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Okay, I have 4 RAM slots in my computer - I'd stocked it with 2 2-GiB sticks, and 2 1-GiB sticks, but my OS only showed 3 GiB. This occurred on 32-bit Windows, 32-bit Linux, and 64-bit Linux.

The BIOS recognizes the RAM perfectly - I went into the BIOS settings, and everything displayed OK. 4 RAM slots, each of them said it had exactly what I'd put in it - total, 6 GiB.
Trouble is, the OS wouldn't see it, even a 64-bit one.

I'm not sure whether I need to enable PAE or something like that, but I'd really like my RAM back.

My computer is a Dell OptiPlex GX620, and I will provide any other specs/logfiles/etc requested.

Also, Memtest86+ returns a LOAD of errors when running it.

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A machine where Memtest86+ returns any errors has hardware problems. Find the bad hardware before you worry about things like OSes. –  Loren Pechtel May 5 '13 at 19:17
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3 Answers

up vote 6 down vote accepted

Your first actions should be to find the bad stick(s) as doing anything further until the stick(s) are removed would be counterproductive until the ram runs error free.

I would suggest pulling all sticks and then test each individualy with what you used (Memtest86+) and get the probelm stick(s) out of the picture and then address the 32 bit OS limitations.

Update:

According to Dell this Optiplex supports a MAX Memory of 4GB. So maybe your issue in memory shortfall is linked to the limiting factor of MAX supported capacity. Althoug, bios updates can and have been done from the manufacturer and they may not have updated the manual...so just a note on possible limit. Quote from Dell Optiplex owners manual:

Addressing Memory With 4-GB Configurations

This computer supports a maximum of 4 GB of memory when you use four 1-GB DIMMs or two 2-GB DIMMs. Current operating systems, such as Microsoft® Windows® XP, can only use a maximum of 4 GB of address space; however, the amount of memory available to the operating system is less than 4 GB. Certain components within the computer require address space in the 4-GB range. Any address space reserved for these components cannot be used by computer memory. The following components require memory address space: l System ROM l APIC(s) l Integrated PCI devices, such as network connectors and SCSI controllers l PCI cards l Graphics card l PCI Express cards (if applicable) At start-up, the BIOS identifies the components that require address space. The BIOS dynamically calculates the amount of reserved address space required. The BIOS then subtracts the reserved address space from 4 GB to determine the amount of usable space. l If the total installed computer memory is less than the usable space, all installed computer memory is available for use only by the operating system. l If the total installed computer memory is equal to or greater than the usable address space, a small portion of installed memory is available for use by the operating system.

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Wow... so it's definitive - 4 GiB of RAM is the maximum for this computer... Thanks! –  JamesTheAwesomeDude May 7 '13 at 2:45
    
... because ... the chipset- the interface between the 64-bit processor, memory, and other components in the system is 32-bit. –  Dustin G. May 8 '13 at 2:29
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The amount of memory in your system depends on several things. First, the processor has to be able to address 64-bit memory space. Second, the chipset on your motherboard has to be 64-bit AND be designed to support 4GB or more as well. Third, your Operating System has to be 64-bit OR utilize PAE to address more than 4GB of memory (Windows 2003 server 32-bit could utilize more than 4GB of RAM).

In your case, your chipset is 32-bit so the maximum amount of memory it can address is 4GB... problem is other devices take a chunk out of available address space in 32-bit world, so you should be seeing 3.2 to 3.5GB available in your current memory configuration because other items such as GPU memory and caches in the system are using up part of that 4GB maximum.

Dell OptiPlex GX620 Specs (Look under Processor Type for Chipset specifics): http://www.dell.com/downloads/global/products/optix/en/spec_optix_gx520-gx620_en.pdf

Intel 945G: (32-bit Chipset) http://ark.intel.com/products/27720/Intel-82945G-Memory-Controller

A brief article to back up my statements: http://www.pcworld.com/article/2013751/why-cant-32-bit-windows-access-4gb-of-ram.html

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I have successfully booted a 64-bit Linux on this before, so I am relatively confident that the chipset is 64-bit... However, taking out all the offending RAM got me up to 3.4-3.5 GiB on all OSes, 32- and 64-bit. –  JamesTheAwesomeDude May 5 '13 at 19:57
    
You can have a 32-bit chipset and run a 64-bit OS since you have a 64-bit processor - they're different things. –  Dustin G. May 6 '13 at 1:53
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As you know, a 32-bit OS won't recognize more than 3.25GB of RAM, because that's all it can address, while a 64-bit OS doesn't have the same problem. Between the fact that a 64-bit Linux isn't seeing all of the RAM, and the errors from Memtest86+, I'd tend to suspect you've got some bad RAM in the machine -- if one of the 2GB and one of the 1GB sticks are bad, that'd leave you with the three gigabytes you're seeing.

Per the Dell support site, the GX620 supports at least one 64-bit OS, which should imply that it has a 64-bit processor and shouldn't need PAE enabled. That said, it shouldn't hurt anything to enable it, and I'd try doing so just to see what result you might get; perhaps it'll help, and at worst it'll do nothing and you'll just need to go back into the BIOS and turn it off.

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I took out everything except the 2 2-GiB sticks, and now I have 3.4 GiB of RAM. I'd like a bit more, but I tried 4 different other sticks, and none of them seem to pass Memtest, if they're installed alongside the 2GiB sticks. –  JamesTheAwesomeDude May 5 '13 at 19:54
    
@JamesTheAwesomeDude Does it work if you replace the 2GB sticks with 4GB sticks, does memtest work then with more than 4GB? Maybe it's actually the RAM slots on your motherboard that are dead, not the sticks themsleves. –  Thomas May 6 '13 at 3:53
    
@Thomas No, the BIOS recognizes any and all RAM I put it, no matter where I put it or how much - even if Memtest and the OS don't. However, CarlB's answer above explains that this computer has a strict 4 GiB limit on usable RAM. (The BIOS will report all the installed RAM, but the OS just can't access it..) –  JamesTheAwesomeDude May 7 '13 at 2:47
    
@JamesTheAwesomeDude I see. I knew BIOS had limits on accessible memory but I had no idea it could be so low (for instance my mobo has max 32GB). That kind of sucks.. sounds like upgrade time :) –  Thomas May 7 '13 at 6:54
    
@Thomas Ya, this is actually a last-gen machine - some school was upgrading their computers, and they were selling these OptiPlex GX620s for dirt-cheap.. course, that was 3 years ago... ;) This thing's single-core, and it's still running off DDR2. –  JamesTheAwesomeDude May 8 '13 at 5:14
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