Segmentation in Operating Systems - Full Explanation

Introduction

Imagine trying to organize a large book by breaking it into chapters and sections so you can find information easily. Computers face a similar problem when managing memory for programs, needing a way to divide memory into meaningful parts.

Explanation

Memory Division by Logical Units

Segmentation divides a program's memory into segments based on logical units like functions, data arrays, or modules. Each segment represents a meaningful part of the program, making it easier to manage and protect.

Segmentation breaks memory into logical parts that match the program's structure.

Segment Table and Addresses

The system keeps a segment table that stores the base address and length of each segment. When a program accesses memory, it uses a segment number and an offset within that segment to find the exact location.

Memory addresses in segmentation use a segment number plus an offset to locate data.

Protection and Sharing

Because segments are separate, the system can protect them individually. It can also share segments like code between programs without copying, saving memory and improving efficiency.

Segmentation allows individual protection and sharing of program parts.

Variable Segment Sizes

Segments can be different sizes depending on the program's needs. This flexibility helps use memory efficiently but can cause gaps or fragmentation between segments.

Segments vary in size, which helps flexibility but can lead to fragmentation.

Real World Analogy

Think of a filing cabinet where each drawer holds folders for different projects. Each folder contains papers related to that project, making it easy to find and protect important documents. Sometimes, folders can be shared between drawers if needed.

Memory Division by Logical Units → Folders in a drawer holding related papers for a project

Segment Table and Addresses → Labels on folders and pages that tell you where to find each paper

Protection and Sharing → Locking a folder to protect its papers or sharing a folder between drawers

Variable Segment Sizes → Folders of different thickness depending on how many papers they hold

Diagram

Diagram

┌───────────────┐
│   Segment 0   │
│  (Code)      │
├───────────────┤
│   Segment 1   │
│  (Data)      │
├───────────────┤
│   Segment 2   │
│  (Stack)     │
└───────────────┘

Segment Table:
[Segment # | Base Address | Length]
[0         | 1000         | 500   ]
[1         | 1500         | 300   ]
[2         | 1800         | 200   ]

This diagram shows memory divided into three segments with a segment table listing their base addresses and sizes.

Key Facts

Segment → A logical unit of a program's memory such as code, data, or stack.

Segment Table → A data structure that stores base addresses and lengths of all segments.

Logical Address → An address consisting of a segment number and an offset within that segment.

Fragmentation → Unused gaps in memory caused by variable-sized segments.

Memory Protection → Preventing unauthorized access to segments by controlling permissions.

Common Confusions

Segmentation is the same as paging.

Segmentation is the same as paging. Segmentation divides memory by logical program parts of variable size, while paging divides memory into fixed-size blocks without regard to program structure.

Logical address is a single number.

Logical address is a single number. In segmentation, a logical address has two parts: a segment number and an offset within that segment.

Summary

Segmentation organizes memory into logical parts that reflect a program's structure, like code and data sections.

Each segment has a base address and length stored in a segment table, enabling precise memory access.

Segmentation supports protection and sharing of program parts but can cause fragmentation due to variable segment sizes.