What is Segmentation in OS?

 

Segmentation:

 

A segment can be defined as a logical grouping of instructions, such as subroutine, array, or data area.  Every program (job) is a collection of these segments. Segmentation is the technique for managing these segments. For ex: consider the following fig:

Fig: Segmented address space

Each segment has a name and a length. The address of the segment specifies both segment name and the offset within the segment. For ex: the length of the segment ‘Main’ is 100 KB, ‘Main’ is the name of the segment. The operating system searches the entire main memory for a free space to load a segment. This mapping is done by a segment table. The segment table is a table where each entry of the segment table has a segment ‘Base’ and a segment ‘Limit’.

 

The segment ‘Base’ contains the starting physical address where the segment resides in memory. The segment ‘Limit’ specifies the length of the segment.

Fig: Segment table

The logical address consists of two parts: segment number(S), and an offset into that segment (D). The segment number(S) is used as an index into the segment table. For ex: consider the fig.

Fig: Segmentation example

The logical address space (a job) is divided into 4 segments, numbered from 0 to 3. Each segment has an entry in the segment table. The ‘Limit’ specifies the size of the segment and the ‘Base’ specifies the starting address of the segment. For ex: segment ‘0’ is loaded in the main memory from 1500 KB to 2500 KB. So, 1500 KB is the base and 2500 - 1500= 1000 KB is the limit.

 

Advantages of Segmentation:

1. Eliminate fragmentation:  by moving segments around, fragmented memory space can be combined into a single free area.
2. Segmentation supports virtual memory.
3. It allows dynamically growing segments.
4. It facilitates shared segments.
5. Dynamic linking and loading of the segments is possible.