P5GD1 Cannot See 4Gb of Memory

Discussion in 'Asus' started by tvc, Aug 19, 2004.

  1. tvc

    tvc Guest

    I recently purchased an ASUS P5GD1 motherboard and installed 4 Samsung
    DDRAM 1GB, PC-3200 DDR400 DIMMs. The BIOS reports only 3200Mb of
    memory.

    I tried each DIMM individually and they all work. I also tried adding
    one at a time and saw 1024Mb, 2048Mb, 3072Mb, and 3200Mb
    respectively.

    Any ideas?

    Thanks.

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    tvc, Aug 19, 2004
    #1
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  2. tvc

    Paul Guest

    I haven't read the 915 datasheet yet, but a typical reason for a
    desktop Northbridge to do this has to do with address space. The
    PCI/AGP address space comes out of the total 32 bit address space.

    Server chipsets that address more than 32 bits, tend to have two
    options. They can either leave the hole for PCI/AGP, below the 4GB
    mark, or some of them can remap memory or move that space for I/O
    higher in the memory map.

    You could experiment a bit, by disabling or removing PCI devices,
    and see if the size of lost memory changes at all. I don't know what
    exact algorithm the BIOS uses - whether it has a fixed allocation
    per PCI device, or is using some info from the config space of each
    PCI device to decide how much to allocate. PCI devices can either
    be plug-in cards or they can be integrated devices (Marvell LAN or
    ITE8212F RAID controller ?). Presumably PCI Express consumes address
    space like an ordinary PCI card would ?

    HTH,
    Paul
     
    Paul, Aug 19, 2004
    #2
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  3. tvc

    Egil Solberg Guest

    Interesting. Went to the datasheet and from page 203 and forward Intel
    describes this.
    As installed system memory approaches 4GB, one can see this behaviour.

    I wonder, will it be almost useless to go for 4GB instead of staying with
    3GB due to this?
     
    Egil Solberg, Aug 19, 2004
    #3
  4. tvc

    Paul Guest

    "Almost useless" - that sounds like something the marketing team could
    use in the ad copy :)

    It would seem that way. Depending on motherboard type, I've seen
    reports of 3.2 to 3.5GB available, and that would seem to be a waste
    of money. 2x1024 + 2x512 might make more sense, except for those who
    need every scrap of memory they can get. For example, if doing
    logic simulation, it might pay to use a PCI video card - maybe
    removing the AGP from the picture will free up more address
    space for use by the ram ? What you do, may depend on how badly
    you need the memory.

    As there are OS and application issues with that much RAM, the
    address space on the Northbridge might not be the only limitation.
    Perhaps the fact that only 3.2GB is available isn't all bad,
    if say the memory footprint for the system is 0.2GB, and the
    OS has a limit of no more than 3GB for a single application.
    For people playing with that much ram, there is plenty of
    research to do. This post covers some of the issues and the
    technical terms used can be used in further Google searches:

    http://groups.google.com/groups?threadm=

    Paul
     
    Paul, Aug 19, 2004
    #4
  5. tvc

    Egil Solberg Guest


    Very interesting, Paul. I actually learn something here. Thanks for an
    informative post.
     
    Egil Solberg, Aug 19, 2004
    #5
  6. tvc

    tvc Guest

    Paul, Egil,

    Thanks for your input. I guess I have some playing around to do. :)

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    tvc, Aug 23, 2004
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