32/64 bit AMD question

Lewis Futrell silicon.vampire at gmail.com
Wed Oct 19 16:25:02 UTC 2005

Maybe put another way, what is the performance difference in 64 bit mode
compared to 32 bit, of the 754 pin and the 939 pin AMD Athlon64's for the
same clock speed?

The funny thing is, it is exactly the memory interface that is different
between the two.  The 754 socket only does single channel memory (64 bit
path) whereas the 939 CPU's are dual channel (128 bit path).  The 64 bit
part of the processor is the same.  The performance difference will be the
memory bandwith.

Thanks Lewis, does that mean that in 32bit mode the chip only does 32bit
memory accesses but in 64 bit mode the 754pin chip does 64 bit and 939 pin
chip does 128bit (memory access not calculations)?

If true, then 64 bit mode should be faster than 32bit in both chips, but the
939 will be faster (by the memory access time) then the 754. Do I have this
understanding correct or am I still missing something?

Regards Russell. 



That is correct.  However, in most tests that I've seen on the review sites,
the performance increase between 32 and 64 bit mode varies upon the
application one is doing.  Sometimes 32 bit mode is just as fast, if not
slightly faster than 64 bit, and vice-versa.  The biggest issue that you may
want to look at is to see if there are 64 bit versions of the
programs/applications that you want to run.  For example, there is not a 64
bit version of Flash that I'm aware of (one can do a chroot environment to
run 32 bit applications in 64 bit Linux, but that is a little over my head).
Also, some drivers do not have a 64 bit version of them, but this is
thankfully being worked on and the likelihood of having a device that won't
work on the 64 bit version of Linux is becoming less and less of an issue.

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