RAM modules work properly only on specific banks?

By mkoskenk ·
I've encountered a situation where computer with 8Gb RAM installed could only use 4Gb of it. BIOS recognized 4Gb of it and Windows told that there is 8Gb installed of which 4Gb is usable and the rest is "Hardware reserved".

After some browsing through forums, I found an advice to swap the modules around in memory slots until full amount is recognized. Believe it or not, this actually worked. After some more digging I found a claim that the memory modules would be configured to work either on bank 0 or bank 1 on dual channel. I know that the modules are most of the time quite finicky about being installed in to the same bank if purchased as e.g. 4Gb kit of 2x2Gb modules, but is it actually true that the modules would be configured to work only on either channel? I couldn't find a confirmation for this theory.

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HAL9000's comment

by mkoskenk In reply to RAM modules work properly ...

This post was moved from Discussion forum, here's what HAL9000 replied there:

"Not really sure how that could work If BIOS could only see 4 GIG I'm at a loss how Windows could see 8 Gig and more importantly unless it was a 64 Bit System how it could use anymore than somewhere around the 3.25 GIG upper Limit of every 32 Bit System though the 32 Bit Vista Systems and I suppose 7 OS lie tot he user about what is available.

Also more than a bit of a loss how you would have fitted the RAM into Bank 0 or 1 and have it work in Dual Chanel Mode but this I would say depends on the M'Board in use in this particular occasion. With Normal Dual Chanel M'Boards you have Paired Sockets which are generally colour coded so you place the RAM into the Yellow or whatever colour RAM Socket that is fitted and leave the other which is generally Black unpopulated. In Theory you could fit the RAM into both Black Sockets and leave the Coloured Sockets unpopulated and it would work in Dual Chanel Mode though depending on the MBOARD and the Chip Set involved this may not work in a lot of cases.

As for swapping the RAM from Socket 2 to 0 and from Socket 0 to 2 all that should do is clean the contact Points in the RAM Sockets and on the Modules which may allow the RAM to work properly when they are refitted. But it would be just as possible that removing and refitting the RAM Modules into the same Sockets would have the same effect.


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Is this on a PC/Laptop or a server?

by scndtnr In reply to RAM modules work properly ...

I just dealt with this on a Dell server; the memory was configured in "mirrored" mode, which meant that of the 8 GBs installed on the board, only 4 GB was available for processing/operations. Using Dell's server admin app, I was able to reconfigure the memory mode to "advanced ECC" ("optimized" was also available, but would have required rearranging the DIMMs). Apparently, this configuration modification was also available in the BIOS, but I chose not to investigate that possibility. At the very least, you should check the hardware user manual for RAM configuration options.

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by mkoskenk In reply to RAM modules work properly ...

We're talking about regular desktop here. Motherboard's ASUSTeK P7P55 LX with 1st generation Intel i7 K 875 processor.

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Common "Issue"

by Seonix In reply to RAM modules work properly ...

This is not uncommon. I'm not quite sure of the logic behind it but you need to use specific DIMM's in a specific order eg A0,B0,A1,B1.
Some motherboards will simply not boot past BIOS if incorrect. Other's will disable to out of order DIMM.
As mentioned above (although it's less likely with desktops), you can have memory configured to mirrored mode. Some desktop mobo's do support it. Very easily configurable in BIOS however.

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RAM kits vs. single DIMMs

by mkoskenk In reply to RAM modules work properly ...

Ok, kind of strange but guess I can live with this :) Maybe this applies only to RAM kits (e.g. 2x2Gb) and single DIMMs purchased separately can be placed into any slot. Anyway, thanks for your replies.

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