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I have 11.10 64bit installed on my Lenovo s205. When I install 2G (confirmed by the BIOS), "Systems Info" only sees 1.6GiB, and when I install 3G, it only sees 2.6GiB. If seen some pages suggesting I run

sudo apt-get install linux-generic-pae linux-headers-generic-pae

after first calling for update, but I get the reply that it is impossible to install this. It might be that these post refer to previous releases, and I get the impression that this is more for people who have installed the 32b version, but is there anything I CAN do?

$ sudo apt-get install linux-generic-pae linux-headers-generic-paeReading package lists... Done
Building dependency tree       
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies.
 linux-generic-pae:i386 : Depends: linux-image-generic-pae:i386 (= but it is not going to be installed
 linux-headers-generic-pae:i386 : Depends: linux-headers-3.0.0-27-generic-pae:i386 but it is not going to be installed
E: Unable to correct problems, you have held broken packages.
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And the 400Mb is not used by your videocard? btw... the PAE kernel is -only- needed with 4Gb and more. Under 4 it is fairly useless. Please provide the results of free -m! – Rinzwind Oct 30 '12 at 14:56

This is likely caused by your computer using main RAM for your graphics card. Based on the amounts reported, I would guess your graphics card is using 384MB specifically. You can usually check and adjust the allocation in your BIOS config - it can't be adjusted from within Ubuntu.

Here's a similar question: Check graphics card memory

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The PAE kernel is only for enabling 32 bit operating systems to support more than 4GB of memory.

That said, there are a few reasons why you don't see the amount you expect to see (ordered from most to least likely).

  1. System Reserved Memory While it's Windows-centric in some parts, this article provides a good description of where your RAM is going, as well as what PAE is. Of specific interest is this:

    The original IBM PC’s processor could access 1024 KB of physical address space, but you could only use 640 KB for RAM. The remaining 384 KB of address space was reserved for memory-mapped hardware and ROM. A similar situation exists with current systems: hardware reserves large chunks of the upper 1 GB of physical address space. Because of these reserved areas, a system with a 32-bit physical address space will be limited to somewhere around 3.1-3.5 GB of RAM.

  2. Onboard video If you're using a video card with shared RAM, then it will eat into your system memory, equivalent to whatever it's set to use. In trying to find the specs for your computer, I've found two conflicting spec, one that says 512MiB (and suggests discrete memory), and one that says 384MiB (and integrated/shared) RAM, so it may depend on the build and where you go it, but it's possible that your video card is sharing your RAM. If you have Catalyst installed, it might be able to give you better information on your card.

  3. SI to Binary conversion This is less of a concern with RAM (it's generally more noticeable and more of concern with hard drives due to OEMs' use of the SI definition when advertising amounts), but I think it's worth mentioning, for educational purposes. Simply put, there is a difference between Gigabye (GB) and Gibibyte (GiB). The "Giga-" prefix is the SI unit for 10^9 (10 to the 9th power), while the binary "Gibi-" is 2^30 (2 to 30th power). For years, "Giga" was used basically for rounding purposes, while we were working in "kilobytes" (1KiB = 1024B, which is pretty close to 1000) and abbreviation purposes (KiB became just K, until we moved into larger units, and we just added the "B" back on). As such, in the off chance that you were given your memory amount in SI, instead of binary, your 4GB would, in fact, be about 3.7GiB. Since we're talking RAM, and the discrepancy is consistent (aways .4GiB), this is highly unlikely, and it's far more likely that it's point 1, and maybe point 2.

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Thank you both for your quick replies. I think the "Onboard Video" explanation is the right one, also because, as ImaginaryRobots remarks, the 'gap' is the same in both occasions. I had a look at the bios, but it does not allow me to change that. Pity. Guess I'll have to upgrade my memory ...

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