§  Segmentation is a memory management scheme in which the physical memory allocated to the process is non-contiguous.

§  In Segmentation logical address space is divided into number of small blocks known as Segment.

§  Segments are of variable and each segment is allocated memory independently.

§  The responsibility for dividing the program into segments lies with the user. The operating system is uninvolved.

§  Each Segment has a name and a length. The addresses specify both the segment name and the offset within the segment.

§  In Segmentation, a logical address consists of two parts: segment number and offset. An offset is the location in a distinct segment in main memory. Segment number is used as an index into a segment table which contains base address of each segment in physical memory and a limit of segment.

§  A Segment table has two fields: base address of the segment and limit of the segment.

§  The base address of a segment is the location of a particular segment in the main memory, from where the segment starts.

§  Segmentation can be implemented using or without using paging.

§  Unlike paging, segment is having varying sizes and thus eliminates internal fragmentation.

§  External fragmentation still exists but to lesser extent.


§  The use of protection bits prevents illegal access to the memory locations. In this way, an attempt to write to a read-only segment fails.

§  The sharing of code or data is possible even in segmentation.


§  As with paging, this mapping also requires two memory references per logical address. This slows down a system by a factor of 2.

§  Segmentation suffers from external fragmentation. This may occurs when all the blocks in the memory are too small to accommodate segment.

§  Another Disadvantage of segmentation is that the programmer must be aware of the maximum size limits of the segments.


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