What is memory segmentationExplain advantages of memory segmentation

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What is memory segmentation?Explain advantages of memory segmentation.

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Memory segmentation is the division of a computer's primary memory into segments or sections. In a computer system using segmentation, a reference to a memory location includes a value that identifies a segment and an offset (memory location) within that segment. Segments or sections are also used in object files of compiled programs when they are linked together into a program image and when the image is loaded into memory.

Segments usually correspond to natural divisions of a program such as individual routines or data tables so segmentation is generally more visible to the programmer than paging alone.[1] Different segments may be created for different program modules, or for different classes of memory usage such as code and data segments. Certain segments may be shared between programs.[1]

Hardware implementation

In a system using segmentation, computer memory addresses consist of a segment id and an offset within the segment. A hardware memory management unit (MMU) is responsible for translating the segment and offset into a physical memory 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. A 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.[2] Paging 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.[3]

Segmentation has been implemented in several different ways on different 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— 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 memory fragmentation results in there being not enough contiguous memory even though there may be enough in total.

Segmentation with paging

Instead of an actual 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 simply 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.



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