OS Structure Overview

Summary

• OS Structure
• Goals of OS
• Commercial OS
• Monolithic Structure
• DOS-like Structure
  • Loss of protection
• Microkernel
  • Downside to Microkernel
    • Why Performance Loss?
• Features of various OSes

OS Structure

OS Structure := the way the OS software is organized wrt applications that it servces and the underlying hardware that it manages.

Goals of OS

• Protection
  • Within and across users and the OS itself
• Performance
  • Time taken to perform the services
• Flexibility
  • Extensibility
  • Not one size fits all
• Scalability
  • performance goes up if hardware resources go up
• Agility
  • Adapting to changes in application needs and/or resources ability
• Responsiveness
  • How quickly OS reacts to external events

Commercial OS

• Linux
• MacOS
• Windows

Monolithic Structure

Each app has its own hardware address space

OS in its own hardware address space

Malfuntioning of an app shouldn’t affect integrity of OS nor other apps

DOS-like Structure

Monolithic structure where there is no protection between the apps and the OS

• Application code can make syscalls directly, like they would any other library

This was in a time where multitasking didn’t really exists and PCs were meant for single users

Loss of protection

Loss of protection is unacceptable for modern OS

Monolithic structures recoup performance losses by consolidating syscalls

• but loses ability to customize for specific apps

Microkernel

Microkernels were designed to focus around customization for a specific application

The microkernel itself handles simple abstractions like address space, IPC, etc.

Downside to Microkernel

• Potential for performance loss
  • Multiple syscalls/IPC calls

Why Performance Loss?

border crossing lead to a change in locality

Features of various OSes

Feature	Monolithic	DOS-like	Microkernel
Extensibility	false	true	true
Protection	true	false	true
Performance	true	true	false