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iMac G5 system optimization via RAM
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Join Date: Mar 2005
Location: Calgary Canada
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From Apple re: iMac G5
http://tinyurl.com/6ttwd p.47
"The throughput of the 400 MHz memory bus is dependent on the DIMMs installed. If only one DIMM is installed, the memory bus is 64-bit. If two non-identical DIMMs are installed, there are two 64–bit memory buses. If two identical DIMMs are installed, the memory bus is 128-bit. Identical DIMM pairs have the same size and composition and provide the fastest and most efficient throughput. "
I gather this means better system performance using "Identical DIMM pairs".
What defines "Identical DIMM pairs" ? Does brand matter ? If I wanted to buy 1 extra 512 without buying from apple what would qualify ? Am I reading too much into this ? buy any 512 and that should do right ?
Thanks in advance ?
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Last edited by trip; Apr 28, 2005 at 11:06 AM.
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"The direct use of force is such a poor solution to any problem, it is generally employed only by small children and large nations". --David Friedman
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Interesting document. Although I cannot contribute any knowledge, I'd like to know the same thing. My iMac G5 (17", 1.6 Ghz) has factory-installed 256 MB of RAM. I added a Crucial 512 MB chip.
Can someone with extensive knowledge in computer memory confirm that 2 identical 512 MB chips running side will increase performace (ignoring the fact that there's an additional 256 MB of RAM, of course)?
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Moderator  Join Date: Apr 2000
Location: Gothenburg, Sweden
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Short answer: using identical chips (as in same size, same number of chips in the module possibly, same manufacturer no) will increase performance.
Longer answer: The RAM chips are 64 bits, rated for 400 MHz DDR. If you use only one chip, that is exactly the bandwidth you get: 64*400*1'000'000 bits/s. If you use two different chips, both of those chips can send data at the same time - they're configured to be on "parallell buses", sort of. The gain in having identical chips is that they are treated as one big 128 bit chip instead of two 64 chips. The gain in that? One read request for 128 bits of data instead of two requests for 64 bits of data each. One less request, one less thing using memory bus bandwidth. The gain isn't huge, but in a bandwidth limited situation it helps some.
That help at all?
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Thanks P, now I know what to buy with the new machine I am ordering !
Would be nice to get an idea on the impact. Would this be a big gain when gaming ?
or so insignificant it's not worth thinking about.
Thanks,
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"The direct use of force is such a poor solution to any problem, it is generally employed only by small children and large nations". --David Friedman
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Forum Regular
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Location: Virginia, USA
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Originally Posted by P
That help at all?
Yes. Thanks 
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