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Subject Name:Operating system Subject Code:10EC35 Prepared By:Remya Ramesan and Kala H.S. Department:ECE Date:24-02-2015
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UNIT 1
INTRODUCTION AND OVERVIEW OF
OPERATING SYSTEM
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Operating system, Goals of an O.S
Operation of an O.S
Resource allocation and related functions
User interface related functions
Classes of operating systems
O.S and the computer system
Batch processing system
Multi programming systems
Time sharing systems
Real time operating systems, distributed operating
systems
CONTENTS
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What is an operating system ?
• An operating system (OS) is a collection of programs that achieve effective utilization of a computer system by providing – Convenient methods of using a computer
• Saves users’ time and boosts their productivity
– Efficient use of the computer
• An OS has several kinds of users – The OS meets diverse requirements of different kinds of
users
– Each user has a different view of what an OS is, and what it does. Each of these views is called an abstract view
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Abstract views
• An abstract view focuses on the essential characteristics of a system – It contains some selected characteristics, and ignores
others • It helps to control complexity in designing and understanding a
system, by hiding unnecessary detail
– For example, two abstract views of a car: • The driver’s view contains the rear-view mirror, steering, gear
shift, and speedometer but ignores engine details
• The mechanic’s view contains fluid levels, electrical system, …
Q: Give examples of abstract views
A: View of an abstract data type, object, ..
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Abstract views
• Abstract views are used in
– Software requirements specification
• Specify what functions a system should perform – The abstract view of a specific function helps in understanding
the system data and fundamental operations on them
– Software design and implementation
• ‘Hiding’ the details of functions and subsystems while designing or implementing other functions and subsystems helps to control complexity
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A designer’s abstract view of an OS
• User interface: Accepts commands from users: GUI, command line interface • Non-kernel programs: Implement user commands • Kernel: Core of the OS─controls the computer, provides functions & services
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A designer’s abstract view of an OS
• The abstract view consists of three components – The kernel programs
• interact with the computer’s hardware and implement the intended operation
– The non-kernel programs • implement creation of programs and use of system
resources by them. These programs use kernel programs to control operation of the computer
– The user interface • interprets the commands of a user and activates non-kernel
programs to implement them
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Goals of an OS
• Two primary goals of an OS are – Efficient use of the computer’s resources
• To ensure cost-effectiveness of the computer
– User convenience • A user should find it easy to use the computer
• These two goals sometimes conflict – Prompt service can be provided through exclusive use
of a computer; however, efficient use requires sharing of a computer’s resources among many users
– An OS designer decides which of the two goals is more important under what conditions
• That is why we have so many operating systems!
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User convenience
• The computing environment influences the notion of user convenience – The computing environment is comprised of
• The computer system • Its interfaces with other systems • Nature of computations performed by its users
– Computing environments change with the architecture and use of a computer, e.g. personal computing, online applications, embedded applications
– Hence the notion of user convenience has several facets that depend on the computing environment
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User convenience
• User convenience has several facets – Fulfillment of a necessity
• Use of programs and files
– Good service • Speedy response
– Ease of Use • User friendliness
– New programming model • e.g., Concurrent programming
– Web-oriented features • e.g., Web-enabled servers
– Evolution • Addition of new features, use of new computers
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Operation of an OS
• During operation, an OS performs two primary functions – Program management
• Perform program initiation/termination
• Provide means through which many programs can work toward a common goal
– Helps in more convenient or faster fulfillment of user requirements
– Resource management • Ensure efficient use of resources
– Use of CPU, I/O devices
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Operation of an OS
• Program management and resource management require three fundamental tasks – Scheduling
• Deciding which program should be serviced by the CPU and for how long
– Resource allocation • Sharing of resources among user programs
– Security and protection • Ensuring non-interference between programs
– Security: guarding against interference by external entities
– Protection: guarding against interference by internal entities
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Operation of an OS
• To facilitate scheduling, resource management, and security and protection – OS maintains various kinds of information to facilitate
operation • Names of registered users • identity of a user who is executing a specific program • status of a program • status of a resource • which users can access a resource
– OS performs several tasks repeatedly to implement scheduling, resource management, and security and protection
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Common tasks performed by operating systems
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Program management
• A computational structure is a configuration of several programs. Three typical computational structures are: – A single program
• Execution of a program on a given set of data
– A sequence of single programs • A program should be executed only if previous programs in the
sequence executed successfully
• Programs should pass their results to other programs through files
– Co-executing programs • The OS must execute these programs at the same time
• The OS must provide an interface between co-executing programs so that their results are available to one another
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A schematic of scheduling
• Programs waiting for CPU attention are entered in a pool • The scheduler picks one program from the pool for execution on the CPU • The CPU may be forcibly taken away from a program. This action is called pre-emption • A pre-empted program is again entered into the pool
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Resource management
• Criteria and techniques of resource management – Criterion: Resource utilization efficiency
• Share the resources wherever possible • Avoid idling of resources • Speedy resource allocation and de-allocation is desirable
– Technique 1: Partitioning of resources • A resource partition is a collection of resources
– A resource partition is allocated to a process – Simplifies resources allocation and also speeds it up
– Technique 2: Allocation from a pool • Resources are allocated individually when needed
Q: Pros & cons of the two techniques?
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Virtual resources
• A virtual resource is an illusion presented to a program – A virtual resource is an abstraction; its
implementation is hidden from a program – Use of virtual resources simplifies resource allocation
• Virtual resources may be created if several programs need a resource. It permits more programs to execute at the same time
– A virtual I/O device is a virtual resource • The OS implements a virtual device as follows:
– When a program performs an operation on a virtual device, the operation may be actually performed on some real resource
• Example: virtual printer – When a program ‘prints’ on a virtual printer, the data is actually
stored in memory; printing takes place sometime later
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Security and Protection
• An OS must ensure non-interference with users’ programs and data – Aspects of non-interference
• Resources must not be used by un-authorized persons • There should be no interference with use of resources by
authorized persons and their programs
– Terminology • An intruder is a person or program that causes interference • Security measures avoid interference by non-users and their
programs • Protection measures avoid interference by users and their
programs
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Security and protection techniques
• Two key techniques of security and protection – Authentication
• Verification of a user’s identity
• Performed when a person logs in (typically through passwords)
– Authorization: has two aspects • The owner of a resource selectively permits other users
to access them
• When a user/program tries to access a resource, the OS verifies whether the user has the permission to perform the access
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Overview of security and protection threats
• Authentication checks whether a person is a registered user • Threats posed by non-users are security threats • Threats posed by a user are protection threats
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OS and the Computer System
• In this module, we study
- Fundamental features of computer systems that are
important to an OS
* Memory hierarchy
* Interrupt structure
* I/O organization
- Fundamentals of how a program interacts with an OS
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Memory utilization during operation of an OS
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Model of a Computer System
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Fields of Program Status Word (PSW)
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(a) Program (b) State of CPU after executing COMPARE instruction
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Memory hierarchy containing cache, main memory and disk
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Memory hierarchy
• Cache memory * Organization – Cache block or cache line -- Inclusive or exclusive -- Direct, fully associative, set associative * Different levels of caches – Why? How many levels? * Cache hit ratio – What factors influence it? • Main memory * Memory protection * Virtual memory?
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Memory protection using bound registers
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A schematic of virtual memory operation
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Input/Output organization
• Involvement of the CPU in I/O operations -- Should be the minimum possible due to imbalance between CPU and I/O speeds -- CPU should be free to execute instructions while I/O operations are in progress • Different I/O modes -- Programmed I/O -- Interrupt I/O -- Direct memory access (DMA)
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Input Output Modes
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Interrupts
• An interrupt signals the occurrence of an event to the CPU
• The CPU is diverted to execution of an OS routine
• Different classes of interrupts convey occurrence of different kinds of events
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Classes of Interrupts
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The interrupt action
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Steps in interrupt action
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An operating system in its computing environment
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Interrupt driven operation of a kernel
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System Call
• A system call is a request made by a program through a
special instruction called a `software interrupt’ instruction
• The software interrupt is a program interrupt • When a software interrupt occurs, the interrupt
hardware transfers control to a routine of the OS • The operand of the software interrupt instruction
indicates what kind of request is being made by a program
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System Calls
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Interrupt processing and scheduling
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Simple and nested interrupt processing
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Classes of Operating Systems
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Efficiency, System Performance, and User Service
• Two of the fundamental goals of an OS: – Efficiency of use
• Of a resource
– User convenience • Measurable aspect: User service
– Turnaround time
– Response time
• To a system administrator, performance of a system in its environment is more important – Typically measured as throughput
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Efficiency, System Performance, and User Service (continued)
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Batch Processing Systems
• Batch: sequence of user jobs formed for processing by the OS
• Batching kernel initiates processing of jobs without requiring computer operator’s intervention
• Card readers and printers were a performance bottleneck in the 1960s – Virtual card readers and printers implemented through
magnetic tapes were used to solve this problem
• Control statements used to protect against interference between jobs
• Command interpreter read a card when currently executing program in job wanted the next card
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Multiprogramming Systems
• Provide efficient resource utilization in a noninteractive environment
• Uses DMA mode of I/O – Can perform I/O operations of some program(s)
while using the CPU to execute some other program
• Makes efficient use of both the CPU and I/O devices
• Turnaround time of a program is the appropriate measure of user service in these systems
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Multiprogramming Systems (continued)
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Multiprogramming Systems (continued)
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Multiprogramming Systems (continued)
• An appropriate measure of performance of a multiprogramming OS is throughput – Ratio of the number of programs processed and the
total time taken to process them
• OS keeps enough programs in memory at all times, so that CPU and I/O devices are not idle – Degree of multiprogramming: number of programs – Uses an appropriate program mix of CPU-bound
programs and I/O-bound programs – Assigns appropriate priorities to CPU-bound and I/O-
bound programs
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Priority of Programs
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Priority of Programs (continued) In multiprogramming environments, an I/O-bound program should have a
higher priority than a CPU-bound program.
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Performance of Multiprogramming systems
• How to improve performance?
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Performance of Multiprogramming systems
(continued)
When an appropriate program mix is maintained, an increase in the
degree of multiprogramming would result in an increase in throughput.
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Time-Sharing Systems
• Provide a quick response to user subrequests – Round-robin scheduling with time-slicing
• Kernel maintains a scheduling queue
• If time slice (δ) elapses before process completes servicing of a subrequest, kernel preempts it, moves it to end of queue, and schedules another process
– Implemented through a timer interrupt
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Time-Sharing Systems (continued)
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Time-Sharing Systems (continued)
• Response time (rt): measure of user service – If processing of a subrequest requires δ CPU seconds
rt = n × (δ + σ) η = δ / (δ + σ)
where η: CPU efficiency, σ: scheduling overhead, n: number of users using system, δ: time required to complete a subrequest
• Actual response time would be different because – Some users may be inactive – Some programs may require > δ CPU seconds
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Time-Sharing Systems (continued)
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Swapping of Programs
• Kernel performs swap-out and swap-in operations
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Real-Time Operating Systems
• In real-time applications, users need computer to perform some actions in a timely manner – To control activities in an external system, or to
participate in them
– Timeliness depends on time constraints
• If application takes too long to respond to an activity, a failure can occur in the external system – Response requirement
– Deadline: time by which action should be performed
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Hard and Soft Real-Time Systems
• A hard real-time system meets response requirements under all conditions – It is typically dedicated to processing real-time
applications
• A soft real-time system makes best effort to meet response requirement of a real-time application – Cannot guarantee that it will be able to meet it
• Meets requirements in a probabilistic manner
– E.g., multimedia applications
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Features of a Real-Time Operating System
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Distributed Operating Systems
• A distributed computer system consists of several individual computer systems connected through a network – Each computer system could be a PC, a
multiprocessor system, or a cluster – Many resources of a kind exist in system
• This feature is used to provide the benefits summarized in Table 3.8
– Handling network or individual computers’ failure requires special techniques
– Users must use special techniques to access resources over the network
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Distributed Operating Systems (continued)
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Special Techniques of Distributed Operating Systems
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Modern Operating Systems
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Summary
• A computing environment consists of a computer system, its interfaces with other systems, and the services provided by its OS to users and programs – Evolved with advances in computer technology:
• Batch processing systems
• Multiprogramming operating system – Priority-based scheduling
• Time-sharing operating systems – Round-robin scheduling with time-slicing
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Summary (continued)
• Evolution (continued) • Real-time operating systems
– Priority-based scheduling and deadline-aware scheduling
• Distributed operating system – Lets programs share resources across network
• Modern operating system – Modern computing environment has elements of all the
classic computing environments
– Uses different techniques for different applications
