Computer only sees 2.5Gb's of my 4Gb's of ram

Discussion in 'OT Technology' started by delussional77, Jul 11, 2008.

  1. delussional77

    delussional77 Active Member

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    I recently built a new computer and originally had 2Gb of RAM in a Gigabyte mobo. I decided that since this was my main rig and HTPC combined that it could benefit from the cheap prices of additional ram.

    I added two more sticks of 1Gb ram (so 2Gb total).

    When I boot the PC the BIOS/setup screen sees all 4Gb.

    The task manager, however, shows 2.6Gb.

    Whats up? Did I do something wrong?
     
  2. skinjob

    skinjob Active Member

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    Do you have 2 video cards installed?
     
  3. delussional77

    delussional77 Active Member

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    actually I have one video card (ATI 3450) and am also using the onboard HDMI on my Gigabyte mobo

    is that an issue?
     
  4. eof

    eof New Member

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    Your OS is 32bit.... You need 64bit
     
  5. skinjob

    skinjob Active Member

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    Video cards take a large chunk of address space for memory-mapped I/O, so it's not surprising that 2 video cards would use that much more. Also, if your onboard video doesn't have it's own video ram, it will allocate system ram for its own use.
     
  6. delussional77

    delussional77 Active Member

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    I checked my BIOS and sure enough I have 512Mb for the onboard video ram

    oh well, guess I'll keep one of the sticks and sell the other....
     
  7. Frequency

    Frequency New Member

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    Chances are you need dual channel memory too
     
  8. gravity

    gravity New Member

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    +1
     
  9. trouphaz

    trouphaz New Member

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    so, what exactly is the deal with that and why is it only apparent when you go up to 4Gb of memory? is it that the cards are using the addresses, but not actually using the memory? so, any memory that happens to overlap those addresses is just unusable?
     
  10. skinjob

    skinjob Active Member

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    That's how memory-mapped I/O works. Memory addresses are allocated beginning at the top end of the address range for CPU-to-peripheral communication. When the CPU needs to communicate with a device, it reads or writes to the memory addresses reserved for that device, and the chipset handles the redirection. This simplifies CPU design by eliminating the need for extra communication facilities, but it renders the physical ram that would normally be mapped to those addresses unusable.

    If the CPU is operating in 64-bit mode, then the I/O mapping gets moved to top end of something like a 16TB address space. So, you won't notice this missing memory unless you find hardware that can take 16TB of physical ram.
     
  11. GOGZILLA

    GOGZILLA Double-Uranium Member

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    open 4 tabs in opera
     

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