Usable RAM on a Dimension 8400

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Usable RAM on a Dimension 8400

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I have a Dimension 8400 with 4GB RAM, running Windows 7 Ultimate 64-bit. When I was using the bundled ATI video card with 128MB RAM, only 3.25GB RAM was reported as usable by Windows. After having just upgraded to a Nvidia video card with 512MB RAM, Windows now reports just 3.00GB of usable RAM Any idea why a full GB of RAM is unusable by Windows? Any way to up the amount of usable RAM?

Verified Answer
  • Although you are using a 64 bit OS, the chipset used in the Dimension 8400 is limited to a 4 GB address space in hardware.  This is not something that can be changed by a BIOS flash or other firmware upgrade.  Since the chipset cannot see anything past 4 GB, it has no way of performing the necessary re-mapping of memory that is overlaid by the OS and hardware support space.  Since the hidden 1 GB of RAM cannot be re-mapped to memory space over the 4 GB limit, memory in that space remains hidden.

    Even though video memory is dedicated to the display adapter, the hardware must still have a way to address it.  Video memory is therefore mapped into the main memory space in the 4th GB.  If you had installed a video card having more than 512 MB of memory you would likely find that you had even less than 3 GB of memory reported.

    Dell Forum member since 2005

All Replies
  • Although you are using a 64 bit OS, the chipset used in the Dimension 8400 is limited to a 4 GB address space in hardware.  This is not something that can be changed by a BIOS flash or other firmware upgrade.  Since the chipset cannot see anything past 4 GB, it has no way of performing the necessary re-mapping of memory that is overlaid by the OS and hardware support space.  Since the hidden 1 GB of RAM cannot be re-mapped to memory space over the 4 GB limit, memory in that space remains hidden.

    Even though video memory is dedicated to the display adapter, the hardware must still have a way to address it.  Video memory is therefore mapped into the main memory space in the 4th GB.  If you had installed a video card having more than 512 MB of memory you would likely find that you had even less than 3 GB of memory reported.

    Dell Forum member since 2005

  • Thanks for your clear and full explanation of what is going on.

  • Nice answer JackShack - I learnt something there (I had assumed if you had installed a 64-bit OS on the 8400, the whole 4gb ram should be usable).

    However interestingly (to me, anyway) it's not a simple sum in terms of how much RAM is reported.

    I had the same experience as tcjdw -

    When I bought my Dimension 8400 (summer 2004) I had a 128MB card  and 3.25Gb of 4Gb was reported.

    Then, I upgraded to a 512MB card and 3Gb of 4Gb was reported (note, even though the card had 384MB more RAM rather than 250MB more).

    Then, I upgraded to a 1Gb card and 3Gb was still reported. (despite increase of video ram on the card).

    I think a few years ago I investigated this and it was something to do with the way the drivers for the card allocate the video memory, and when you play a game it can take allocate more.. something technical like that.

    Anyway it's a bit of an aside - thanks for the useful info that the chipset is limited to 4Gb in hardware.