Operating System Structures

Introduction

• Discussing the structures of operating systems.
• Previous discussion on multi-programming and multi-tasking systems.
• Emphasis on the need for careful engineering of operating systems.
• Overview of different structures used in operating system design.

Simple Structure

• Early operating systems like MS-DOS.
• Lack of defined structure; all layers access base hardware.
• Vulnerability to crashes due to direct hardware access.
• Example: Microsoft Disk Operating System (MS-DOS).
• Simple structure leads to potential system crashes.
• Limitations due to hardware like Intel 8088, which lacked hardware protection.

Monolithic Structure

• Used by earlier Unix systems.
• Contains a large kernel with many functionalities.
• Structure: Kernel, Shells/Commands, System Libraries.
• Difficult to implement and maintain due to complex single-level packing.
• Challenges in adding or debugging functionalities.

Layered Structure

• Operating system divided into layers.
• Each layer has specific functionalities.
• Easier to debug and maintain (focus on specific layers).
• Challenges in design and efficiency.
  • Service requests pass through multiple layers, causing delays.
• Protected hardware access.
• Example of layering: Backing storage below memory management.

Microkernels

• Reduces kernel size by removing non-essential components.
• Core functionalities in the microkernel; others as system programs.
• Communication through message passing.
• Advantages: More functions in user mode; less risk of system crashes.
• Disadvantages: Performance decrease due to system function overhead.

Modular Structure

• Best current methodology using object-oriented programming.
• Core kernel with dynamically loadable modules.
• Resembles both layered and microkernel approaches.
• More flexible: Any module can communicate with any other module directly.
• Avoids overhead of message passing used in microkernels.

Summary

• Structures: Simple, Monolithic, Layered, Microkernel, Modular.
• Each has its own advantages and disadvantages.
• Modular approach combines flexibility and efficiency, widely used in modern operating systems.