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We’re never satisfied with the performance level of our computing equipment and neither are the processor designers. Two-way superscalar processors were very successful around 1994. Many designs were able to execute 1.6–1.8 instructions per cycle on average, using all of the tricks described so far. As we became able to manufacture chips with an ever-increasing transistor count, it seemed that we would naturally progress to four-way and then eight-way superscalar processors. The fundamental problem we face when trying to keep four functional units busy is that it’s difficult to find contiguous sets of four (or eight) instructions that can be executed in parallel. It’s an easy cop-out to say, “the compiler will solve it all.”

The solution to these problems that will allow these processors to effectively use four functional units per cycle and hide memory latency is out-of-order execution and speculative execution . Out-of-order execution allows a later instruction to be processed before an earlier instruction is completed. The processor is “betting” that the instruction will execute, and the processor will have the precomputed “answer” the instruction needs. In some ways, portions of the RISC design philosophy are turned inside-out in these new processors.

Speculative computation

To understand the post-RISC architecture, it is important to separate the concept of computing a value for an instruction and actually executing the instruction. Let’s look at a simple example:


LD R10,R2(R0) Load into R10 from memory ... 30 Instructions of various kinds (not FDIV)FDIV R4,R5,R6 R4 = R5 / R6

Assume that (1) we are executing the load instruction, (2) R5 and R6 are already loaded from earlier instructions, (3) it takes 30 cycles to do a floating-point divide, and (4) there are no instructions that need the divide unit between the LD and the FDIV. Why not start the divide unit computing the FDIV right now, storing the result in some temporary scratch area? It has nothing better to do. When or if we arrive at the FDIV, we will know the result of the calculation, copy the scratch area into R4, and the FDIV will appear to execute in one cycle. Sound farfetched? Not for a post-RISC processor.

The post-RISC processor must be able to speculatively compute results before the processor knows whether or not an instruction will actually execute. It accomplishes this by allowing instructions to start that will never finish and allowing later instructions to start before earlier instructions finish.

To store these instructions that are in limbo between started and finished, the post-RISC processor needs some space on the processor. This space for instructions is called the instruction reorder buffer (IRB).

The post-risc pipeline

The post-RISC processor pipeline in [link] looks somewhat different from the RISC pipeline. The first two stages are still instruction fetch and decode. Decode includes branch prediction using a table that indicates the probable behavior of a branch. Once instructions are decoded and branches are predicted, the instructions are placed into the IRB to be computed as soon as possible.

Questions & Answers

what is the stm
Brian Reply
How we are making nano material?
LITNING Reply
what is a peer
LITNING Reply
What is meant by 'nano scale'?
LITNING Reply
What is STMs full form?
LITNING
scanning tunneling microscope
Sahil
how nano science is used for hydrophobicity
Santosh
Do u think that Graphene and Fullrene fiber can be used to make Air Plane body structure the lightest and strongest. Rafiq
Rafiq
what is differents between GO and RGO?
Mahi
what is Nano technology ?
Bob Reply
write examples of Nano molecule?
Bob
The nanotechnology is as new science, to scale nanometric
brayan
nanotechnology is the study, desing, synthesis, manipulation and application of materials and functional systems through control of matter at nanoscale
Damian
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
How can I make nanorobot?
Lily
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
how can I make nanorobot?
Lily
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
Difference between extinct and extici spicies
Amanpreet 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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