[RC5] Cuda client

Joseph Kaye jkaye at isd.net
Thu Jan 31 15:26:06 EST 2013


On 1/31/2013 12:18 PM, Décio Luiz Gazzoni Filho wrote:
> Just to clear up a common misconception: Intel never *removed* an 
> instruction from a new processor that was present in an earlier 
> processor, including rol. That would break up backward compatibility 
> which is a main selling point of the x86 architecture (and by 
> extension the Wintel platform). What did happen is that there is a 
> piece of hardware used for efficient (usually single-cycle) 
> implementation of variable-sized shifts and rotations (the shl, shr, 
> sar, rol, ror, rcl and rcr instructions) -- that hardware is called a 
> barrel shifter. it has historically been implemented on every Intel 
> processor since the 80386 or so, but don't quote me on that. Certainly 
> the classic 1993-era Pentium did have it. The barrel shifter is what 
> wasn't present on the Pentium 4, and the reason why the rol 
> instruction executed slower (I believe it took 4 cycles). So the 
> instruction has always existed, even on the Pentium 4 -- it had to 
> because of backward compatibility reasons -- but the hardware for 
> efficiently implementing it didn't exist only on the P4 chips, and 
> Intel has added it back on the newer Core chips, which is why they 
> perform better. Décio _______________________________________________ 
> rc5 mailing list rc5 at lists.distributed.net 
> http://lists.distributed.net/mailman/listinfo/rc5 

Thanks Décio.   I knew it was something like that, but I was bit foggy 
on the particulars.




--Joseph


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