[gmx-users] SSE on AMD

Erik Lindahl lindahl at csb.stanford.edu
Tue Oct 7 18:43:01 CEST 2003

Hi Alan,
> Would that so the whole mysterious problem I've been facing with 
> Athlons XP?  And I would say more, I found out new AMD XP faster 
> clocks (2000+ and 2400+) not much powerful than my old Athlons XP 
> 1800+.  What does not happen with Pentiums.
> If so, I really would like to test it, thus how do I do to setup GMX 
> to use 3Dnow?  And more, since we have heterogenous Openmosix cluster, 
> how could I keep SSE and SSE2 working on Pentiums and 3Dnow on AMDs.

Look in src/gmxlib/detectcpu.c. By default we check for SSE first, then 
3DNow if we dind't find SSE.

You can find a check for the CPU-type in the old 3.0.X code, but here's 
how to do it (check the C function call in detectcpu.c)

1. Execute CPUID with EAX set to 0.
2. Copy the four bytes in EBX to the beginning of a string
3. Add four bytes from EDX to the end
4. Add four bytes from ECX to the end
5. Terminate the string with 0 if you want to print it...

For an Intel CPU, the string is "GenuineIntel", while AMD presents 

If this sounds way too complicated you can also just grep for the name 
in /proc/cpuinfo, but that will only work on Linux.



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