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0 pointsastrodust12y ago0 comments

Neither RISC nor CISC won in the end. Intel's current generation processors, like most others of similar performance specifications, break down the incoming instruction stream into micro ops (http://en.wikipedia.org/wiki/Micro-operation), the units of processing that are actually executed. This renders the difference between CISC and RISC a case of semantics.

Before this, the idea was that RISC was simpler to implement and could be optimized more easily, ultimately be more cost effective. What wasn't factored in was how good Intel is at optimizing, and how hard they'd push their process, beating the RISC side despite all the disadvantages CISC had.

Now it's the GPU that's eating Intel's lunch, high performance floating point code on the CPU is several orders of magnitude slower than a high-end GPU, so Intel's trying to fight back with their "pile of CPUs" strategy (http://en.wikipedia.org/wiki/Larrabee_(microarchitecture)). It's not working out very well so far.

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Symmetry12y ago

In defence of Intel here, if you look at performance per watt GPUs aren't all that far ahead of Intel. It's mostly that every instruction an modern CPU executes is predicted by a branch predictor, makes it's way through several levels of cache, is run through a reorder buffer and register renaming and a reservation station before finally being executed. And all of that takes energy, though it speeds up the rate at which sequential instructions can be issued by the processor quite a bit.

As to RISC vs CISC, well, it's true that x86 instructions are decoded to micro Ops inside a modern processor but the fact that the instruction was complicated does have a cost even for a modern processor. The act of just decoding four instructions in a clock cycle and transforming them into uOps is quite a bit of work, on the same order as finally executing them if they're simple additions or such. And the uOps that make up an instruction have to be completed all together or else when the processor is interrupted by a page fault or such it will resume in an inconsistent state. And the first time you run through a segment of code you can only run one instruction at a time since figuring out where instruction boundaries are is hard, though you can store the location of those boundaries with just another bit per byte when they're in the L1 instruction cache.

On the other hand, complex variable length instructions mean that you don't need as many bytes to express some piece of code both since you're using less bytes per instruction on average and because complex instructions mean you sometimes use fewer of them.

Of course, Intel is the biggest CPU vendor out there and has correspondingly large and brilliant design teams working hand in hand with the most advanced fabs in the industry.

Now, there are many RISC instruction sets that have taken on x86 before, but they all attacked it from the high end, from upmarket. Doing just the opposite of what ARM is doing now. Will it succeed in dethroning x86 from the low end the way x86 did to it's rivals? Who knows. But I think that previous fights don't tell us much about this one.

scholia12y ago

Quite. But there was plenty of "Intel is doomed" hype in the 1980s when RISC chips first appeared. Indeed, Microsoft didn't write either of its home grown operating systems -- NT and CE -- on x86 processors.

Of course, "Intel is doomed" (and "Microsoft is doomed") have been staples of clueless fanboy hype for 40 years. I'm still waiting for one of them to be right....

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