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Old September 18, 2013, 08:54 PM
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Default RISC cooler than CISC?

This the right place for this kind of question?

Do RISC processors run cooler than CISC?

Someone here trying to tell us that an Ipad is superior to a desktop pc because, amongst other things, they don't overheat like pc's.

We're trying to tell him that's because they've got underpowered little chips and aren't capable of doing much.

He's coming back and saying a calculator is a calculator and the Ipad calculator can do anything any other cpu can do - it just might take it longer...

and it will work night and day without all the enormous mass of heatsink and fan...

whereas the pc cpu won't/can't...

Why? They're all RISC aren't they?

How many pc's are RISC now?

Aren't modern cpu's RISC core within a sort of CISC envelope?

Does RISC run cooler than CISC? Why?
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Old September 19, 2013, 01:29 AM
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Errrr, no, modern CPU's are all CISC. It started by adding all these MMX and SSE and 3DNow! extensions to the x86 instruction set, that the CISC dependancy.

RICS, on the other hands, have few instructions, but can push them thru really fast. To make the RISC work using x86 code, you need a plenty of microcode emulation for it. Check VIA C3 Nehemiah, Ezra CPU's for example. RISC's by internal, microcode emulation of x86. Not exactly fast, but can be passively cooled back in 2006 or so...

Hence note the principialy different approach - RISC - few transistors fast, CISC - complex set of many transistors.
That determine the heat output. More transistors = more heat.

RICS CPU's have their day when the CISC ones cannot get into higher clocks. Todays the technical problems achieving 5 or 6GHz limits are pretty much covered and IIRC WR is around 8GHz (lame Pentium 4 Netburst architecture designed for maximum clock, yet zero effectivity per clock...). So if a CISC can do 8GHz, then RISC have to be at least twice or more faster by clock, so it can compete with it. But it can't reach such speeds, so the idea is now pretty much abandoned in favor of even more complex CISC cpu's and even more cores (transistors = heat).

Of course a good RISC CPU can make wonders, because few transistors = fewer power requirments and small generated heat.
But CISC CPU's are closing in with the smaller and smaller transistors (knocking on the 10nm gate) and even complex CISC CPU's, such as Atom, can live with few watts and output only a few watts.
Of course they are nowhere near fast, but that it not the point of their existence.

Hope that helps.

Of course it will be todays possible create a 10GHz RISC CPU. But what it would be good for w/o a x86 set? And futhermore, how it can overpower the standard CPU's? Not even in desktop - much less in massively parallel server enviroment - it can get even close to the performance of todays fastest x86 CPU's.
But it can be great for some portable devices that run only their applications. No doubt about it. But as soon, as we hit and make the 10nm process working for CISC's in few years, the reason to have a special (non x86 RISC) CPU (power and heat requirments) will be all over.
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Last edited by trodas; September 19, 2013 at 01:36 AM.
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Old September 19, 2013, 02:56 AM
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Educational!

I almost understood all of that.
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Old September 19, 2013, 02:42 PM
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Exact reason why I didn't respond to this thread because I don't understand the topic completely. If you look at AMD's ARM push, they are targeting low end server environment where you can trade off processing power for lower energy and heat. It's pretty poor logic to argue that an IPad is better simply it's cooler to run. The amount of heat / energy you save per hour doesn't mean squat if at the end of the computation task it eats up close to what a CISC consumed. In addition, he seem to ignored that there's a value to time. If you are doing research, ie cancer / AIDS research, would you put up with a chip that takes 4 times as long to do something just because it runs cooler and eats less energy? He obviously doesn't understand that there's more to watching cat videos on YouTube.
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Old September 19, 2013, 03:16 PM
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Quote:
Originally Posted by moocow View Post
...there's more to watching cat videos on YouTube.
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Old September 19, 2013, 03:29 PM
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A RISC is a Reduced Instruction Set Computer, such as something that has an ARM processor inside, whereas a CISC is a Complex instruction set computer such as an x86 processor.

The good thing about a RISC is that it's designed to do a specific task extremely well, efficiently and low power (if possible). However, it CANNOT do anything aisde what it's designed for (usually, some slight exceptions with hacks and such). A CISC isn't as good as a RISC when it comes to what the RISC specifically does, but anything else it is far superior as it has the ability to perform different sorts of tasks either simultaneously or just faster.

A RISC shouldn't overheat as it's being designed as the core component and everything is revolves around it, whereas a CISC can well be pushed beyond it's normal limits, runs faster, uses more power, etc which generates more heat.

Hopefully some of this makes sense, just quick reiterating some of the stuff we were 'taught' at school.
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Old September 19, 2013, 03:34 PM
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Quote:
Originally Posted by trodas View Post
Errrr, no, modern CPU's are all CISC. It started by adding all these MMX and SSE and 3DNow! extensions to the x86 instruction set, that the CISC dependancy.

RICS, on the other hands, have few instructions, but can push them thru really fast. To make the RISC work using x86 code, you need a plenty of microcode emulation for it. Check VIA C3 Nehemiah, Ezra CPU's for example. RISC's by internal, microcode emulation of x86. Not exactly fast, but can be passively cooled back in 2006 or so...

....snip...
Of course it will be todays possible create a 10GHz RISC CPU. But what it would be good for w/o a x86 set? And futhermore, how it can overpower the standard CPU's? Not even in desktop - much less in massively parallel server enviroment - it can get even close to the performance of todays fastest x86 CPU's.
But it can be great for some portable devices that run only their applications. No doubt about it. But as soon, as we hit and make the 10nm process working for CISC's in few years, the reason to have a special (non x86 RISC) CPU (power and heat requirments) will be all over.
Not quite! Most modern CPU's AREN'T CISC, they're RISC! Why? Because most are ARM! ARM is a RISC architecture. By having a limited set of instructions, it means that they have to spend less on the "space" to be able to understand long instructions, which x86 does - and most of it is simply for legacy compatibility, even though no modern program would use. ARM removes those legacy instructions with each iteration, keeping their instruction set light and without too much unnecessary space.

These short instructions are easier to decode and require less instruction cache, hence they use less transistors at more or less every level outside of the execution units themselves. Does this have it's fair share of downsides? Absolutely! For one thing, CISC has legacy compatibility and and convention, at least as far as programming Assembly code for it goes. ARM is a mishmash and very, very difficult to work with often - it's not uncommon to see performance enhancing instruction extensions (like NEON) go unused on ARM because they can be very model-specific. CISC processors are also generally faster because they have an instruction for every situation - perhaps ARM cut a corner, and can't do an FMUL operation, which means that they need to take ~6x the instructions to perform the same operation that a CISC does in 1-2 cycles!

In short, the smaller front-end of the RISC architecture doesn't require as many transistors, and therefore it uses (usually) less power than CISC equivalents - however to be honest, most power requirements are process specific rather than architecture specific. I don't think it's possible to design a low power CPU anymore, but rather it's possible to go with the lowest power process from TSMC/GF(/Intel).
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Old September 19, 2013, 04:12 PM
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Quote:
Originally Posted by Arinoth View Post
A RISC is a Reduced Instruction Set Computer, such as something that has an ARM processor inside, whereas a CISC is a Complex instruction set computer such as an x86 processor.

The good thing about a RISC is that it's designed to do a specific task extremely well, efficiently and low power (if possible). However, it CANNOT do anything aisde what it's designed for (usually, some slight exceptions with hacks and such). A CISC isn't as good as a RISC when it comes to what the RISC specifically does, but anything else it is far superior as it has the ability to perform different sorts of tasks either simultaneously or just faster.

A RISC shouldn't overheat as it's being designed as the core component and everything is revolves around it, whereas a CISC can well be pushed beyond it's normal limits, runs faster, uses more power, etc which generates more heat.

Hopefully some of this makes sense, just quick reiterating some of the stuff we were 'taught' at school.
So from what I'm taking from this, in the future with more die shrinks there's a possibility of CPUs with different RISC cores instead of multiple CISC cores?
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Old September 20, 2013, 01:34 AM
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Sushi Warrior -
Quote:
Not quite! Most modern CPU's AREN'T CISC, they're RISC! Why? Because most are ARM! ARM is a RISC architecture. By having a limited set of instructions...
Ah! You are right, of course. When I say "CPU", I meant a CPU's used exclusively in computers
That is a bit that I should include, because once we don't limit to computers, then there is a CPU in each pocket calculator, washing machine, airconditioning... you name it. And indeed, most (if not all) of them are RICS

On the contrary, I believe, that the Intel Atom line of CPU's proves, that low power CPU can be done. It is just not any fast... yet it is a complete CISC (or it at least looks like that way to the rest of the world) and it can run Windows XP pretty well.
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Old September 20, 2013, 06:45 AM
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Quote:
Originally Posted by supaflyx3 View Post
So from what I'm taking from this, in the future with more die shrinks there's a possibility of CPUs with different RISC cores instead of multiple CISC cores?
There is the possibility of that, though if we'll go that route, software programmers are going to need to hurry up and catch up to what hardware we have now, as you and many may know that we do not have a lot of applications or games that actively multi-thread across multiple cores to reduce the time it takes to perform an operation (ergo load a game, process something, etc).
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