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Segmentation or Paging scheme that ubuntu 12.04 use to allocate the main memory? Does it use both of it? How?

If it doesn't use both scheme, so what the scheme that should be use? why?

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Intel introduced the concept of memory segmentation in the 286, but the only OS I know of that really used it was OS/2. All modern operating systems just use paging as the memory protection mechanism. Why? Because it is far more flexible. It isn't easy or convieient to have to divide memory up into entirely disjoint segments and address them separately. It is far more simple to use a flat memory space that you may or may not configure with different access permissions on different pages.

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Like all Linux-based systems running on Intel or similar hardware, Ubuntu allocates and manages main memory in pages, and even the chips themselves are set up to manage memory in pages. For more information see for example http://www.redhat.com/magazine/001nov04/features/vm/. The article is old so some details have changed, but the concepts are still the same.

Linux (including Ubuntu!) on other hardware architectures with virtual memory also use pages in all cases I am aware of, for example the ARM architecture.

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