Editing Memory segmentation (section)

==Hardware implementation==
In a system using segmentation, computer memory addresses consist of a segment id and an offset within the segment.<ref name="glaser1965"/> A hardware [[memory management unit]] (MMU) is responsible for translating the segment and offset into a [[physical address]], and for performing checks to make sure the translation can be done and that the reference to that segment and offset is permitted.

Each segment has a length and set of permissions (for example, ''read'', ''write'', ''execute'') associated with it.<ref name="glaser1965"/> A [[process (computing)|process]] is only allowed to make a reference into a segment if the type of reference is allowed by the permissions, and if the offset within the segment is within the range specified by the length of the segment. Otherwise, a [[hardware exception]] such as a [[segmentation fault]] is raised.

Segments may also be used to implement [[virtual memory]]. In this case each segment has an associated flag indicating whether it is present in main memory or not. If a segment is accessed that is not present in main memory, an exception is raised, and the [[operating system]] will read the segment into memory from secondary storage.

Segmentation is one method of implementing [[memory protection]].<ref name="ostep-1">{{cite book |title=Operating Systems: Three Easy Pieces |chapter=Segmentation |chapter-url=http://pages.cs.wisc.edu/~remzi/OSTEP/vm-segmentation.pdf |publisher=Arpaci-Dusseau Books |date=2014 |first1=Remzi H. |last1=Arpaci-Dusseau |first2=Andrea C. |last2=Arpaci-Dusseau}}</ref> [[Page (computer memory)|Paging]]<!-- please, do not "simplify" or "fix" the link. The [[paging]] article is about a virtual memory technique, not about memory protection --> is another, and they can be combined. The size of a memory segment is generally not fixed and may be as small as a single [[byte]].<ref>{{cite book |publisher=Intel Corporation |title=Intel 64 and IA-32 Architectures Software Developer's Manual Volume 3 (3A, 3B & 3C): System Programming Guide |date=2012 |pages=3–13 |url=http://download.intel.com/products/processor/manual/325384.pdf}}</ref>

Segmentation has been implemented several ways on various hardware, with or without paging. Intel [[x86 memory segmentation]] does not fit either model and is discussed separately below, and also in greater detail in a separate article.

===Segmentation without paging===
Associated with each segment is information that indicates where the segment is located in memory&mdash; the ''segment base''. When a program references a memory location, the offset is added to the segment base to generate a physical memory address.

An implementation of virtual memory on a system using segmentation without paging requires that entire segments be swapped back and forth between main memory and secondary storage. When a segment is swapped in, the operating system has to allocate enough contiguous free memory to hold the entire segment. Often [[fragmentation (computing)|memory fragmentation]] results if there is not enough contiguous memory even though there may be enough in total.

===Segmentation with paging===
Instead of a memory location, the segment information includes the address of a [[page table]] for the segment.
When a program references a memory location the offset is translated to a memory address using the page table. A segment can be extended by allocating another memory page and adding it to the segment's page table.

An implementation of [[virtual memory]] on a system using segmentation with paging usually only moves individual pages back and forth between main memory and secondary storage, similar to a paged non-segmented system. Pages of the segment can be located anywhere in main memory and need not be contiguous. This usually results in a reduced amount of input/output between primary and secondary storage and reduced memory fragmentation.