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Community Newsletter: Q&A forum: 7/28/06 Computer doesn't recognize the new RAM I installed

by: Lee Koo (ADMIN) July 27, 2006 9:41 AM PDT

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7/28/06 Computer doesn't recognize the new RAM I installed

by Lee Koo (ADMIN) ModeratorCNET staff - 7/27/06 9:41 AM


I recently purchased an HP Media Center PC, 2.8GHz dual-core processor with 1GB of RAM. I installed another 512MB memory stick with the exact same specifications that are required for the PC. However, the PC does not seem to recognize the memory, even though it is installed correctly and the PC turns on and works fine. Why does the PC not recognize the memory, and is there a way to make the PC recognize it? Please give me a checklist of ways I can solve this issue. Thanks!

Submitted by: Jake K.



Jake, it would be helpful to have more information on the original memory. Was the original 1GB of RAM comprised of one 1GB module or two 512MB modules; is it SDRAM, DDR or DDR2; how many total memory sockets does the computer have; and what is the computer's CPU chipset? It would also be nice to know the exact description of every module in the system--every number on every label on the various modules. This is all relevant information that may be necessary to answer the question.

With the information given, there is not enough to answer the question with certainty, but hopefully, I can help you by suggesting things that you need to look at. And even if your memory addition had worked, I can suggest to you why this was probably not a good addition, and how you could have better upgraded the system.

Now lets cover a few reasons why your additional memory might not be recognized:

First, of course, the new memory module may simply be defective. Although not terribly likely, this does happen.

Second, you could have the memory installed wrong. Remove it and reseat it, checking very carefully that you have it seated correctly. The notches in the memory module and socket are supposed to prevent incorrect insertion, but believe me, they dont make it impossible. Im a professional, and Ive managed to install a module backwards (notches not withstanding, and I couldnt believe I had done it .... or was able to have done it), and also with one end not completely seated.

Third, you could have bought the wrong type of memory. Four types of memory have been used in Pentium 4 systems: SDRAM, RDRAM, DDR and DDR2 (SDRAM and RDRAM are not currently being used in new systems, but are still found in older Pentium 4 systems). Note that this is the TYPE of memory, each type is totally different and totally incompatible with the others because they work completely differently. Any given system will generally take one and only one memory type. Subtle differences in the connectors are supposed to prevent inserting the wrong type of module, but as I noted above, its still possible to do it. So, the first question you need to be certain of is what type of memory do I need and you must buy the correct type.

Fourth, even if you have the right type of memory (a module that would otherwise work in your system), when adding a module to an existing system that already has installed modules, you can run into compatibility issues between the modules. You said that you bought a module with the exact same specifications that are required for the PC, but as a professional I know that most computer owners dont truly understand what such a statement should mean. To many people, if they buy a 512MB DDR2 memory module, they bought a module with the exact same specifications that are required for the PC. But as a professional, I know that there is much more to it than that.

To be more specific and to point out just a few of the ways in which the wrong memory could be bought, memory modules have a variety of timing specifications. For example, a fully labeled module (and some modules are not fully labeled) may state on its label 533MHz 444-12. Those 12 characters specify five distinct timing specifications for that module, yet two different 512MB DDR2 modules could be totally different in any or all 5 of those specifications, and yet would still be properly described as 512MB DDR2 modules. However, the computer itself cares about those specs, and may not work if they are wrong. And by no means are these 5 timing specs the total extent of the issue, there are more specifications that may matter, but I use these as an obvious example.

However, there is another issue, you are adding memory to a computer that already has installed memory. In this situation, not only must the added module be compatible with the computer, it must also be compatible with the modules that you already have. To take our example, supposed you bought a 444-12 module but the existing modules are 333-10. Now it may be that both 444-12 modules and 333-10 modules would both work by themselves, but that in some systems, they will not work if you try to mix them. So when you state that you bought a memory stick with the exact same specifications, I have to ask just how certain you are that ALL of the relevant specifications are the same.

But wait, theres more!: Even modules that are the same size, same type and have the same timing can be architecturally different internally. For example, it is possible to build a 512MB memory module from eight chips having 64MB each, or from a module using four chips of 128MB each. And a given chip can be different from another chip of the same size, that is a 64MB memory chip (not module) could have 32 million words each 16 bits wide, or 16 million words each 32-bits wide. And, again, while a typical consumer looks only at the overall gross module spec (512MB of DDR2, for example), to the computer these details matter. And they matter a lot when you are adding memory to a system already containing memory, because very often you cannot mix different modules in the same system, even if both modules would work by themselves or used with other truly identical modules.

And that brings, us, finally, to my fifth area of concern, which is the matter of dual channel memory systems and channel symmetry. Many of todays best and highest performance systems use dual channel memory systems, in which there are two memory channels that are interleaved to double the speed of the overall memory system. In order for a dual channel memory system to function, the memory modules installed in both channels must be architecturally identical and symmetrical. (They dont have to be absolutely identical: I have such a system with one Hynix and one Infineon module that works fine, but when the Hynix module was added, it was chosen VERY carefully with all of these factors in mind).

If the module composition in a dual channel memory system become mismatched or asymmetrical, even if the system still works and recognizes all of the memory, it will revert to single channel operation which will cut the speed of the memory system by half, slowing the entire system. Consequently, even if it otherwise works, you never want to actually change or add to the memory in a system in a manner that creates such a mismatch, because of the performance impacts. A dual channel memory system typically has either two or four memory sockets, and each socket belongs to one channel or the other, and it matters, also, not only what you install but in which sockets you install it. So you can have a set of four memory modules (two pairs of two different sizes, for example ... say two 512MB modules and two 256MB modules) that would work properly in the system in all regards, yet if you simply install those 4 modules into the wrong sockets the system may not run at all, or may run with the memory system speed cut by half.

My concern with what you did is that you started with a system that had two modules of 512MB each, and you added a 3rd module of 512MB. By definition this is a problem in a dual channel memory system, simply because its an odd number of modules. For dual channel operation In a dual channel memory system you must ALWAYS install memory in MATCHED PAIRS, so any odd number of modules always a sub-optimal configuration (if it will work at all). If you install a configuration of memory modules other than matched pairs, any one of three things can happen:

-The system wont boot (at all)
-The system runs but reverts to single channel operation
-The system doesnt recognize all of the memory (AND it MAY revert to single channel operation).

So your addition was, at best, sub-optimal and not recommended. But please note that all of the above factors apply in addition to the considerations added by the likelihood that your dual-core system supports dual channel memory. [However, most recent systems use dual channel memory even if the processor is single core. There is no connection between dual-core processors and dual-channel memory other than that dual core processor systems are of very recent vintage, and therefore likely to also have dual channel memory systems.]

From this discussion you can see that there are huge number of considerations and factors that come into play when adding memory to an existing system. Because the issues of matching memory types is so potentially complex, people who are adding memory to a dual channel memory system are always advised to buy the memory only in pairs, and many people take the [perhaps overly] conservative approach of buying all new memory rather than trying to find memory that matches and is compatible with previously installed memory, whose full specifications may be difficult or impossible to determine.

I hope that this helps you in resolving your problems.

Submitted by: Barry W. of North Canton, Ohio

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