P5AD2-E Premium not seeing all of memory

Discussion in 'Asus' started by Norman E. Powroz, Feb 4, 2005.

  1. I just finished building a new system for a customer based on a
    P5AD2-E Premium (a really nice board!).

    Since the system will be used as a web and database server, I have
    maxed it out in memory and disc configuration, but for some reason,
    all of the memory isn't visible. I have 4 1GB Kingston DDR2 modules
    installed, which theoretically takes the board to its rated 4GB
    maximum limit.

    However, the BIOS can only see 3.25GB of memory. I know that each
    stick of memory is good. If I only install a pair of DIMMs, the full
    2GB in the pair is accessible. Each pair combinaton works perfectly,
    but with all 4, we only get 3.25GB.

    Is this a known limitation that I will simply have to live with, or
    have I overlooked something in the BIOS? I think I am running the
    latest BIOS flash, but I haven't gone into that area as yet.

    Any advice appreciated.

    Norman E. Powroz, Feb 4, 2005
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  2. Norman E. Powroz

    Paul Guest

    Page 159 of the 925X Express Chipset document, shows an example
    of what is going on. The address space available is 32 bits,
    and some of the address space must be made available for PCI or
    PCI Express devices (memory mapped I/O).


    What this means is, the Northbridge "throws away" some of the
    DRAM memory, and addresses falling into the top most area of
    memory, go out on the PCI or the PCI Express bus.

    There are some computing products, that support the remapping
    of DRAM memory. What they do, is they leave a space of, say
    1GB or so (from 3GB to 4GB), for access to the various busses
    on the motherboard, and then, they lift the GB of displaced
    RAM and map it from 4GB to 5GB. (To get up there, of course
    the processor has to have more address bits available, and
    some mechanism in the OS to use them. Google on PAE, PSE,
    36 bit, address space, as search terms.)

    Section 4.4 pg 128 of this document, illustrates a chipset
    that supports the concept. I just happened to have this
    doc in my collection, and there are more capable chipsets
    than this one.


    There could well be a similar function supported
    on Athlon64 motherboards that accept an Opteron processor.
    Page 80 of this document discusses how it works.


    At least one Asus AMD motherboard doesn't seem to support that
    "hoisting" function, and I cannot tell you whether the
    Northbridge has anything to say in the matter or not,
    on Athlon64. (The memory controller is in the processor,
    so all that should be needed is a compliant BIOS.)

    There is some info here, on OS support for large memory.
    The main message of this posting (the second half of the posting),
    is a lot of things have to be working right, before you get to
    use a large memory.


    Paul, Feb 4, 2005
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  3. To see all of that RAM you will need to enable PAE support in your OS -
    the address space is only 4GB, and some of that is used by things like
    PCI memory space, AGP aperture, etc. so you need PAE to see the rest of
    the RAM.
    Robert Hancock, Feb 5, 2005
  4. Whoa!! I have to say that I never expected an answer as well
    researched or presented as the one you just gave. Quite honestly, I
    feel overwhelmed. The amount of information, and the obvious depth of
    your knowledge and research, are impressive to say the least.

    You have provided me with everything I need to know about the problem,
    as well as highlighting the fact that there is nothing I can do about
    it, since it's a hardware imposition. It kind of harks back to the old
    8088 days in which memory between the 640KB and 1MB boundaries was
    needed for hardware communication areas, such as video or other
    direct-memory and memory-mapped devices. And of course, everyone used
    memory managers such as QEMM in order to maximize the "high memory"

    Thanks again, Paul.

    Norman E. Powroz, Feb 5, 2005
  5. A given app is going to be limited to 2GB of memory space (or 3GB if you
    use the /3GB switch on Windows). However, the entire system can use up
    to (I believe) 64GB if you turn PAE on.
    Robert Hancock, Feb 6, 2005
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