously stored pattern that most closely resembles
the current pattern.

Types of Associative memory
 Hetero-associative
Hetero-associative memory, the retrieved pattern is in
general, different from the input pattern not only in
content but possibly also in type and format.
Neutral networks are used to implement these
associative memory models called NAM (Neutral
associative memory).

Advantages of Associative memory
This is suitable for parallel searches. It is also used
where search time needs to be short
Associative memory is often used to speed up
databases, in neural networks and in the page tables
used by the virtual memory of modern computers.
CAM-design challenge is to reduce power
consumption associated with the large amount of
parallel active circuitry, without sacriﬁcing speed or
memory density

Disadvantages of Associative memory
An associative memory is more expensive than a
random access memory because each cell must have
an extra storage capability as well as logic circuits for
matching its content with an external argument.
Usually associative memories are used in applications
where the search time is very critical and must be
very short.

Associative memory 14208

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