[RC5] core questions

Décio Luiz Gazzoni Filho decio at revistapcs.com.br
Tue Mar 12 06:54:34 EST 2002


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On Monday 11 March 2002 23:04, you wrote:
> I just did a processor changeout in a linux box on my lan. I had an old
> cyrix 166 mhz clone chip and replaced it with a pentium P5mmx at the same
> speed. (running Mandrake Linux 8.0 w/64mb memory) I see the core changed,
> and the speed of processing went way up in comparison to the old chip. The
> core running now is a "jasonp" I believe..
>
> Questions: Is there actually THAT much improvement in handling the RC5 task
> I should be seeing the little indicator turning that much faster?
>

Yes.

>
> and, does the mmx functions actually contribute something to the processing
> speed? I always thought they had something to do with video processing, not
> raw processing power.
>

MMX implements the SIMD (single instruction, multiple data) processing 
paradigm. That means, for this specific case, that you can simultaneously 
process two 32-bit integers. Since this instruction set contains the 
necessary instructions for RC5 (i.e. ADD, XOR, and shifts/OR to ``emulate'' a 
rotate instruction), it can be used to implement RC5 just as Jason did. If 
the timings for integers and equivalent MMX instructions were the same -- and 
if there was a real MMX rotate instruction -- you might even achieve 
near-100% speedup.

>
> In any case, I want to thank "jasonp" for a core thats visibly more
> powerful than the old cyrix one, amazing what just a little tweaking can do
> huh?

You can reach him at jasonp at boo.net if you really mean to. (:

Décio
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