Segmentation systems

1. Segmentation systems • In segmentation, programs are divided into variable size segments. • Every logical address is formed of a segment number and an offset within that segment. • Programs are segmented automatically by the compiler or assembler.

2. Segmentation systems • 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, symbol table, arrays

3. User’s View of a Program

4. Segmentation Architecture • Logical address consists of a two tuple: <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. • For logical to physical address mapping, a segment table (ST) is used. When a logical address <segment #,d> is generated by the processor: • Check if ( 0 ≤ d < limit ) in ST. • If o.k., then the physical address is calculated as Base + d, and the physical memory is accessed at memory word ( Base + d ).

5. Segmentation Hardware

6. Example of Segmentation

7. Example of Segmentation • For example, assume the logical address generated is <1,123> • Check ST entry for segment #1. The limit for segment #1 is 400. Since 123<400,we carry on. • The physical address is calculated as: 9300 + 123 = 9423, and the memory word 9423 is accessed.

8. Sharing of Segments