Operating Systems (A) (Honor Track) Lecture 1: What is An OS Tao Wang School of Electronics Engineering and Computer Science http://ceca.pku.edu.cn/wangtao Fall 2012 Acknowledgements: Prof. Xiangqun Chen at PKU and Prof. Yuanyuan Zhou at UCSD 1 A Typical Computer from a Hardware Point of View CPU Memory ... CPU Chipset Network 2 Why do we need an OS? A modern computer consists of: • • • • • One or more processors Main memory Disks Printers Various input/output devices Managing all these components requires a layer of software – the operating system 3 What Is an OS? “Code” that:  Sits between programs & hardware  Sits between different programs  Sits betweens different users But what does it do? to provide an orderly and controlled allocation of the processors, memories and I/O devices among the various programs competing for them Real life analogy:  Government? 4 OS is…  The operating system is the software layer between user applications and the hardware  The OS is “all the code that you didn’t have to write” to implement your application ? 5 And as an Extended Machine Figure 1-2. Operating systems turn ugly hardware into beautiful abstractions. Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved. 0-13-6006639 6 OS and Hardware  The OS abstracts/controls/mediates access to hardware resources (what resources?)  Computation (CPUs)  Volatile storage (memory) and persistent storage (disk, etc.)  Communication (network, modem, etc.)  Input/output devices (keyboard, display, printer, camera, etc.)  The OS defines a set of logical resources (objects) and a set of well-defined operations on those objects (interfaces)  Physical resources (CPU and memory)  Logical resources (files, programs, names) 7 Benefits to Applications  Simpler  no tweaking device registers  Device independent  all network cards look the same  Portable  Across Windows95/98/ME/NT/2000/XP/Vista/Windows 7  Worry less about interference from other applications 8 What does an OS do? Resources Services  Allocation  Abstraction  Protection  Simplification  Reclamation  Convenience  Virtualization  Standardization Makes computers simpler 9 What Is an OS? Government Resources Finite resources  Allocation Competing demands Land,  Protection  Reclamation  Virtualization Limited budget, Examples: Oil,  CPU Gas,  Memory …  Disk  Network 10 What Is an OS? Government Resources  Allocation You can’t hurt me I can’t hurt you Law and order  Protection  Reclamation  Virtualization Implies some degree of safety & security 11 What Is an OS? Government Resources  Allocation The OS giveth The OS taketh away Income Tax  Protection  Reclamation  Virtualization Voluntary at run time Implied at termination Involuntary Cooperative 12 What Is an OS? Government Resources  Allocation Illusion of infinite, private resources Social security  Protection  Reclamation Memory versus disk Timeshared CPU  Virtualization More extreme cases possible (& exist) 13 Some Questions to Ponder  What is part of an OS? What is not? Is the window system part of an OS? Java?  Popular OSes today are Windows, Linux, and OS X  How different/similar do you think these OSes are?  Somewhat surprisingly, OSes change all of the time  Consider the series of releases of NT, Linux, OS X…  What are the drivers of OS change? • New hardware, new applications  What are the most compelling issues facing OSes today? 14 Summary  Why do we need an OS?  What  Next does an OS do? lecture: OS History 15