How much memory can XP use?

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Thread: How much memory can XP use?

  1. #1
    Goldfish
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    How much memory can XP use?

    There it is. Straight and simple. How much ram can Microsoft XP home and Pro edition actually utilize? I have heard the answer to be 4gb from people I work with.

    I have a friend that has 4 gig installed on a very modern mobo but he says it can only see 2.5gb of the total 4gb he has installed. He can then take out that ram and stick it into an older mobo and it will see 3.5gb which is that type of mobo's max ram anyway. What could be his problem with the new mobo only seeing 2.5gb? Thanks.
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  2. #2
    Hammerhead Shark Brahma's Avatar
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    It should be 4GB, but it seems most of the time, anything over 3 gigs won't be recognized.

    Have him try turning page file off to see if that makes any difference. Although MS doesn't recommend leaving it that way.

    There is a change to the boot.ini that can be made to see 3gig of ram called the /3G switch....the first link discusses it.

    Here's some relevant MS pages on the subject:

    http://www.microsoft.com/whdc/system...AE/PAEmem.mspx

    http://support.microsoft.com/default...b;en-us;555223

  3. #3
    Great White Shark
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    Win XP can see and use 4GB memory. That amount is reduced by the motherboard architecture and the chip set being used. Most current Intel desktop motherboards can hold up to 8GB memory. Most current AMD desktop motherboards can hold up to 4GB memory.

    The default Windows memory organization is 2GB for the OS and 2GB for the application. The /3G switch simply reallocates that to 1GB for the OS and 3GB for the application.

    The 32bit Intel chips have used 36bit addressing since the Pentium processor.

  4. #4
    :::Cynical Shark::: vairox's Avatar
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    as much as you can feed it.

    may I quote the scorpions here...

    The b**** is hungry
    she needs to tell
    so give her inches
    and feed her well
    Last edited by vairox; 11-24-2006 at 02:17 PM.
    Quote Originally Posted by ua549
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  5. #5
    Reef Shark
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    32 bit XP SP2 cannot see and use 4GB. See http://support.microsoft.com/kb/888137 .

    This was done to "save" you from badly written drivers.

    In effect, if you have 4GB of RAM, then memory mapped motherboard resources will hide a significant chunk of main memory (making the memory unusable to Windows) and SP2 will eliminate another big chunk, so a typical result would be 2.75GB usable from Windows.

    I suspect the other PC that you put your memory in was SP1, which doesn't have this "fix".

    I had 4GB in my machine, but as I said, Windows only recognized 2.75GB and it dropped the memory speed down from DDR400 to DDR333 (because of the four sticks of RAM). I decided to take the two sticks out, because an additional 750MB wasn't worth the slower speed.

    If you really want lots of memory, go for a 64bit CPU and a 64 but operating system. Then the issues disappear, but get replaced by new ones - i.e. the lack of drivers. Which will get better as time goes on.
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  6. #6
    Great White Shark
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    My WinXP box recognizes more than 2.75 GB memory. With a 32bit OS it recognizes approximately 3.5 GB memory. The system also uses 8 sticks of memory.
    The issue with 4 sticks of memory is strictly an AMD problem. The speed problem is also an AMD issue. They do not apply to Intel mobos and chip sets.

  7. #7
    Reef Shark
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    Is your XP box Service Pack 1? Service Pack 2 introduces the limit. It prevents drivers from using PAE to address the upper areas of memory, meaning that all drivers have to use the < 4GB address space, making the real RAM unaddressable.

    I only commented on the speed issue because it had direct relevence on whether I kept the additional 2 sticks of ram for only a 750MB benefit. I was making no claims that it affected everybody, only that it affected me, and was a factor in my decision.

    I personally prefer AMD's solution of an on-chip memory controller. It gives me lower memory latencies and dedicated memory bandwidth that doesn't have to be shared with the rest of the system, with the disadvantages that you've already pointed out.

    And anyway, I was only responding to the question, as in what's the maximum that Windows XP sees, and why would one PC see 3.5GB and the other 2.75GB.
    Last edited by gzunk; 12-03-2006 at 07:51 AM.
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  8. #8
    Great White Shark
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    The .75GB memory difference is tied to the chip set and the way it is implemented by the mobo design. Perhaps you are referring to user memory. If a driver is loaded, the system sees the memory it is using. Just because it isn't reported does not mean it is not seen and used by the system. At any rate any single application is limited to 3GB.

  9. #9
    Mako Shark
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    ua549

    In Vista Business with a C2D + 975X mobo will the system see and use 4gb (4x1gb)? Thanks.

  10. #10
    Great White Shark
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    The 64 bit version will. I don't know about the 32bit version since I'm only running Vista as a guest OS with 2GB memory on a Win2k3 box.

  11. #11
    Reef Shark
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    Basically the memory mapped IO *is* the problem, and it's equally a problem on Intel and AMD motherboards.

    With the introduction of the No Execute Bit for Data Pages in SP2, Windows XP now runs in PAE mode (Page Address Extension, to enable the addressing of >4GB memory) even when running with 4GB or less. There are reasons for this that I won't go into because it's not relevant.

    What this means is that all drivers that need to access the memory mapped IO, through DMA for example, would have to understand PAE in order to work correctly with SP2. Microsoft determined that this was not the case, and modified the way that PAE works so that the memory mapped IO is all under 4GB, and badly written drivers are safe to use.

    So, to summarise, XP SP1 does not support the no-execute bit, and therefore does not need to turn on PAE if the memory size <= 4GB. If you turn on PAE with XP SP1, then it can re-map some of the IO addresses > 4GB, exposing more of your real memory for use both by the kernel and userspace, but with the risk that badly written drivers might not be able to use the re-mapped IO addresses. XP SP2 prevents that re-mapping, so hiding more of your memory, from both the kernel and userspace.

    This has nothing to do with the process address space, which is always either 2GB or 3GB depending upon the /3GB switch you pass to the kernel on boot up. Your processes will always have this much "virtual" address space. Some if it may be real memory, some if it may be virtual memory on the hard disk. The real memory is backed up with memory addressable by Windows, which, unfortunately DOES NOT include the memory hidden by the IO address map.

    32 bit Vista is likely not affected by this, since the driver model is changing anyway. I would expect that in order to get a vista driver certified it will have to behave correctly in PAE mode.

    Anyway thats me done. Only trying to help and provide a bit of background information.
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  12. #12
    Catfish adam_chevyss's Avatar
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    Quote Originally Posted by ua549
    My WinXP box recognizes more than 2.75 GB memory. With a 32bit OS it recognizes approximately 3.5 GB memory. The system also uses 8 sticks of memory.
    The issue with 4 sticks of memory is strictly an AMD problem. The speed problem is also an AMD issue. They do not apply to Intel mobos and chip sets.


    How did you get eight sticks of memory and is it worth it over 4 at the same end number ie: 4 gb or let's say 2 gb?
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  13. #13
    Mako Shark
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    Quote Originally Posted by adam_chevyss
    How did you get eight sticks of memory and is it worth it over 4 at the same end number ie: 4 gb or let's say 2 gb?
    ua549 does different stuff with his massive servers than we do playing games on our little computers.

    For most of us 2gb is fine for now, probably worth going to 4gb if you are rendering video or something that intensive.

  14. #14
    LOLWUT ImaNihilist's Avatar
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    After 2GB, Windows XP doesn't seem to really manage the extra RAM very well. You can go up to 3GB, but you can't actually go up to 4GB as I understand it because some memory addressing space is used up by other things, such as the video card memory. So if you have a 512MB video card, you can't really go higher than 3.5GB of system RAM.

    I suppose you can go up to 3GB if you really want, but more than that isn't worth it. I guess if you really want to max out Windows XP the best way to do it would be use two 1GB DIMMs and two 512MB DIMMs. I'd say that's the absolute maximum you'd really want to go up to in XP. You also might have trouble doing that with an AMD based system, because they can be kind of picky about populating all four DIMMs and using different size/latency memory.
    Last edited by ImaNihilist; 12-07-2006 at 04:51 PM.

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