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· Memory-management scheme that supports user view of memory.

· A program is a collection of segments. A segment is a logical unit such as:

§ Main program,

§ Procedure,

§ Function,

§ Method,

§ Object,

§ Local variables, global variables,

§ Common block,

§ Stack,

· Segmentation is a memory management scheme that supports this user view of memory.

· A logical address space is a collection of segments. Each segment has a name and a length.

· The addresses specify both the segment name and the offset within the segment.

· The user therefore specifies each address by two quantities such as segment name and an offset. For simplicity of implementation, segments are numbered and are referred to by a segment number, rather than by a segment name.

· Logical address consists of a two tuples:

§ <segment-number, offset>

· Segment table – maps two-dimensional physical addresses; each table entry has:

Base – contains the starting physical address where the segments reside in memory.

Limit – specifies the length of the segment.

· Segment-table base register (STBR) points to the segment table’s location in memory.

· Segment-table length register (STLR) indicates number of segments used by a program;

§ Segment number s is legal if s< STLR.

· When the user program is compiled by the compiler it constructs the segments.

· The loader takes all the segments and assigned the segment numbers.

· The mapping between the logical and physical address using the segmentation technique is shown in above figure.

· Each entry in the segment table as limit and base address.

· The base address contains the starting physical address of a segment where the limit address specifies the length of the segment.

· The logical address consists of 2 parts such as segment number and offset.

The segment number is used as an index into the segment table. Consider the below example is given below.

Segmentation with Paging

· Both paging and segmentation have advantages and disadvantages, that’s why we can combine these two methods to improve this technique for memory allocation.

· These combinations are best illustrated by architecture of Intel-386.

· The IBM OS/2 is an operating system of the Intel-386 architecture. In this technique both segment table and page table is required.

· The program consists of various segments given by the segment table where the segment table contains different entries one for each segment.

· Then each segment is divided into a number of pages of equal size whose information is maintained in a separate page table.

· If a process has four segments that is 0 to 3 then there will be 4 page tables for that process, one for each segment.

· The size fixed in segmentation table (SMT) gives the total number of pages and therefore maximum page number in that segment with starting from 0.

· If the page table or page map table for a segment has entries for page 0 to 5.

The address of the entry in the PMT for the desired page p in a given segment s can be obtained by B + P where B can be obtained from the entry in the segmentation table.
Using the address (B +P) as an index in page map table (page table), the page frame (f) can be obtained and physical address can be obtained by adding offset to page frame