Lecture Operating systems: A concept-based approach (2/e): Chapter 14 - Dhananjay M. Dhamdhere

Chapter 14 - Structure of operating systems. This chapter discusses three methods of structuring an operating system. The layered structure of operating systems simplifies coding, the kernel-based structure provides ease of implementation on different computer systems, and the microkernel-based structure permits modification of an operating system’s features to adapt to changes in the computing environment and also provides ease of implementation on different computer systems.

PROPRIETARY MATERIAL. ©  2007 The McGraw­Hill Companies, Inc. All rights reserved. No part of this PowerPoint slide  may be displayed, reproduced or distributed 

– Protection and Security

• OS should be structured so that it can cope with

changes in computer systems and computing

environments

Operation of an OS

• Three key aspects of OS operation are

– Booting

* Determines configuration of the system, viz CPU, I/O devices, etc.

* Loads the kernel, initializes its data structures and hands over control

* A guiding principle that should be used to manage a class of entities

* For example, round-robin scheduling

– Mechanism

* A specific operation needed to implement a policy

* For example, dispatching

Portability and extensibility of OS

• For cost-effectiveness, OS lifetime should be >10 years

– Several changes take place during this time

* Computer architecture, I/O device technology, computing environments change

– The OS should be able to cope with these changes

 It should be possible to adapt to changes

 For example, modify OS functionalities or add new ones

Operating Systems Dhamdhere: Operating Systems—A Concept­Based Approach, 2 ed Copyright © 2008Slide No: 8

Problems with monolithic OS structure

• OS code did not have modules with well-defined

interfaces

– Functionalities knew each other’s data and could access directly – The OS layer had both architecture dependent and architecture independent code

* Hence portability was poor

Alternative OS structures

• Problems with monolithis structure led to developments

of three methods of structuring

Operating Systems Dhamdhere: Operating Systems—A Concept­Based Approach, 2 ed Copyright © 2008Slide No: 10

Layered design of operating systems

• An OS is structured into a number of layers

– Each layer provides a set of services

– A higher layer uses the services of the immediate lower layer

* Helps in controlling complexity and cost of developing and maintaining an OS by reducing the semantic gap

Layers of THE multiprogramming system

•  Layers 0 and 1 perform processor and memory management, respectively

•  Layer 3 hides intricacies of device­level I/O

– It creates several virtual machines

* A virtual machine is like a virtual resource

* It has the same architecture as the real computer, i.e the host machine, but does not contain some of its features

 For example, it may not support some instructions of the host computer

– VM OS interleaves operation of several virtual machines

* Instructions of the selected virtual machine are executed on the host machine, which ensures efficiency of its operation

– Each virtual machine has its own operating system, called a

guest OS

– This way, several operating systems can operate on the host

computer concurrently

VM OS

•  VM/370 is an early widely known VM OS; it worked on IBM/370

* Application processes uses the capabilities of a virtual machine

– Virtualization is the process by which capabilities of host

machine are made available in virtual machine (VM)

* Full virtualization: all capabilities of host are available in VM

 May lead to lack of security as VM OS cannot prevent an application process from using privileged instructions

* If full virtualization is not used, guest OS must invoke the VM OS to use some capabilities of the host machine

Mechanisms in a kernel

Operating Systems Dhamdhere: Operating Systems—A Concept­Based Approach, 2 ed Copyright © 2008Slide No: 20

Dynamically loadable kernel modules

• Kernel modules have well-specified interfaces

– Facilitates dynamic loading of kernel modules

* Conserves memory by loading only the required modules

* Provides extensibility as kernel modules can be modified separately

Architecture of Windows

•  The HAL provides abstractions of I/O interfaces, interrupt controllers and interprocess    communication processes in a multiprocessor system

•  HAL and the kernel are together equivalent of the conventional kernel