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Re: [Qemu-devel] ARM CPU Speed simulated by Qemu?

From: Laurent DESNOGUES
Subject: Re: [Qemu-devel] ARM CPU Speed simulated by Qemu?
Date: Tue, 12 Sep 2006 16:44:37 +0200 (CEST)

> Now, CPUs is where I have only a vague idea of what would be needed to 
> simulate. I know there are up to three levels of caches and main memory, 
> which all have different access times. The CPU itself has a pipeline and 
> branch prediction and such which could invalidate the contents of 
> pipeline up to a given point (of branching).
> I think the most time consuming operation which should be properly 
> simulated is memory access. For this to work properly, all levels of 
> caches must be emulated, too.
> How much do misses on the branch prediction level cost? How much 
> pipeline interlocks? I don't think those would be _that_ dramatic. Since 
> today's compilers are said to be optimizing quite well...

The most complex thing to accurately simulate a modern
CPU (including ARMs) is the data cache and by far.  In
comparison, getting accurate core pipeline simulation
is *very* easy.

There is a company that claims to be able to accurately
simulate an at 200 Mhz (http://www.vastsystems.com).  I
bet there are using statistical cycle counting and so
are probably very wrong :)


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