I'm just getting started with cs. In school I heard about that modern micro processor are not perfect. So what are the issues? Are does problems related to energy(power draw)/ time /speed(clock speed)? Or is the design faulty? So if you could design the perfect cpu what would it look like?

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    $\begingroup$ This question is way too broad. You ask for a summary of the whole field of microprocessor design. $\endgroup$
    – adrianN
    Oct 27 '16 at 15:16

There are inherent tradeoffs in targeting a given use. An implementation optimized for one workload, power budget, and cost (at a particular volume of sales) will necessarily be less than optimal for some other use.

Binary compatibility places another constraint on optimization. An ISA which is a good/easy compiler target and which is not strongly tied to a particular set of implementation techniques and technologies will be less than optimal for a given implementation technology (i.e., some optimizations are hindered by the abstraction presented in the ISA) and will sacrifice some benefits in performance, energy-efficiency, etc. for these other goals.

(Redesigning the ISA for every implementation is not a solution, even when using an intermediate-level software distribution format to provide compatibility. The cost and delay of ISA design (including developing an optimizing compiler for the software distribution format) constrains how extensively changes can be made to optimize for particular targets. Even microarchitectures are typically extensively reused because of development costs, including time to market and risk.)

Time to market also constrains optimization. High performance designs targeting new manufacturing processes begin work years before the process characteristics are defined. (This also applies to application targets. Algorithms and use patterns change.) There is also a limited amount of design effort that can be reasonably applied. Even if the value of optimizations could justify the development costs, worse but available products can establish market momentum.


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