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In addition to the bank stall hazard, single-word references made directly to a multibanked memory system carry a greater latency than those of (successfully) cached memory accesses. This is because references are going out to memory that is slower than cache, and there may be additional address translation steps as well. However, banked memory references are pipelined. As long as references are started well enough in advance, several pipelined, multibanked references can be in flight at one time, giving you good throughput.

The CDC-205 system performed vector operations in a memory-to-memory fashion using a set of explicit memory pipelines. This system had superior performance for very long unit-stride vector computations. A single instruction could perform 65,000 computations using three memory pipes.

Software managed caches

Here’s an interesting thought: if a vector processor can plan far enough in advance to start a memory pipe, why can’t a RISC processor start a cache-fill before it really needs the data in those same situations? In a way, this is priming the cache to hide the latency of the cache-fill. If this could be done far enough in advance, it would appear that all memory references would operate at the speed of the cache.

This concept is called prefetching and it is supported using a special prefetch instruction available on many RISC processors. A prefetch instruction operates just like a standard load instruction, except that the processor doesn’t wait for the cache to fill before the instruction completes. The idea is to prefetch far enough ahead of the computation to have the data ready in cache by the time the actual computation occurs. The following is an example of how this might be used:


DO I=1,1000000,8 PREFETCH(ARR(I+8))DO J=0,7 SUM=SUM+ARR(I+J)END DO END DO

This is not the actual FORTRAN. Prefetching is usually done in the assembly code generated by the compiler when it detects that you are stepping through the array using a fixed stride. The compiler typically estimate how far ahead you should be prefetching. In the above example, if the cache-fills were particularly slow, the value 8 in I+8 could be changed to 16 or 32 while the other values changed accordingly.

In a processor that could only issue one instruction per cycle, there might be no payback to a prefetch instruction; it would take up valuable time in the instruction stream in exchange for an uncertain benefit. On a superscalar processor, however, a cache hint could be mixed in with the rest of the instruction stream and issued alongside other, real instructions. If it saved your program from suffering extra cache misses, it would be worth having.

Post-risc effects on memory references

Memory operations typically access the memory during the execute phase of the pipeline on a RISC processor. On the post-RISC processor, things are no different than on a RISC processor except that many loads can be half finished at any given moment. On some current processors, up to 28 memory operations may be active with 10 waiting for off-chip memory to arrive. This is an excellent way to compensate for slow memory latency compared to the CPU speed. Consider the following loop:

Questions & Answers

Is there any normative that regulates the use of silver nanoparticles?
Damian Reply
what king of growth are you checking .?
Renato
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
?
Kyle
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
Adin
why?
Adin
what school?
Kyle
biomolecules are e building blocks of every organics and inorganic materials.
Joe
anyone know any internet site where one can find nanotechnology papers?
Damian Reply
research.net
kanaga
sciencedirect big data base
Ernesto
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.
Bharti
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
Daniel
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
Maciej
characteristics of micro business
Abigail
for teaching engĺish at school how nano technology help us
Anassong
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
NANO
what is fullerene does it is used to make bukky balls
Devang Reply
are you nano engineer ?
s.
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.
Tarell
what is the actual application of fullerenes nowadays?
Damian
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.
Tarell
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.
Virgil
is Bucky paper clear?
CYNTHIA
carbon nanotubes has various application in fuel cells membrane, current research on cancer drug,and in electronics MEMS and NEMS etc
NANO
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.
Harper
Do you know which machine is used to that process?
s.
how to fabricate graphene ink ?
SUYASH Reply
for screen printed electrodes ?
SUYASH
What is lattice structure?
s. Reply
of graphene you mean?
Ebrahim
or in general
Ebrahim
in general
s.
Graphene has a hexagonal structure
tahir
On having this app for quite a bit time, Haven't realised there's a chat room in it.
Cied
what is biological synthesis of nanoparticles
Sanket Reply
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
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Source:  OpenStax, High performance computing. OpenStax CNX. Aug 25, 2010 Download for free at http://cnx.org/content/col11136/1.5
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