OS UNIT-I Introdution to Operating systems.ppt

Operating Systems
(R1631055)
by
Mrs. T.Satya Kumari
Assistant Professor
Department of CSE
Aditya College of Engineering
Approved by AICTE, Accredited by NAAC
and Permanently affiliated to JNTUK, Kakinada
1
Department of CSE, Aditya College of Engineering, Surampalem

UNIT-I
Introduction to Operating System Concept
 Types of operating system
 Operating system concepts
 Operating system services
 Introduction to system call
 System call types
Department of CSE, Aditya College of Engineering, Surampalem 2

What is an Operating System?
 An operating system is a program that manages the
computer hardware.
 A program that acts as an intermediary between a
user of a computer and the computer hardware
 Operating system goals:
 Execute user programs and make solving user problems
easier
 Make the computer system convenient to use
 Use the computer hardware in an efficient manner

Operating System Definition
 OS is a resource allocator
 Manages all resources
 Decides between conflicting requests for efficient and fair
resource use
 OS is a control program
 Controls execution of programs to prevent errors and improper
use of the computer
 No universally accepted definition
 “The one program running at all times on the computer” is the
kernel. Everything else is either a system program (ships with the
operating system) or an application program
Department of CSE, Aditya College of Engineering, Surampalem
4

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

Computer System Organization
 Computer-system operation
 I/O Structure
 Storage structure

Computer System Organization
 Computer-system operation
 One or more CPUs, device controllers connect through common bus providing access to
shared memory
 Concurrent execution of CPUs and device controllers competing for memory cycles

Computer Startup
 bootstrap program is loaded at power-up or reboot
 Typically stored in ROM or EEPROM, generally known as
firmware
 Initializes all aspects of system
 Loads operating system kernel and starts execution
 The operating system then starts executing the first process,
such as "init," and waits for some event to occur.

Computer-system operation (cont.)
 occurrence of an event is signaled by an interrupt
from either the hardware or the software.
 Hardware may trigger an interrupt at any time by
sending a signal to the CPU, usually by way of the
system bus.
 Software may trigger an interrupt by executing a
special operation called a system call (also called a
monitor call).

Common Functions of Interrupts
 When the CPU is interrupted, it stops what it is doing
and immediately transfers execution to a fixed
location.
 The fixed location usually contains the starting
address where the service routine for the interrupt is
located.
 The interrupt service routine executes;
 on completion, the CPU resumes the interrupted
computation.

Common Functions of Interrupts
 Interrupt transfers control to the interrupt service routine
generally, through the interrupt vector, which contains the
addresses of all the service routines.
 Interrupt architecture must save the address of the interrupted
instruction.
 Incoming interrupts are disabled while another interrupt is being
processed to prevent a lost interrupt.
 A trap is a software-generated interrupt caused either by an error
or a user request.
 An operating system is interrupt driven.

Interrupt Handling
 The os preserves the state of the CPU by storing
registers and the program counter.
 After the interrupt is serviced, the saved return
address is loaded into the program counter
 And the interrupted computation resumes as
though the interrupt had not occurred.

Interrupt Timeline

I/O Structure
 A general-purpose computer system consists of CPUs and
multiple device controllers that are connected through a
common bus.
 Each device controller is in charge of a specific type of
device.
 Depending on the controller, there may be more than
one attached device.
For instance, seven or more devices can be attached
to the small computer-systems interface (SCSI) controller.

I/O Structure
 Device controller maintains some local buffer storage
and a set of special-purpose registers.
 Device controller is responsible for moving the data
between the peripheral devices that it controls and its
local buffer storage.
 Typically, operating systems have a device driver for
each device controller.
 Device driver understands the device controller and
presents a uniform interface to the device to the rest of
the operating system.

I/O Structure
 To start an I/O operation, device driver loads appropriate registers
within device controller.
 Device controller examines contents of the registers to determine
what action to take (such as "read a character from the keyboard").
 The controller starts transfer of data from device to its local buffer.
 Once transfer of data is complete, device controller informs the
device driver via an interrupt.
 Then device driver returns control to the operating system, possibly
returning the data or a pointer to the data if the operation was a
read.
 For other operations, the device driver returns status information.

Device-Status Table

Direct Memory Access Structure
 Used for high-speed I/O devices able to transmit information at
close to memory speeds
 Device controller transfers blocks of data from buffer storage
directly to main memory without CPU intervention
 Only one interrupt is generated per block, rather than the one
interrupt per byte

Storage Structure
 Main memory – only large storage media that the CPU can access directly
 Secondary storage – extension of main memory that provides large
nonvolatile storage capacity
 Magnetic disks – rigid metal or glass platters covered with magnetic
recording material
 Disk surface is logically divided into tracks, which are subdivided into sectors
 The disk controller determines the logical interaction between the device and
the computer

Storage Hierarchy
 Storage systems organized in hierarchy
 Speed
 Cost
 Volatility
 Caching – copying information into faster storage system; main memory
can be viewed as a last cache for secondary storage

Caching
 Important principle, performed at many levels in a computer
(in hardware, operating system, software)
 Information in use copied from slower to faster storage
temporarily
 Faster storage (cache) checked first to determine if
information is there
 If it is, information used directly from the cache (fast)
 If not, data copied to cache and used there
 Cache smaller than storage being cached
 Cache management important design problem
 Cache size and replacement policy

Performance of Various Levels of Storage
 Movement between levels of storage hierarchy can be explicit or
implicit

Operating System Structure
 Multi Programmed Systems
 Time Sharing Systems
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
 One job selected and run via job scheduling
 When it has to wait (for I/O for example), OS switches to
another job

Memory Layout for Multiprogrammed System

 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
 Response time should be < 1 second
 Each user has at least one program executing in memory
process
 If several jobs ready to run at the same time  CPU
scheduling
 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

 Timesharing (multitasking)

Operating-System Operations
 Interrupt driven by hardware
 Software error or request creates exception or trap
 Division by zero, request for operating system service
 Other process problems include infinite loop, processes
modifying each other or the operating system

Operating-System Operations
 Dual-mode operation allows OS to protect itself and other system
components
 User mode and kernel mode
 Mode bit provided by hardware
Provides ability to distinguish when system is running user
code or kernel code
Some instructions designated as privileged, only executable
in kernel mode
System call changes mode to kernel, return from call resets it
to user

Transition from User to Kernel Mode
 Timer to prevent infinite loop / process hogging resources
 Set interrupt after specific period
 Operating system decrements counter
 When counter zero generate an interrupt
 Set up before scheduling process to regain control or terminate program that
exceeds allotted time

Functions of operating system
 Process Management
 Memory Management
 Storage Management
 Protection and Security

Process Management
A process is a program in execution. It is a unit of work within the
system. Program is a passive entity, process is an active entity.
 Process needs resources to accomplish its task
 CPU, memory, I/O, files
 Initialization data
 Process termination requires reclaim of any reusable resources

Process Management
 Single-threaded process has one program counter specifying
location of next instruction to execute
 Process executes instructions sequentially, one at a time,
until completion
 Multi-threaded process has one program counter per thread
 Typically system has many processes, some user, some operating
system running concurrently on one or more CPUs
 Concurrency by multiplexing the CPUs among the processes /
threads

Process Management Activities
The operating system is responsible for the following
activities in connection with process management:
 Creating and deleting both user and system processes
 Suspending and resuming processes
 Providing mechanisms for process synchronization
 Providing mechanisms for process communication
 Providing mechanisms for deadlock handling

Memory Management
 All data in memory before and after processing
 All instructions in memory in order to execute
 Memory management determines what is in memory
 Optimizing CPU utilization and computer response to users
 Memory management activities
 Keeping track of which parts of memory are currently being
used and by whom
 Deciding which processes (or parts thereof) and data to
move into and out of memory
 Allocating and deallocating memory space as needed

Storage Management
 OS provides uniform, logical view of information storage
 Abstracts physical properties to logical storage unit - file
 Each medium is controlled by device (i.e., disk drive, tape drive)
Varying properties include access speed, capacity, data-
transfer rate, access method (sequential or random)
 File-System management
 Files usually organized into directories
 Access control on most systems to determine who can access
what
 OS activities include
 Creating and deleting files and directories
 Primitives to manipulate files and dirs
 Mapping files onto secondary storage
 Backup files onto stable (non-volatile) storage media

Mass-Storage Management
 Usually disks used to store data that does not fit in main memory or data
that must be kept for a “long” period of time
 Proper management is of central importance
 Entire speed of computer operation hinges on disk subsystem and its
algorithms
 OS activities
 Free-space management
 Storage allocation
 Disk scheduling
 Some storage need not be fast
 Tertiary storage includes optical storage, magnetic tape
 Still must be managed
 Varies between WORM (write-once, read-many-times) and RW (read-
write)

Performance of Various Levels of Storage
 Movement between levels of storage hierarchy can be explicit or implicit

Storage Management
 Caching
 I/O systems

Protection and Security
 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
Huge range, including denial-of-service,
worms, viruses, identity theft, theft of
service

Protection and Security
 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
Privilege escalation allows user to change to
effective ID with more rights

Types of Operating Systems
 Simple Batch Systems(Batch operating system)
 Time-Sharing Operating Systems(Multi-tasking)
 Distributed Systems
Network Operating Systems
 Special-Purpose Systems
Real-Time Embedded Systems
Multimedia Systems
Handheld Systems

Types of Operating Systems
 Simple Batch Systems(Batch operating system)
 In this type of system, there is no direct interaction
between user and the computer.
 The user has to submit a job (written on cards or tape)
to a computer operator.
 Then computer operator places a batch of several jobs
on an input device.
 Jobs are batched together by type of languages and
requirement.
 Then a special program, the monitor, manages the
execution of each program in the batch.
 The monitor is always in the main memory and
available for execution.

Simple Batch Systems(Batch operating system)
(cond.)
The disadvantages with Batch Systems are as
follows −
 Lack of interaction between the user and
the job.
 CPU is often idle, because the speed of the
mechanical I/O devices is slower than the
CPU.
 Difficult to provide the desired priority.

Time-Sharing Operating Systems(Multi-tasking)
 Each task is given some time to execute, so that all the tasks
work smoothly.
 Each user gets time of CPU as they use single system.
 These systems are also known as Multitasking Systems.
 The task can be from single user or from different users also.
 The time that each task gets to execute is called quantum.
 After this time interval is over OS switches over to next task.
Advantages of Time-Sharing OS:
 Each task gets an equal opportunity
 Less chances of duplication of software
 CPU idle time can be reduced

Distributed Systems
 A distributed system is a collection of physically
separate, possibly heterogeneous computer
systems that are networked to provide the users
with access to the various resources that the
system maintains.
 Access to a shared resource increases
computation speed, functionality, data
availability, and reliability.

Distributed Systems
Advantages of Distributed Operating System:
 Failure of one will not affect the other network
communication, as all systems are independent
from each other.
 Electronic mail increases the data exchange
speed
 Since resources are being shared, computation is
highly fast and durable.
 Load on host computer reduces
 These systems are easily scalable as many
systems can be easily added to the network.
Delay in data processing reduces.

Network operating system
 An operating system that provides features such as file
sharing across the network and includes a communication
scheme that allows different processes on different
computers to exchange messages.
 A computer running a network operating system acts
autonomously from all other computers on the network.
Advantages:
 Centralized servers are highly stable.
 Security is server managed.
 Upgrades to new technologies and hardware can be easily
integrated into the system.
 Remote access to servers is possible from different
locations and types of systems.

Special-Purpose Systems
 Real-Time Embedded Systems
Embedded computers are the most prevalent form of
computers in existence. These devices are found
everywhere, from car engines and manufacturing robots
to VCRs and microwave ovens. They tend to have very
specific tasks.
Embedded systems almost always run real-time
operating systems. A real time operating system time
interval to process and respond to inputs is very small.
Examples: Military Software Systems, Space Software
Systems.

Real-Time Embedded Systems
 A real-time system has well-defined, fixed time
constraints. Processing must be done within the
defined constraints, or the system will fail.
 For instance, it would not do for a robot arm to
be instructed to halt after it had smashed into
the car it was building.
 A real-time system functions correctly only if it
returns the correct result within its time
constraints.
 Two types
Hard real time and soft real time

Multimedia Systems
 Multimedia data consist of audio and video files
as well as conventional files. These data differ
from conventional data in that multimedia data—
such as frames of video—must be delivered
(streamed) according to certain time restrictions
 Multimedia describes a wide range of
applications that are in popular use today. These
include audio files such as MP3 DVD movies, video
conferencing, and short video clips of movie
previews or news stories downloaded over the
Internet.

Handheld systems
 Handheld systems include personal digital
assistants (PDAs), such as Palm and Pocket-PCs,
and cellular telephones, many of which use
special-purpose embedded operating systems.
 Many handheld devices have between 512
KB and 8 MB of memory. As a result, the
operating system and applications must manage
memory efficiently. This includes returning all
allocated memory back to the memory manager
once the memory is no longer being used.

Handheld systems
 Currently, many handheld devices do not use virtual
memory techniques, thus forcing program developers to
work within the confines of limited physical memory.
 Processors for most handheld devices often run at a fraction
of the speed of a processor in a PC. Faster processors
require more power. To include a faster processor in a
handheld device would require a larger battery that would
have to be replaced more frequently.
 The last issue confronting program designers for handheld
devices is the small display screens typically available. One
approach for displaying the content in web pages is web
clipping, where only a small subset of a web page is
delivered and displayed on the handheld device.

Operating System Services
 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.

 One set of operating-system services provides functions that are
helpful to the user (Cont):
 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

A View of Operating System Services

 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, main memory, 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

 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
If a system is to be protected and secure,
precautions must be instituted throughout it. A
chain is only as strong as its weakest link.

System Calls
 Programming interface to the services provided by the OS
 Typically written in a high-level language (C or C++)
 Mostly accessed by programs via a high-level Application Program
Interface (API) rather than direct system call use
 Three most common APIs are Win32 API for Windows, POSIX API for
POSIX-based systems (including virtually all versions of UNIX,
Linux, and Mac OS X), and Java API for the Java virtual machine
(JVM)
 Why use APIs rather than system calls?
(Note that the system-call names used throughout this text are
generic)

Example of System Calls
 System call sequence to copy the contents of one file to another file

Example of Standard API
 Consider the ReadFile() function in the
 Win32 API—a function for reading from a file
A description of the parameters passed to ReadFile()
 HANDLE file—the file to be read
 LPVOID buffer—a buffer where the data will be read into and written from
 DWORD bytesToRead—the number of bytes to be read into the buffer
 LPDWORD bytesRead—the number of bytes read during the last read
 LPOVERLAPPED ovl—indicates if overlapped I/O is being used

System Call Implementation
 Typically, a number associated with each system call
 System-call interface maintains a table indexed according to
these numbers
 The system call interface invokes intended system call in OS kernel
and returns status of the system call and any return values
 The caller need know nothing about how the system call is
implemented
 Just needs to obey API and understand what OS will do as a
result call
 Most details of OS interface hidden from programmer by API
Managed by run-time support library (set of functions built
into libraries included with compiler)

API – System Call – OS Relationship

Standard C Library Example
 C program invoking printf() library call, which calls write() system call

System Call Parameter Passing
 Often, more information is required than simply identity of desired
system call
 Exact type and amount of information vary according to OS and
call
 Three general methods used to pass parameters to the OS
 Simplest: pass the parameters in registers
 In some cases, may be more parameters than registers
 Parameters stored in a block, or table, in memory, and address
of block passed as a parameter in a register
This approach taken by Linux and Solaris
 Parameters placed, or pushed, onto the stack by the program
and popped off the stack by the operating system
 Block and stack methods do not limit the number or length of
parameters being passed

Parameter Passing via Table

Types of System Calls
 Process control
 File management
 Device management
 Information maintenance
 Communications
 Protection

Examples of Windows and Unix System Calls

OS UNIT-I Introdution to Operating systems.ppt

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