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• Segmentation

Storage Management

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Recap

In the last class, you have learnt:

• Fragmentation

• Paging concepts

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Objective

On completion of this class, you will be able to

know

• Segmentation

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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, etc..

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User’s View of a Program

Subroutine

Sqrt

Main Program

Symbol table

Stack

Logical address space

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Logical View of Segmentation

1

3

2

4

user space

1

4

2

3

physical memory space

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Segmentation

• Each segment has a name and length

• Address specify both the segment name and

offset

• Instead of using name the segments are

numbered

• The logical address consists of a tuple:

– <segment-number,offset>

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Segmentation Architecture

• 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

• Use of segment table is illustrated in the next

slide

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Segmentation Hardware (contd..)

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• Logical address generated by CPU consists of two

parts

– Segment no. ‘s’:-used as a index into segment table

– Offset ‘d’:- must be between ‘0’ and segment limit

– A trap is generated if offset exceeds the segment limit

– Correct offset + segment base = address in physical

memory

• Segment table is an array of base-limit register pair

Segmentation Hardware

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Paging Hardware

HARDWARE -- Must map a tuple (segment / offset) into one-dimensional address

CPU

MEMORY

+<

No

LogicalAddress Yes

PhysicalAddress

Segment Table

Limit Base

S D

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Example of Segmentation

• Five segments numbered from 0 through 4.

• The segments are stored in physical memory as

shown in fig.1

• Segment table gives the details of limit and base

of each segment

– Ex. Segment 2 is 400 bytes long and begins at

location 4300

– A reference to 53 of segment 2 is mapped onto

location 4300+53=4353

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Example of Segmentation

segment 0

segment 3

segment 2

segment 4

segment 1

limit base

1000

400

400

1100

1000

1400

6300

4300

3200

4700

Subroutine

Sqrt

Main Program

Symbol table

Stack

Logical address space

segment 0

segment 3

segment 4

segment 2

segment 1

1400

3200

4300

4700

5700

6300

6700

2400

Physical Memory

Figure :1

Ex: Segment 2 = 4300+53=4353

Segment table

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Segmentation, Paging - Comparison

• Segments are variable-size; Pages are fixed-size

• Segmentation requires more complicated

hardware for address translation than paging

• Segmentation suffers from external fragmentation

• Paging only yields a small internal fragmentation

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Summary

In this class you have learnt

• Segmentation

– Pages

– frames

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Frequently Asked Questions

• What is segmentation?

• Explain the segmentation hardware with a neat

diagram

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Quiz

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

a) Segmentationb) Paging scheme C) None

2. The logical address consists of a a) Page offset b) Hole c) Tupled) None

Other subject materials

• Web designing

• Micro processors

• C++ tutorials

• java

home

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Quiz

3. ____________used as a index into segment

table

a) Page number

b) Segment no. ‘s’

c) Compaction

d) None

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Quiz

4. _________must be between 0 and segment

limit

a) External fragmentation

b) Offset ‘d’

c) Compaction

d) None