Some time ago we talked about using more than 3 GB of RAM in Ubuntu 32bit which involved installing the server kernel. Well, this is no longer possible in Ubuntu Karmic Koala (9.10). According to the Ubuntu wiki:

Note that the i386-server flavour is being dropped. I can think of no good reason to continue to support a 32 bit server. The 32 bit -generic kernel ought to suffice for those headless implementations that have used the -server flavour in the past, such as home gateways.

Installing the server kernel was needed so that you'd had a kernel with PAE enabled (alternatively, you could just compile the normal kernel and enable PAE).

But because many OEMs and users requested a 32 bit PAE enabled kernel, Karmic Koala now comes with linux-generic-pae meta package which enables PAE in Ubuntu 32 bit, thus allowing you to use more than 3GB of RAM.

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Ubuntu 32bit only supports 3 GB of RAM so if you have 4, only 3 will be used. You can use the 64bit version for all 4GB of RAM memory to work, or follow the instructions in this post to make the 32bit version of Ubuntu to work with 4GB or RAM.

Firstly, please note that to achieve this, you must install the Ubuntu Server Kernel which has Phisical Extencion Address Enabled (PAE) enabled thus allowing Ubuntu to see all 4 GB of RAM. You can also compile the 32 Kernel desktop version yourself, and enable PAE but that's difficult to explain so I'll not cover that :). Once, that was easily done with Kernelcheck, but now kernel.org changed something which made Kernelcheck to stop working. Another solution would be to just install the 64bit Ubuntu.

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