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PC memory sticks for motherboard - mixing sizes and # of sticks

i thought that one needed to match IN PAIRS memory sticks for motherboards.
(I have an intel DZ77RE-75K  with g.skill ripshawx 8G X 2 sticks 1600 9-9-9)
One went bad, so i'm running with 1 X 8G now, and this brings up some questions.

Gskill tech support replied:   (but this confuses the situation)

Mixing memory kits, even of the same model number, may result in compatibility issue. G.SKILL will guarantee the compatibility for single memory kit only

1. If i get one warranty replacement stick, then it's not a kit any more.  So what good is a replacement warranty for 1 of 2 sticks if this is true.

The following questions are Assuming there is some latitude WITH THE SAME MODEL #

2. Is there likely to be a performance difference btwn (same-speed sticks):
   1 X 8G  stick         AND        2 X 4G sticks           AND       4 X 2G sticks

3. With the 2X 8G pair back in place, is it beneficial (speed; other than just more RAM)  to add  a second pair but of 2 X 4G ?
(or does it REALLY need to match  2X 8G)


thanks!
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willsherwood
Asked:
willsherwood
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2 Solutions
 
rindiCommented:
Matching pairs of RAM is no requirement at all. But if you do use matching pairs the performance should be slightly better. But, that improvement in performance is usually not noticeable. You would probably only see a small difference if you run benchmarks, but not in real life.

If you use different speeds of memory, then the overall speed will be set to that of the RAM with the lowest speed, so if you have one slow module and the others are faster, the speed all the RAM will run at will be that of the slowest module.

But also here you generally won't notice much differences in real life. The speed of the RAM is irrelevant compared to other parts of a system.

More RAM should always be a bigger benefit than faster RAM. The only issue I can think of here is that some OS versions, particularly if they come from m$, have limits in how much RAM can be used (they made those limits for marketing purposes mainly)
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garycaseCommented:
"1. If i get one warranty replacement stick, then it's not a kit any more.  So what good is a replacement warranty for 1 of 2 sticks if this is true."

==>   Note that there is no such thing as "dual channel memory" => although the memory makers sell "dual channel pairs"  (kits)  which are often thought of as paired modules, they are simply packages that have two modules with the same specs.

If you replaced your failed module with another module with the same specs, you'll then have a proper pair of modules to run in the optimal dual channel mode.


" 2. Is there likely to be a performance difference btwn (same-speed sticks):
   1 X 8G  stick         AND        2 X 4G sticks           AND       4 X 2G sticks"

==>  Yes.   Of those specific choices, the best would be 2 x 4GB modules.   With only one module installed, your memory will run in single channel mode.   With 2 matching modules installed, it will run in synchronous dual-channel mode ... with twice the memory bandwidth  [Note that this doesn't mean the memory runs twice as fast, since there are a lot of latency cycles, but on cycles when data is being transferred, it'll transfer twice as fast]


"3. With the 2X 8G pair back in place, is it beneficial (speed; other than just more RAM)  to add  a second pair but of 2 X 4G ?   (or does it REALLY need to match  2X 8G)"

==>  If you have one pair of 8GB modules, and want to add another pair, that pair can be any size modules you want.    As long as the 2nd pair is the same size modules, everything will still run in the optimal dual channel mode.    Note that even if your modules are mixed sizes, most modern memory controllers will still work in a lower-performance dual channel mode.

HOWEVER ... your memory system will be more reliable if you only install 2 modules.   On unbuffered boards, you get far more reliable signaling on the memory bus with only 2 modules installed.    When you install 4 modules, the signaling is degraded, and the system needs to compensate for this -- by either reducing the speed; adding latency cycles; or sometimes bumping the voltage a small amount.

Finally, when you replace the failed 8GB module, be sure you get a module of the same (or higher) speed as the one you have now.   If you install a lower-speed modules, then both of the modules will operate at the slower speed.
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willsherwoodAuthor Commented:
bravo! excellent help - makes sense!
and i did not realize (but totally understand) about degraded signal integrity - MORE is NOT better in this case.
so i'll just leave it at 2 X 8G.  that should be enough for all the photoshop usages!

may i add a follow-up question,   does it really matter  which of the paired slots i use  (1+2 OR 3+4, see below)?
on the physical motherboard card, they are numbered one way,  but in BIOS set up
it shows a slightly different numbering scheme (off by 1, different "origin" (0 vs 1) ).
* are the slots i was/will using for a matched pair.

physical              BIOS(as i recall)
black  3                   2
blue    1                   0      *
black  4                   3
blue    2                   1      *

i assume the non-consecutive spacing/ordering is to help either or both of:
-  heat dissipation when only 2 are used (spaced apart)
-  PC card routing (wiring) proximity or signal integrity for dual channel
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garycaseCommented:
In general it doesn't matter which pair you use, as long as the modules are in different channels.    On most boards, that means installing a pair of modules in the same-color slots -- so you would, for example, use the blue slots or the black slots.

You can confirm this by checking the manual;  or by simply installing them; then running CPU-Z and confirming that the memory is in dual channel mode.   [If you installed them both in the same channel it would show single channel]
http://www.cpuid.com/softwares/cpu-z/versions-history.html
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willsherwoodAuthor Commented:
wow  more superb information,  MUCH appreciated.
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