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My specific question is with the ASUS M3N78-VM motherboard that my home server uses. I currently have 4x2GB installed for a total of 8GB but I was hoping to upgrade to say 16GB (4x4GB). The official specifications of that motherboard says it only supports up to 8GB but it was released back when only 2GB sticks were available. What, if any, would prevent it from working with 4GB sticks now that 4GB sticks are available?

My general question is with motherboards in general... what would prevent a motherboard from working with bigger sticks if its all just the same type (DDR2 for example and same speeds) of memory?

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3 Answers 3

up vote 19 down vote accepted

In short, this is a mixture of limitations. One is in how many "address lines" the CPU itself supports (different from the 64/32-bit address space/register size), 32 lines would allow 4GiB of RAM, having a 33rd line connected would allow 8GiB of ram and so on.

The other problem is in how many of those address lines the manufacturer actually bothers to wire up to the memory controller.

In order to simplify the design the manufacturers tend to decide on a current realistic amount of memory and wire up as many address lines as are needed to support that amount of memory.

Routing all those address lines on a PCB is painstaking work as all the track lengths have to be as near identical as possible (as at the high frequencies that these things now operate at having a slightly different length can mean that data on one line arrives at a different time to the data sent on another line even though the sender sent them both at the same time) and so manufacturers will keep the amount of work as small as possible and so supporting 65TiB of RAM is nowhere near the same priority as getting the motherboard out this year.

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I really want my motherboard to support 65TiB of RAM :( –  Flimzy Jul 8 '11 at 22:54
    
+1 for mentioning that address lines are different from a processor's architecture. On that note, it would be easier for a memory extension on a 32-bit processor to just "double up" and go to a full 64-bit (requiring double the space for pointers). Great answer. –  Breakthrough Jul 9 '11 at 0:58
    
I want my entire PC on one chip, completely solid state, only ports to connect external devices and a power cord. I only hope I live long enough to see it. –  Moab Jul 9 '11 at 2:44
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@Moab: Already exists. It's called System-on-a-chip. Some even run desktop OSes like Linux. (OK, most use some external chips, e.g. RAM, but they're close)... –  sleske Jul 21 '11 at 11:09
    
Don't forget that each memory slot has a limited number of memory address lines going to it as well. This means that each slot will have its own maximum. Nearly always each slots' maximum is the same as the others. The only exception to that rule I've seen was a motherboard that could use DDR2 and DDR3 memory (but not at the same time). So you may have a system with 4 slots that supports up to 8GB of RAM, but no slot can take more than a 2GB stick as each slot is only given 31 address lines plus some system to activate that particular slot. Bits 32-33 just control which RAM stick is active. –  BeowulfNode42 Aug 16 at 7:19

Disclaimer: This is my understanding, I may be wrong, but I have been around the block a bit..

In my experience the people who write the final "specifications" that customers get to read, are often not necessarily involved with the actual design of the board. As such, you CAN run in to occasions where the docs say things like "4GB max supported memory". I've found on many occasions that I can cram more memory on a board than the docs claim is supported, and have things actually work out rather well.

My advice to you is to try it. There's a store I like (just in my small city) that'd let me buy RAM and return it, so I actually could just try it, maybe you can find a store by you that's similarly awesome.

There obviously are various architectural limits of how much memory a machine can address. The most obvious one is weather or not the platform supports 64bit operation - if so, then the addressable memory limit will be fairly massive (~4PB for x86). I think it's safe to assume your board supports 64bit. So then it we come to things like Tomas mentioned, like the memory controller within the CPU. As Tomas also said, it makes sense that BIOS may play a role here too. You should therefore check your motherboard's product page to see if they had any BIOS updates that yap about memory limits.

At the end of the day, there may be various things that limit how much memory your machine will be able to use. We're talking about several variables. And, in my opinion, the best & fastest way to know, is to just try it (and run MemTest86 to verify).

Take care & good luck.

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If the slot maximum * number of slots is more than the motherboards' quoted maximum total then it is reasonable to assume that more will work and to try filling all slots with the maximum for that slot. –  BeowulfNode42 Aug 16 at 7:31

There can be other issues, a friend was doing QA on Apple hardware, the system was certified for 768MB max ram (12 64MB modules), in theory, you could double that, but when they were certifying the system, getting 12 128MB was not practical, so it was not certified in that configuration, but it should work.

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