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I have a relatively clean installation of Ubuntu which reads this under system details:

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I've read everywhere that 32-bit OSs don't support more than 4 GB of RAM (some sources say 3 GB), so what's going on in my system? Does it recognize but not use all the RAM I'm providing it (BTW, the System Monitor also shows 5,8 GiB) or what? Unless the system installed PAE automatically, I'm not using it. I've taken a look at the answers on Does Ubuntu Desktop 32Bit Support more than 4GB Memory with default installation?, but am still looking for an answer to my case.

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Waldir Leoncio
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    Why don't you fire up lots of applications and use free to check for yourself? But yes, 32-bit systems can use more than 4GB of RAM. With 32 bits, theoretically you only have address space for 4GB of memory, but like you said, with PAE for example, this limit can be raised. Microsoft Windows servers have been supporting more than 4GB of memory for years as well. It's no magic. – Jakke Jun 27 '14 at 11:40
  • I've actually though about that, but didn't know about the smart way to conduct such a test. – Waldir Leoncio Jun 27 '14 at 11:45
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    There's nothing much to plan about it... any application uses memory and can be used for this test. If you want an easy test, just open all of your bookmarks in different tabs in browsers. For a lot of people that alone will fill up your memory ;) – Jakke Jun 27 '14 at 11:53

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If it's showing that, you're probably using PAE. It's enabled by default.

Why is it 5.8GB instead of 6GB? Do you have onboard graphics? It's probably reserving a chunk.

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Oli
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  • I do have an Intel onboard. Should I disable it? How? – Waldir Leoncio Jun 27 '14 at 11:39
  • Aren't you using it? If you aren't, it usually disables itself if there's another video card but otherwise you should be able to disable it through BIOS. I'd be surprised if it's eating more than 256MB. How much RAM do you expect to see? – Oli Jun 27 '14 at 11:40
  • Theoretically, I understand it should show 6 GB, but I always assumed it showed less because actual memory space is always a little under the advertised amount. This is my gaming and video-watching rig, so I guess I can leave the GeForce to handle all the graphics; I'll take a look at my BIOS, thanks for the tip. – Waldir Leoncio Jun 27 '14 at 11:43
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    This is only true for disks, RAM always comes as advertised. If you're lacking a few MB, it means that portion of your RAM is used as shared memory for other devices than your CPU. If you want to use all of your memory, you'll have to disable your GPU, which means you will only have access to the console and possibly SSH. – Jakke Jun 27 '14 at 11:50
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The sentence "32-bit operating systems don't support more than 4 GB of RAM" is false in general. The Physical Address Extension, which is CPU feature, allows the 32-bit operating system to address more than 4GB of RAM. It is true, however, that some editions of 32-bit Microsoft Windows system don't use Physical Address Extension due to marketing reasons and hence don't support more than 4GB of RAM. The myth of only 4GB of RAM in 32-bit systems have grown on popularity of such Windows editions.

32-bit Ubuntu makes use of Physical Address Extension, so you can use more than 4GB of RAM in your 32-bit system.