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My desktop is an old home-built machine circa 200[5-6] running Ubuntu 11.10 (but this is not relevant because I'm reading available ram from BIOS loading screen), with an ASUS P5GPL motherboard, not X or X-SE - it has four slots. I'm mainly a laptop person, but keep this around for running a server from if needed, backing up to, seeding Ubuntu to people from, etc…

It has four (DDR) ram slots, two black and two blue, in the order black-blue-black-blue (I will call them D, C, B, and A, respectively) with some space in the middle. The blue ones are the closest to the processor. I used to have two 512MB chips in the two blue slots.

I just got a 1GB chip and plugged it into one of the black slots; my system didn't recognize it. I messed around and discovered that it will not recognize chips in many positions, and I couldn't get it to recognize all three of these chips at the same time. In particular, if I put the 512MB chips in A and B it would only use 1, but AC, AD, BD, and CD worked. I didn't try BC, I believe. Only some of these continue to work when I switch the 1GB chip into one of these positions.

Can I have some advice as to how to position these chips to get all 2GB used? How about if I get another 1GB chip - where should I put the two? And what about the RAM maximum Crucial says? Can I go above 2GB, if I get another 1GB chip?

Right now, I have a 512MB chip in A and the 1GB chip in C.

EDIT: I read some other posts and tried dmidecode in Ubuntu to clarify the max memory question, that wasn't a major part anyways. It says my max memory module size is 1024M (OK) and my max memory size is 4096M (doesn't agree with Crucial OR the Asus web site, maybe it will only work while in Linux and BIOS won't OK it?).

EDIT2: My 2 512MB chips in either A and B or C and D cause a message in BIOS that that configuration is not supported and it only uses 1. Likewise, if the 1GB chip is in another slot at this time, it will only go to 1.5GB.

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up vote 5 down vote accepted

BTW what you are calling a memory "chip" is a DIMM (dual inline memory module) or simply a module. It's a module and not just a "chip" because the DIMM is constructed of a printed circuit board and chips.

Your motherboard is designed for dual-channel operation of the installed DIMMs. The owners manual describes the required sockets that DIMMs must be used for proper operation. The two blue sockets comprise one memory channel, let's call it the "X" channel; the two black sockets comprise the other "Y" memory channel. One pair of blue & black sockets is for the low address end (starting at 0); the other pair of blue & black sockets is for the upper address range.

When installing just a single DIMM (which of course will result in single-channel operation), the blue socket you call "C" should be used. (I'm guessing that this "C" socket is the low "X" channel socket.)

A matched pair of DIMMs (for dual-channel operation): one module should be installed in the blue socket you call "C" and the other module should be installed in the black socket you call "D". (I'm guessing that the "C" & "D" sockets are the low sockets for both channels.)

Two matched pairs of DIMMs (for dual-channel operation): one matched pair should be installed in the blue & black sockets you call "A" and "B" and the other matched pair should be installed in the blue & black sockets you call "C" and "D". (This uses both the high & low sockets of both "X" & "Y" channels.)

Two mismatched DIMMs (for single-channel operation) should be installed in the blue sockets you call "A" and "C". If you obtain another 1GB module, then install the two 1GB modules in "A" and "C", since they are unlikely to be matched. (I'm guessing that the "A" & "C" sockets are the sockets for the first channel, i.e. if there's only one channel in use then use "X" rather than "Y".)

Operation with other combinations of DIMMs is unpredictable.

I used to have two 512MB chips in the two blue slots.

This can work (obviously), but in single-channel mode and with a small but unnecessary performance penalty if they are actually matched DIMMs.

If the two 512MB modules are matched, then you might be able to install all 2GB worth of memory by installing the 512MB modules in "C" & "D" and the 1GB module in "A". This configuration would operate in single-channel mode if it does work.

To confirm proper operation of the memory, you should run Memtest86+. It should be available while Ubuntu is booting, although Grub2 will probably hide its boot menu. Pressing the Esc key during/after the PC's POST screen should cause the Grub2 boot menu to appear.

You can use the info from Memtest86+ to see the performance improvement or degradation by switching between single-channel and dual-channel memory operation. In order to obtain dual-channel operation (and the best memory throughput), the pair of memory modules must have identical CAS latency, or even better, sold as a matched pair by the manufacturer.

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This looks like it will work. I'm Away From Desktop ATM and will check this (and A-B:512 C:1024) when I get home and mark you as correct assuming it works. – Andrew Mar 21 '12 at 14:55
Didn't work. I may not have a matched set of 512MB chips. Any other reasons? – Andrew Mar 28 '12 at 19:37
I doubled checked that this works on an ASUS motherboard, although it uses an AMD memory controller. Perhaps Intel implemented a more restrictive dual-channel scheme than AMD? As for your 512MB modules, which are not chips, use Memtest86+ in the dual channel setup (you previously wrote that "CD worked" but not "AB"). A main-memory transfer rate of about 1700MB/sec would indicate dual channel versus about 1200MB/sec for single-channel operation. – sawdust Mar 29 '12 at 4:25
Sorry to keep calling them chips. I'll test it and get back to you as soon as I finish downloading my dropbox folder onto this computer. – Andrew Mar 29 '12 at 20:14

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