[RC5] Differing processor speeds

Bruce Ford b.ford at qut.edu.au
Fri Jul 17 10:24:51 EDT 1998

> Date: Wed, 15 Jul 1998 10:15:38 -0600
> From: Steve Bird <sbird at UATC.com>
> Subject: [RC5] Differing processor speeds
> Ok, here's my set-up.
> At home I have 2 machines....a P200MMX and a P2-266, both of them are
> Intel-based processors. When I benchmark both machines I get
> 1.9M/keys/second on the P200MMX and 2.15M/keys/second on the P2-> 
> 266.  Both of these figures are from the DES benchmarking section of the 
> test.
> My questions are:
> 1) Are these results unusual?

The results are not unusual.  I used Pentium pairing rules when creating the 
MMX sboxes (which by the way are at 
http://www.tals.dis.qut.edu.au/staff/bruce/des-ii.htm ).  The need to do alot of 
op mmreg,mem instructions appears to affect the PII's performance, probably 
because they generate 2 micro-ops.

There was no time to develop specialized x86 processor cores as are 
present for RC5.

> 2)If so, is the P200MMX score too high or is the P2-266 score too low?


> 3) In almost every other application I have (ranging from games to the
> RC5-64 - based benchmark), the P2-266 outshines the P200MMX to the 
> tune of about 2.5:1 in raw performance(actually 3:1 in RC5-64), Why isn't
> this same level of performance represented in the DES benchmark.  To
>  me, it almost seems as if the results would be consistent with a
> (hypothetical) Pentium 266(ie...the extra instructions of the P2 aren't even
> used).  So whats up with this?

See above. Also,  in RC5, the rotate left instuction is used heavily.  On the 
Pentium the rol reg,cl takes 4 clocks non-pairable(don't believe what the Intel 
docs say), on a PII - 1 clock pairable.  It makes a huge difference.

Maybe by DES-II-3 we will have PII specific bitslicing MMX cores.

Bruce Ford

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