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For every memory reference that the CPU makes, the specific line that would hold the reference (if it is has already been copied into the cache) is determined. The tag held in that line is checked to see if the correct block is in the cache

b/ Operations

- Each main memory block is assigned to a specific line in the cache:

i = j modulo C, where i is the cache line number assigned to main memory block j

– If M=64, C=4:

Line 0 can hold blocks 0, 4, 8, 12, ...

Line 1 can hold blocks 1, 5, 9, 13, ...

Line 2 can hold blocks 2, 6, 10, 14, ...

Line 3 can hold blocks 3, 7, 11, 15, ...

– Example:

Memory size of 1 MB (20 address bits) addressable to the individual byte

Cache size of 1 K lines, each holding 8 bytes:

Word id = 3 bits

Line id = 10 bits

Tag id = 7 bits

Where is the byte stored at main memory

location $ABCDE stored?

$ABCDE=1010101 1110011011 110

Cache line $39B, word offset $6, tag $55

c/ Remarks

  • Advantages of direct mapping

+ Easy to implement

+ Relatively inexpensive to implement

+ Easy to determine where a main memory reference can be found in cache

  • Disadvantage

+ Each main memory block is mapped to a specific cache line

+ Through locality of reference, it is possible to repeatedly reference to blocks that map to the same line number

+ These blocks will be constantly swapped in and out of cache, causing the hit ratio to be low.

2.2 associative mapping

a/ Organization

A set-associative cache or associative mapping is inhenrently more complicated than a direct-mapped cache because although the correct cache entry to examine can be computed from the memory address being referenced, a set of n cache entries must be checked to see if the need line is present.

Associative mapping can overcomes direct mapping’s main of the direct mapping, the associate cache organization is illustrated in Figure 10.3.

Figure 10.3. Associate Cache Organization

– Operation must examine each line in the cache to find the right memory block

+ Examine the line tag id for each line

+ Slow process for large caches!

– Line numbers (ids) have no meaning in the cache

+ Parse the main memory address into 2 fields (tag and word offset) rather than 3 as in direct mapping

– Implementation of cache in 2 parts:

+ Part SRAM: The lines themselves in SRAM

+ The tag storage in associative memory

– Perform an associative search over all tags

b/ Operation example

With the same example: Memory size of 1 MB (20 address bits) addressable to the individual byte. Cache size of 1 K lines, each holding 8 bytes:

Word id = 3 bits

Tag id = 17 bits

Where is the byte stored at main memory location $ABCDE stored?

$ABCDE=10101011110011011 110

Cache line unknown, word offset $6, tag $1579D.

c/ Remarks

  • Advantages

- Fast

- Flexible

  • Disadvantage
  • Implementation cost

Example above has 8 KB cache and requires 1024 x 17 = 17,408 bits of

associative memory for the tags!

2.3 set associative mapping

a/ Organization

The associative mapping is considered as compromise between direct and fully associative mappings that builds on the strengths of both

– Divide cache into a number of sets (v), each set holding a number of lines (k)

– A main memory block can be stored in any one of the k lines in a set such that

set number = j modulo v

– If a set can hold X lines, the cache is referred to as an X-way set associative cache

Most cache systems today that use set associative mapping are 2- or 4-way set


Figure. 10. 4 Set associative cache organization

b/ Example

Assume the 1024 lines are 4-way set associative:

1024/4 = 256 sets

Word id = 3 bits

Set id = 8 bits

Tag id = 9 bits

Where is the byte stored at main memory

Location $ABCDE stored?

$ABCDE=101010111 10011011 110

Cache set $9B, word offset $6, tag $157

3. replacement algorithms

As we known, cache is the fast and small memory. when new block is brough into the cache, one of the blocks existing must be replaced by the new block that is to be read from memory.

For direct mapping, there is only one possible line for any particular block and no choice is possible. For the associative cache or a set associative cache, a replacement algorithm is needed.

A number of algorithms can be tried:

– Least Recently Used (LRU)

– First In First Out (FIFO)

– Least Frequently Used (LFU)

– Random

Probably the most effective algorithm is least recently used (LRU): Replace that block in the set that has been in the cache longest with no refrence.

4. performances

4.1 write policy

– When a line is to be replaced, must update the original copy of the line in main memory if any addressable unit in the line has been changed

– Write through

+ Anytime a word in cache is changed, it is also changed in main memory

+ Both copies always agree

+ Generates lots of memory writes to main memory

– Write back

+ During a write, only change the contents of the cache

+ Update main memory only when the cache line is to be replaced

+Causes “cache coherency” problems -- different values for the contents of an address are in the cache and the main memory

+ Complex circuitry to avoid this problem

4.2 block / line sizes

– How much data should be transferred from main memory to the cache in a single memory reference

– Complex relationship between block size and hit ratio as well as the operation of the system bus itself

– As block size increases,

+ Locality of reference predicts that the additional information transferred will likely be used and thus increases the hit ratio (good)

+ Number of blocks in cache goes down, limiting the total number of blocks in the cache (bad)

+ As the block size gets big, the probability of referencing all the data in it goes down (hit ratio goes down) (bad)

+ Size of 4-8 addressable units seems about right for current systems

Number of caches

Single vs. 2-level cache:

- Modern CPU chips have on-board cache (Level 1 – L1):

L1 provides best performance gains

- Secondary, off-chip cache (Level 2) provides higher speed access to main memory

L2 is generally 512KB or less more than this is not cost-effective.

Questions & Answers

Is there any normative that regulates the use of silver nanoparticles?
Damian Reply
what king of growth are you checking .?
What fields keep nano created devices from performing or assimulating ? Magnetic fields ? Are do they assimilate ?
Stoney Reply
why we need to study biomolecules, molecular biology in nanotechnology?
Adin Reply
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
what school?
biomolecules are e building blocks of every organics and inorganic materials.
anyone know any internet site where one can find nanotechnology papers?
Damian Reply
sciencedirect big data base
Introduction about quantum dots in nanotechnology
Praveena Reply
what does nano mean?
Anassong Reply
nano basically means 10^(-9). nanometer is a unit to measure length.
do you think it's worthwhile in the long term to study the effects and possibilities of nanotechnology on viral treatment?
Damian Reply
absolutely yes
how to know photocatalytic properties of tio2 nanoparticles...what to do now
Akash Reply
it is a goid question and i want to know the answer as well
characteristics of micro business
for teaching engĺish at school how nano technology help us
Do somebody tell me a best nano engineering book for beginners?
s. Reply
there is no specific books for beginners but there is book called principle of nanotechnology
what is fullerene does it is used to make bukky balls
Devang Reply
are you nano engineer ?
fullerene is a bucky ball aka Carbon 60 molecule. It was name by the architect Fuller. He design the geodesic dome. it resembles a soccer ball.
what is the actual application of fullerenes nowadays?
That is a great question Damian. best way to answer that question is to Google it. there are hundreds of applications for buck minister fullerenes, from medical to aerospace. you can also find plenty of research papers that will give you great detail on the potential applications of fullerenes.
what is the Synthesis, properties,and applications of carbon nano chemistry
Abhijith Reply
Mostly, they use nano carbon for electronics and for materials to be strengthened.
is Bucky paper clear?
carbon nanotubes has various application in fuel cells membrane, current research on cancer drug,and in electronics MEMS and NEMS etc
so some one know about replacing silicon atom with phosphorous in semiconductors device?
s. Reply
Yeah, it is a pain to say the least. You basically have to heat the substarte up to around 1000 degrees celcius then pass phosphene gas over top of it, which is explosive and toxic by the way, under very low pressure.
Do you know which machine is used to that process?
how to fabricate graphene ink ?
for screen printed electrodes ?
What is lattice structure?
s. Reply
of graphene you mean?
or in general
in general
Graphene has a hexagonal structure
On having this app for quite a bit time, Haven't realised there's a chat room in it.
what is biological synthesis of nanoparticles
Sanket Reply
Difference between extinct and extici spicies
Amanpreet Reply
While the American heart association suggests that meditation might be used in conjunction with more traditional treatments as a way to manage hypertension
Beverly Reply
Researchers demonstrated that the hippocampus functions in memory processing by creating lesions in the hippocampi of rats, which resulted in ________.
Mapo Reply
The formulation of new memories is sometimes called ________, and the process of bringing up old memories is called ________.
Mapo Reply
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Source:  OpenStax, Computer architecture. OpenStax CNX. Jul 29, 2009 Download for free at http://cnx.org/content/col10761/1.1
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