What is an Operating System?

• A program that controls the execution of application programs

• An interface between applications and hardware• Manages computer resources.• Masks the details of the hardware• Operating system goals:

– Execute user programs and make solving user problems easier.

– Make the computer system convenient to use. (Primary goal)

– Use the computer hardware in an efficient manner.(Secondary goal)

Operating System Objectives

• Convenience– Makes the computer more convenient to use

• Efficiency– Allows computer system resources to be used

in an efficient manner

• Ability to evolve– Permit effective development, testing, and

introduction of new system functions without interfering with service

Computer System Structure

• Computer system can be divided into four components– Hardware – provides basic computing resources

• CPU, memory, I/O devices

– Operating system• Controls and coordinates use of hardware among various

applications and users

– Application programs – define the ways in which the system resources are used to solve the computing problems of the users

• Word processors, compilers, web browsers, database systems, video games

– Users• People, machines, other computers

Four Components of a Computer System

Functions of an Operating System / Services

provided by an Operating System • One set of operating-system services provides

functions that are helpful to the user:– User interface - Almost all operating systems have a user

interface (UI)• Varies between Command-Line (CLI), Graphics User Interface

(GUI), Batch– Program execution - The system must be able to load a program

into memory and to run that program, end execution, either normally or abnormally (indicating error)

– I/O operations - A running program may require I/O, which may involve a file or an I/O device.

– File-system manipulation - The file system is of particular interest. Obviously, programs need to read and write files and directories, create and delete them, search them, list file Information, permission management.

Functions of an Operating System / Services provided by an Operating System

– Communications – Processes may exchange information, on the same computer or between computers over a network

• Communications may be via shared memory or through message passing (packets moved by the OS)

– Error detection – OS needs to be constantly aware of possible errors

• May occur in the CPU and memory hardware, in I/O devices, in user program

• For each type of error, OS should take the appropriate action to ensure correct and consistent computing

• Debugging facilities can greatly enhance the user’s and programmer’s abilities to efficiently use the system

Functions of an Operating System / Services provided by an Operating System

• Another set of OS functions exists for ensuring the efficient operation of the system itself via resource sharing– Resource allocation - When multiple users or multiple

jobs running concurrently, resources must be allocated to each of them

• Many types of resources - Some (such as CPU cycles,mainmemory, and file storage) may have special allocation code, others (such as I/O devices) may have general request and release code.

– Accounting - To keep track of which users use how much and what kinds of computer resources A good operating system keeps usage statistics for various resources and monitor performance parameters such as response time. For example in windows it gives you information about performance, disk usage or job queues for printing.

Functions of an Operating System / Services provided by an Operating System

– Protection and security - The owners of information stored in a multiuser or networked computer system may want to control use of that information, concurrent processes should not interfere with each other

• Protection involves ensuring that all access to system resources is controlled

• Security of the system from outsiders requires user authentication, extends to defending external I/O devices from invalid access attempts

• Protection – any mechanism for controlling access of processes or users to resources defined by the OS

• Security – defense of the system against internal and external attacks– including denial-of-service, worms, viruses, identity theft, theft of service

• Systems generally first distinguish among users, to determine who can do what– User identities (user IDs, security IDs) include name and associated number,

one per user– User ID then associated with all files, processes of that user to determine access

control– Group identifier (group ID) allows set of users to be defined and controls

managed, then also associated with each process, file

Operating system • Operating system can be explored from two

viewpoints: – User View– System View

• The user view of the computer system varies by the interface being used. – System designed for one user to monopolize its resources,

to maximize the work that the user is performing. Operating system is designed for ease of use.

– Users which access the same computer through other terminals share resources and exchange information. The operating system is designed to maximize resources utilization- to assure that all available CPU time, memory, and I/O are used efficiently, and that no user takes more than her fair share.

System View• From the computer’s point of view, the operating system is the

program that is most intimate with the hardware. The OS can be viewed as:

• OS is a resource allocator– Manages all resources– Decides between conflicting requests for efficient and fair resource use– Must be fair; not partial to any process, specially for process in the

same class– Must discriminate between different class of jobs with different service

requirements– Do the above efficiently

• Within the constraints of fairness and efficiency, an OS should attempt to maximize throughput, minimize response time, and accommodate as many users as possible

• OS is a control program– Controls execution of programs to prevent errors and improper use of

the computer

Classes of OSOS Class Periods Prime Concern

Batch Processing

60’s CPU idle time

Multiprogramming

70’s Resource utilization

Time sharing 70’s Good response time

Real time 80’s Meet the deadlines

Distributed 90’s Resource sharing

Batch Operating System

• Batch jobs together with similar needs and run them through the computer as group.

• Batch processing generally requires the program, data and appropriate system commands to be submitted together in the form of job

• Batch operating systems usually allow little or no interaction between the users and the executing programs.

Batch Operating System

• Scheduling in batch system is very simple. Jobs are typically processed in the order of submission that is in first-come, first - served.

• Memory management in batch systems is also very simple. Memory is usually divided into two areas.

• In this execution environment, the CPU is often idle, because the speeds of the mechanical I/O devices are intrinsically slower than are those of the electronic devices.

Multiprogramming OS

• When one job needs to wait for I/O, the processor can switch to the other job

Multiprogramming OS

• Multiprogramming needed for efficiency– Single user cannot keep CPU and I/O devices busy at

all times– Multiprogramming organizes jobs (code and data) so

CPU always has one to execute– A subset of total jobs in system is kept in memory– If several jobs ready to run at the same time CPU

scheduling– When it has to wait (for I/O for example), OS switches

to another job– Process Scheduling, disk storage, and memory

management

Time Sharing OS• Timesharing (multitasking) is logical extension in

which CPU switches jobs so frequently that users can interact with each job while it is running, creating interactive computing

• Processor’s time is shared among multiple users– Response time should be < 1 second– Each user has at least one program executing in memory

process– If processes don’t fit in memory, swapping moves them

in and out to run– Virtual memory allows execution of processes not

completely in memory

Time Sharing OS

• In time-sharing systems, several jobs must be kept simultaneously in memory (job-scheduling technique – Round Robin method), the system must have memory management and protection. To obtain reasonable response time virtual memory concept is used.

• To ensure orderly execution, the system must provide mechanisms for job synchronization and communication, and it may ensure that the job do not get stuck in a deadlock.

Real Time systems

• Real – time computing may be defined as that type of computing in which the correctness of the system depends not only on the logical result of the computation but also on the time at which results are produced. – A hard real – time – A soft real – time

Real Time systems• A hard real-time task must meet its deadline;

otherwise it will cause undesirable damage or fatal error to the system.

• Secondary storage is usually limited or missing, data stored in ROM

• Virtual memory is absent (Time Sharing)• Also called as special purpose OS• A soft real – time task has an associated deadline,

which is desirable but not mandatory; it still makes sense to schedule and complete the task even if it has passed its deadline. (E.g. Digital audio or multimedia system)

Multiprocessor Systems • Multiprocessor systems (also known as parallel

systems or tightly coupled systems) have more than one processor in close communication, sharing the computer bus, memory and peripheral devices

– Increased throughput: By increasing the number of processors, more work is done in less time

– Economy of scale: Multiprocessor systems can save more money than multiple single – processor systems, because they can share peripherals, mass storage, and power supplies.

– Increased – reliability: • graceful degradation (Fault tolerant)

Multiprocessor Systems• Symmetric multiprocessing (SMP) in which each processor

run on an identical copy of the operating system, and these communicate with one another as needed. SMP means that all processor are peers; no master – slave relationship exists between processors.

• Asymmetric multiprocessing, in which each processor is assigned a specific task. A master processor controls the system; the other processor either look to the master for instruction or have predefined tasks. This defines a master-slave relationship. The master processor schedules and allocates work to the slave processors. The disadvantages of this approach are:

a) A failure of the master brings down the whole system. b) The master can become a performance bottleneck.

Distributed System

• A distributed computer system is a collection of autonomous computer systems capable of communication and cooperation via their hardware and software interconnections – Client-Server Computing– Peer-to-Peer System

Distributed System

• In client/server environment the client machines are generally single- user PCs or workstations that provide a highly user friendly interface to the end user. Server provides the set of shared user services to the clients.

• Compute-server provides an interface to client to request services (i.e. database)

• File-server provides interface for clients to store and retrieve files

Distributed System

– P2P does not distinguish clients and servers• Instead all nodes are considered peers• May each act as client, server or both• Node must join P2P network

• These systems are usually referred to as loosely coupled systems.