Chapter 1 Introduction Copyright © 2008

Operating Systems, by Dhananjay Dhamdhere Copyright © Introduction Abstract Views of an Operating System Goals of an OS Operation of an OS Preview of the Book

Operating Systems, by Dhananjay Dhamdhere Copyright © Abstract Views of an Operating System What is an OS? –Abstract views To a college student: SW that permits access to the Internet To a programmer: SW that makes it possible to develop programs on a computer system To a user of an application package: SW that makes it possible to use the package To a technician in a computerized chemical plant: invisible component of a computer system that controls the plant –An abstract view focuses only on essential characteristics

Operating Systems, by Dhananjay Dhamdhere Copyright © Abstract Views of an Operating System (continued) Operating system designer also has an abstract view OS is a collection of routines that facilitates execution of user programs and use of resources

Operating Systems, by Dhananjay Dhamdhere Copyright © Abstract views We use abstract views to present design of OS components. It has two benefits: –Managing complexity Abstract view contains only selected features of a system –Presenting a generic view For example, user interface of Figure 1.2 has many variants in practice –Command line interface –Graphical user interface (GUI)

Operating Systems, by Dhananjay Dhamdhere Copyright © Goals of an OS Fundamental goals of an operating system –Efficient use of computer resources –User convenience –Noninterference in the activities of its users When these goals conflict, designer makes a trade-off for –Efficient use –User convenience Notion of effective utilization –Each OS provides a different flavor of effectiveness

Operating Systems, by Dhananjay Dhamdhere Copyright © Efficient Use OS ensures efficient use of memory, CPU, and I/O devices –Poor efficiency can result if a program does not use a resource allocated to it OS itself consumes CPU and memory resources, which constitutes overhead –It reduces resources for user programs OS can monitor use of resources to ensure efficiency –It would increase the overhead OS uses policies that ensure efficiency

Operating Systems, by Dhananjay Dhamdhere Copyright © User Convenience

Operating Systems, by Dhananjay Dhamdhere Copyright © Noninterference User can face interference in computational activities –Program execution or operation of OS can be disrupted by actions of other persons OS prevents interference by allocating resources for exclusive use of programs and OS services, and preventing illegal accesses to resources –Illegal file access OS knows which user files can be accessed by whom –Achieved through authorization

Operating Systems, by Dhananjay Dhamdhere Copyright © Operation of an OS Principal functions of OS: –Program management –Resource management –Security and protection

Operating Systems, by Dhananjay Dhamdhere Copyright © Operation of an OS (continued)

Operating Systems, by Dhananjay Dhamdhere Copyright © Program Management OS can interleave execution of programs on a fast CPU –Scheduling decides which program should be given the CPU at any time Policy influences efficient CPU use and user service Preemption: Taking away the CPU from a program

Operating Systems, by Dhananjay Dhamdhere Copyright © Resource Management Resource allocations and deallocations can be done with a resource table –Entry: name, address and status of a resource unit –Constructed by the boot procedure, maintained during operation

Operating Systems, by Dhananjay Dhamdhere Copyright © Resource Management (continued) Popular resource allocation strategies: –Resource partitioning OS decides a priori what resources to allocate to each user program; divides system resources into partitions –A resource partition is a collection of resources Resource table contains entries for partitions Simple to implement, but lacks flexibility –Pool-based OS allocates resources from a pool of resources –Consults table and allocates the resource if it is free Less overhead of allocating and deallocating resources Achieves more efficient use of resources

Operating Systems, by Dhananjay Dhamdhere Copyright © Resource Management (continued) A virtual resource is a fictitious resource –Abstract view of a resource taken by a program –Supported by OS through use of a real resource –Same real resource may support several virtual ones –Started with the use of virtual devices E.g., a print server Provides effect of having more resources Most OSs provide virtual memory –May execute a program bigger than size of RAM Some OSs create virtual machines –Each virtual machine can be allocated to a user

Operating Systems, by Dhananjay Dhamdhere Copyright © Security and Protection Security counters threats of interference or illegal use posed by persons/programs outside OS control –Authentication: only registered user can use a computer system Protection counters threats posed by users of an OS –Memory protection is a HW feature used by OS to thwart disruption of programs and OS services –Authorization thwarts interference with files

Operating Systems, by Dhananjay Dhamdhere Copyright © Security and Protection (continued)

Operating Systems, by Dhananjay Dhamdhere Copyright © Security and Protection (continued) Intruders are outsiders who can cause interference –May use or create malicious programs Trojan horses Viruses Worms Methods of addressing security threats –Authentication techniques –Plugging security holes –Internet firewalls

Operating Systems, by Dhananjay Dhamdhere Copyright © Preview of the Book The book discusses: –Part 1: Introduction to Operating Systems –Part 2: Managing User Computations –Part 3: Management of Memory –Part 4: Management of Files and I/O Devices –Part 5: Distributed Operating Systems Parts 1-4 discuss conventional computing environments –Single computer system having a single CPU

Operating Systems, by Dhananjay Dhamdhere Copyright © Introduction to Operating Systems Part 1 (Chapters 1–4) –How OS interacts with the computer and with user programs through events and interrupts (Chapter 2) See Figure 1.5 –Effective Utilization of a Computer System (Chapter 3) Techniques used to ensure effective utilization –Portability and Extensibility of Operating Systems (Chapter 4) Structuring of an OS achieve these goals

Operating Systems, by Dhananjay Dhamdhere Copyright © Introduction to Operating Systems (continued)

Operating Systems, by Dhananjay Dhamdhere Copyright © Managing User Computations Part 2 (Chapters 5–10) covers program management –Processes and Threads (Chapter 5) How programs are executed –Process Synchronization (Chapter 6) See Figure 1.6 –Scheduling (Chapter 7) How to achieve effective utilization –Deadlocks (Chapter 8) See Figure 1.7 –Message Passing (Chapter 9) –Multiprocessor OSs (Chapter 10) Exploiting multiple CPUs for effectiveness

Operating Systems, by Dhananjay Dhamdhere Copyright © Managing User Computations (continued) Processes Credit and Debit must access the balance in an account without interference Process Generate produces some data; Analyze uses it

Operating Systems, by Dhananjay Dhamdhere Copyright © Managing User Computations (continued) A process has to wait if a required resource is not available Deadlocks arise when processes wait for each others resources

Operating Systems, by Dhananjay Dhamdhere Copyright © Management of Memory Part 3 (Chapters 11-12) covers memory management –Memory reuse techniques and noncontiguous memory allocation (Chapter 11) and virtual memory (Chapter 12)

Operating Systems, by Dhananjay Dhamdhere Copyright © Management of Files and I/O Devices Part 4 (Chapters 13-15) discusses management of files and I/O devices

Operating Systems, by Dhananjay Dhamdhere Copyright © Management of Files and I/O Devices (continued) Part 4 (Chapters 13-15) (Contd.) –File Systems (Chapter 13) Allows users to create, use, and share files –Implementation of File Operations (Chapter 14) IOCS implements file operations by transferring data between memory and I/O devices –Security and Protection (Chapter 15) Techniques of thwarting security and protection threats

Operating Systems, by Dhananjay Dhamdhere Copyright © Distributed Operating Systems Part 5 (Chapters 16-21) discusses distributed OSs: –Distributed Operating Systems (Chapter 16) Model of a distributed computer system Networking HW and SW Distributed computation paradigms –Theoretical Issues in Distributed Systems (Chapter 17) Theoretical issues that arise from networking delays and how to tackle them –Distributed Control Algorithms (Chapter 18) Algorithms for performing OS functions in a distributed manner

Operating Systems, by Dhananjay Dhamdhere Copyright © Distributed Operating Systems (continued) Part 5 (Chapters 16-21) (continued): –Recovery and Fault Tolerance (Chapter 19) Issues in recovery and fault tolerance in distributed systems –Distributed File Systems (Chapter 20) Reliability and performance improvement techniques used in distributed file systems –Distributed System Security (Chapter 21) Techniques used to address security issues in distributed systems

Operating Systems, by Dhananjay Dhamdhere Copyright © Summary OS services programs simultaneously by allocating some of the resources to each program and interleaving their execution on the CPU –Requirements to ensure effectiveness of computing: Efficient use User convenience Non-interference –Primary functions Management of programs Management of resources Security and protection