Memory Management – Segmentation

Concept Of Segmentation in Operating System

 

In segmentation, programs are divided into variable size segments, instead of fixed size pages. Every logical address is formed of a segment name and an offset within that segment. In practice, segments are numbered. Programs are segmented automatically by the compiler or assembler.

 

For example, a C compiler will create separate segments for:
1. the code of each function
2. the local variables for each function
3. the global variables.

 

segmentation

 

For logical to physical address mapping, a segment table is used. When a logical address <s, d> is generated by the processor:
1. Base and limit values corresponding to segment s are determined using the segment table
2. The OS checks whether d is in the limit. (0 ≤ d < limit)
3. If so, then the physical address is calculated as (base + d), and the memory is accessed.

 

segmentation-2

 

Sharing Segments
Also sharing of segments is applicable as in paging. Shared segments should be read only and should be assigned the same segment number.

 

Example :
Consider a system in which 3 users executing an editor program which is 1500 KB in size, each having their own data space.

example-3-6

 

3. Paged segmentation

The idea is to page the segments and eliminate the external fragmentation problem. In paged segmentation the logical address is made of <s,p,d> triplet. The ST entry for segment S now, contains:

  •  the length of segment S
  •  the base address of the PT for segment S.

 

 

There is a separate PT for every segment. On the average, now there is half a page of internal fragmentation per segment. However, more table space is needed. In the worst case, again three memory accesses are needed for each memory reference. The flowchart for accessing a word with logical address <s,p,d> is shown below.

 

paged-segmentation

 

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