Segmentation

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

User’s View of a Program

Segmentation Architecture

Logical address consists of a two tuple: <segment-number, offset>,

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

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

    2. 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.

Relocation.

     1. dynamic

     2. by segment table

     3. Sharing.

     4. shared segments

     5. same segment number

     6. Allocation.

     7. first fit/best fit

     8. external fragmentation

Protection. With each entry in segment table associate:

     1. validation bit = 0 _ illegal segment

     2. read/write/execute privileges

Protection bits associated with segments; code sharing occurs at segment level.

Since segments vary in length, memory allocation is a dynamic storage-allocation problem.

A segmentation example is shown in the following diagram

Segmentation Hardware

Example of Segmentation

Sharing of Segments