UNIT – 1 INTRODUCTION AND OVERVIEW OF OPERATING SYSTEMS ECEA

 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 ECEA

 Operating system designer also has an abstract view ECEA OS is a collection of routines that facilitates execution of user programs and use of resources

 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 has many variants in practice  Command line interface  Graphical user interface (GUI) ECEA

 Logical and physical organization ECEA

 Fundamental goals of an operating system 1)Efficient use of computer resources 2)User convenience 3)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 ECEA

 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 ECEA

 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 ECEA

 Principal functions of OS: ◦ Program management ◦ Resource management ◦ Security and protection ECEA ConcernOS responsibility Programs Initiation and termination of programs. Providing convenient methods so that several programs can work towards a common goal. Resources Ensuring availability of resources in the system and allocating them to programs. Scheduling Deciding when and for how long, to devote the CPU to a program. ProtectionProtect data and programs against interference from other users and their programs.

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 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 ECEA

 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 ECEA

 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 ECEA

 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 ECEA

 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 ECEA

 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 ECEA