I just discovered that Synaptic on my Oneiric installation displays about 55 thousand packages instead of 35. About 20 of these have a name ending with :i386. I also recently read in the Oneiric Beta release notes that the AMD-64-version of has better handling of 32-bit code. I guess these two facts are related. Seamless installation of 32-bit coden on 64-bit installation is explained here.

Are there any optimizations (kernel specific I guess) regarding its execution?

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    Welcome to Ask Ubuntu, since you've asked two questions I've integrated your answer into your first one since it seems to be a duplicate and edited your question to be about the kernel, if you have any more multiarch questions please ask them separately so we can build a FAQ of them, thanks! – Jorge Castro Sep 28 '11 at 13:58
  • Related: askubuntu.com/questions/49674/… – Jorge Castro Sep 28 '11 at 14:00

No. The multiarch changes only relate to the way executables and libraries are packaged and installed to the system. There is no change to the way the code is executed, nor any changes to the kernel.

In a nutshell:

Ubuntu has 64 bit and 32 bit versions. In order to run 32 bit software (the main one being Adobe Flash) on the 64 bit version of Ubuntu, you need some 32 bit support libraries.

Under the old system, all these libraries existed in 32 bit form in the 32 bit version of Ubuntu, but you could not use those packages as they would overwrite your 64 bit versions. So these 32 bit libraries had to be specially packaged for the 64 bit version, into a single package called ia32-libs. This package was huge (the source was several gigabytes) and constantly needed new libraries adding to it.

Under multiarch, you just install packages from the 32 bit and 64 bit repositories side by side, which is much simpler for everyone.

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