Segmentation is one of the most common ways to achieve memory protection; another common one is paging. Segmentation means that a part or parts of the memory will be sealed off from the currently running process, through the use of hardware registers. If the data that is about to be read or written to is outside the permitted address space of that process, a segmentation fault will result.
This usage should not be confused with that of the memory segments used by early x86 processor architectures.
For details of x86's implementation of segmentation in both 16-bit and 32-bit mode, please see the article on memory segments.
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