os module i

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MODULE I

Introduction: Operating System Batch, Multiprogrammed, Time-sharing and

Real timesystemsOperating system structureOperating system operations

System Structures: Operating system serviceSystem callsSystem Programs

SystemstructureSimple structure, Layered approachKernel, Shell.

Introduction: Operating System

What is an Operating System?

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.

Computer System Components

A computer system can be divided roughly into four components: the hardware,

the operating system, the application programs, and the users.

Hardware provides basic computing resources (CPU, memory, I/Odevices).

Operating systemcontrols and coordinates the use of the hardware amongthe various application programs for the various users.

Applications programsdefine the ways in which the system resources areused to solve the computing problems of the users (compilers, database

systems, video games, business programs).

Users (people, machines, other computers).


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Abstract view of the components of a computer system

Operating systems from two viewpoints: that of the user and that of the system.

User View

The user's view of the computer varies according to the interface being used.

Most computer users sit in front of a PC, consisting of a monitor, keyboard, mouse,

and system unit. Such a system is designed for one user to monopolize its

resources. The goal is to maximize the work (or play) that the user is performing.

In this case, the operating system is designed mostly for ease of use, with someattention paid to performance and none paid to resource utilizationhow various

hardware and software resources are shared.

System View

From the computer's point of view, the operating system is the program most

intimately involved with the hardware. In this context, we can view an operating

system as a resource allocator.A computer system has many resources that may be

required to solve a problem: CPU time, memory space, file-storage space, I/O

devices, and so on. The operating system acts as the manager of these resources.

Facing numerous and possibly conflicting requests for resources, the operating

system must decide how to allocate them to specific programs and users so that it

can operate the computer system efficiently and fairly.


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Evolution of Operating Systems

Early Systems (1950)

Mainframe Computer Systems were the first computers used to tackle

many commercial and scientific applications.

Simple Batch Systems (1960)

Batch jobs together with similar needs and run them through the computer asgroup.

Batch processing generally requires the program, data and appropriatesystem commands to be submitted together in the form of job.

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

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

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

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

electronic devices.

Memory Layout for a Simple Batch System


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Multiprogrammed Batch Systems (1970)

Multiprogramming needed for efficiency. The O.S. picks and begins to execute one job from memory. Once this job

needs an I/O operation the O.S. switches to another job (CPU or O.S. alwaysbusy).

The number of jobs in memory is less than the number of jobs in disk (JobPool).

If several jobs are ready to be brought into memory and there is not enoughroom for all of them, then the system chooses jobs among them (Job

Scheduling).

If several jobs are ready to run at the same time, the system must chooseamong them (CPU Scheduling).

Having several programs in memory at the same time requires memorymanagement.

In non-multiprogrammed system, CPU sits idle. In multiprogramming system, CPU will never be idle.

Memory Layout for a multiprogramming System

Timesharing

Timesharing (Interactive Computing) is logical extension in which CPUswitches jobs so frequently that users can interact with each job while it is

running, creating interactive computing.


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Processors time is shared among multiple usersResponse time should be < 1 second

Each user has at least one program executing in memory process

If processes dont fit in memory, swapping moves them in and out to

runVirtual memory allows execution of processes not completely in

memory

In time-sharing systems, several jobs must be kept simultaneously inmemory (job scheduling techniqueRound Robin method), the system must

have memory management and protection. To obtain reasonable response

time virtual memoryconcept is used.

To ensure orderly execution, the system must provide mechanisms for jobsynchronization and communication, and it may ensure that the job do notget stuck in a deadlock.

Real Time Systems

Note that not all Operating Systems are general-purpose systems. Real-Time (RT) systems are dedicated systems that need to adhere to

deadlines , i.e., time constraints

Real time computing may be defined as that type of computing inwhich the correctness of the system depends not only on the logical result ofthe computation but also on the time at which results are produced.

A hard realtime Hard real-time system must meet its deadline. Conflicts with time-sharing systems, not supported by general-

purpose OSs.

Often used as a control device in a dedicated application: Industrial control

Robotics Secondary storage limited or absent, data/program is stored in

short term memory, or Read-Only Memory (ROM).

Virtual memory is absent (Time Sharing). Also called as special purpose OS


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A soft realtime Deadlines desirable but not mandatory. Limited utility in industrial control or robotics. Useful in modern applications (multimedia, video conference,

virtual reality) requiring advanced operating-system features

Multiprocessor Systems (1980)

System with several CPUs in close communication: processors share memory and a clock. communication usually takes place through the shared memory.

Also known as parallel systems, tightly-coupled systems. Multiprocessors systems growing in use and importance advantages

include: Increased throughput Economy of scale- Increased reliabilitygraceful degradation or fault tolerance

Multiprocessor Architecture


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Multiprogramming vs. Multiprocessing

Types of Multiprocessor Systems

Asymmetric Multiprocessing master processor schedules and allocates work to slave processors.

Symmetric Multiprocessing (SMP) Each processor runs an identical copy of the operating system. Typically each processor does self-scheduling form the pool of

available process.

- Most modern operating systems support SMP.Symmetric Multiprocessing (SMP)

Each processor can perform the same functions and share same mainmemory and I/O facilities (symmetric).

The OS schedule processes/threads across all the processors (realparallelism).

Existence of multiple processors is transparent to the user. Incremental growth: just add another CPU. Robustness: a single CPU failure does not halt the system, only the

performance is reduced.


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Networked/Distributed Systems (1980)

Networked Systems

Distribute resources and the computation among several physical processors. Loosely coupled system:

each processor has its own local memory. processors communicate with one another through various

communications lines.

Advantages: Resources Sharing Computation speed upload sharing Reliability

Node1

Node2

network

. . .

Node3 node n

Distributed Systems

Distributed system is collection of loosely coupled processorsinterconnected by a communications network.


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Processors variously called nodes, computers, machines, hosts. Reasons for distributed systems:

Resource sharing: sharing and printing files at remote sites. processing information in a distributed database. using remote specialized hardware devices.

Computation speedupload sharing. Reliability detect and recover from site failure, function transfer,

reintegrate failed site.

Communicationmessage passing.Client-Server Systems

Dumb terminals supplanted by smart PCs. Many systems are now servers, responding to requests generated by clients-Computer-server provides an interface

to client to request services (i.e., database)

File-server provides interface for clients to store and retrieve files.GeneralStructure of a Client-Server System

Peer to Peer systems

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


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Registers its service with central lookup service on network, or Broadcasts request for service and responds to requests for service via

discovery protocol

Web-based Systems (1990)

Web has become ubiquitous. PCs most prevalent devices. More devices becoming networked to allow web access. New category of devices to manage

Web traffic among similar servers:

Load Balancers.

Basis for Grids/Cloud Computing.

Grid Computing Systems

Collection of computer resources, usually owned by multiple parties and inmultiple locations, connected together such that users can share access totheir combined power:

Can easily span a wide-area network

Heterogeneous environment

Crosses administrative/geographic boundaries Supports Virtual Organizations (VOs)

Examples: EGEE - Enabling Grids for E-SciencE (Europe), Open ScienceGrid (USA).

Cloud Computing Systems

Collection of computer resources, usually owned by a single entity,connected together such that users can lease access to a share of theircombined power:

Location independence: the user can access the desired service fromanywhere in the world, using any device with any (supported) system.


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Cost-effectiveness: the whole infrastructure is owned by the providerand requires no capital outlay by the user.

- Reliability: enhanced by way of multiple redundant sites, thoughoutages can occur, leaving users unable to remedy the situation

Hand held Systems

Handheld systems are also dedicated.

- Personal Digital Assistants (PDAs).- Cellular telephones

Issues:

- Limited memory- Slow processors- Small display screens

Operating system structure

One of the most important aspect of OS is the ability to Multiprogramming

Single user cannot keep CPU and I/O devices busy at all times 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


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Multiprogramming

Jobs are stored in Job Pool Virtual memory is the useful technique for the limitation size of physical

memory

Operating system operations

Interrupt (driven by hardware) vs.Trap (called by software or error)

Trap example: divide by 0; invalid memory access

We need to make sure that an error in a user program could cause problemsonly for the one program that was running (such as infinite loop) .

Dual mode operation

Dual-mode operation allows OS to protect itself and other systemcomponents

It contains user mode (mode bit:1) and Kernel mode (mode bit:0) The operating system loaded user application starts in user mode. Whenever

a trap or interrupt occurs, hardware switches to kernel mode.

Transition from user to kernel mode


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Privileged instructions: some machine instructions that may cause harm, canonly be executed in kernel mode.

Such as instruction switch to user mode; I/O control; timer management, andinterrupt management.

Timer

To prevent the OS out of control by CPU

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

System Structures: Operating system service

One set of operating-system services provides functions that are helpfulto the user :

User interface - Almost all operating systems have a user interface (UI)o Varies between Command-Line (CLI), Graphics User Interface

(GUI), Batch

Program execution - The system must be able to load a programintomemory and to run that program, end execution, either normally

orabnormally (indicating error)


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

File-system manipulation - The file system is of particularinterest.Obviously, programs need to read and write files and directories,

create and delete them, search them, list file Information, permission

management

Communications Processes may exchange information, on the samecomputer or between computers over a network

o Communications may be via shared memory or through messagepassing (packets moved by the OS)

Error detectionOS needs to be constantly aware of possible errorso May occur in the CPU and memory hardware, in I/O devices, in user

programo For each type of error, OS should take the appropriate action toensure

correct and consistent computing

o Debugging facilities can greatly enhance the users and programmersabilities to efficiently use the 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 runningconcurrently, resources must be allocated to each of them.

o Many types of resources - Some (such as CPU cycles, mainmemory,and file storage) may have special allocation code, others

(such asI/O devices) may have general request and release code.

Accounting - To keep track of which users use how much and whatkinds ofcomputer resources

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

o Protection involves ensuring that all access to system resources iscontrolled.


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o Security of the system from outsiders requires userauthentication,extends to defending external I/O devices from invalid

access attempts.

o If a system is to be protected and secure, precautions must beinstituted 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 forPOSIX-based systems (including virtually all versions of UNIX, Linux, and

Mac OS X), and Java API for the Java virtual machine (JVM).

Example of System Calls

System call sequence to copy the contents of one file to another file.


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Example of Standard API

Consider the ReadFile() function in the

Win32 APIa function for reading from a file

A description of the parameters passed to ReadFile()HANDLE filethe file to be read

LPVOID buffera buffer where the data will be read into and written from

DWORD bytesToReadthe number of bytes to be read into the buffer

LPDWORD bytesReadthe number of bytes read during the last read

LPOVERLAPPED ovlindicates if overlapped I/O is being used

System Call Implementation

Typically, a number associated with each system call.o System-call interface maintains a table indexed according to these

numbers.

The system call interface invokes intended system call in OS kernel andreturns status of the system call and any return values.

The caller need know nothing about how the system call is implemented.o Just needs to obey API and understand what OS will do as a result callo Most details of OS interface hidden from programmer by API .


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Managed by run-time support library (set of functions built intolibraries included with compiler.

API System Call OS Relationship

System Call Parameter Passing

Often, more information is required than simply identity of desired systemcall.

o Exact type and amount of information vary according to OS and call. Three general methods used to pass parameters to the OS.

o Simplest: pass the parameters in registers. In some cases, may be more parameters than registers.

o Parameters stored in a block, or table, in memory, and address ofblock passed as a parameter in a register .

This approach taken by Linux and Solaris.o Parameters placed, or pushed, onto the stack by the program and

popped off the stack by the operating system.


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o Block and stack methods do not limit the number or length ofparameters being passed.

Parameter Passing via Table

Types of System Calls

Process control File management Device management Information maintenance Communications

Process Control

end, abort load, execute create process, terminate process get process attributes, set process attributes wait time wait event, signal event allocate and free memory


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File Management

Common systems calls dealing with files and directories:

create file, delete file

open, close read, write, reposition get file attributes, set file attributes

Device Management

Similarity between files and I/O devices results in similar device system calls

(some OS even merges two into a combined filedevice structure):

request device, release device read, write, reposition get device attributes, set device attributes logically attach or detach devices

Information Maintanence

System calls for transferring information between the user program and OS:

get time or date, set time or date get system data, set system data get process, file, or device attributes set process, file or device attributes

Communication

System calls for transferring information between the user program and OS:

create, delete communication connection send, receive message transfer status information attach or detach remote devices


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System Programs

System programs provide a convenient environment for programdevelopment and execution. The can be divided into:

o File manipulationo Status informationo File modificationo Programming language supporto Program loading and executiono Communicationso Application programs

Most usersview of the operation system is defined by system programs, notthe actual system calls.

Provide a convenient environment for program development and execution.o Some of them are simply user interfaces to system calls; others are

considerably more complex.

File management -Create, delete, copy, rename, print, dump, list, andgenerally manipulate files and directories.

Status information.o Some ask the system for info -date, time, amount of available

memory, disk space, number of users.o Others provide detailed performance, logging, and debugging

information.

o Typically, these programs format and print the output to the terminalor other output devices.

o Some systems implement a registry -used to store and retrieveconfiguration nformation.

File modificationo Text editors to create and modify fileso Special commands to search contents of files or perform

transformations of the text.

Programming-language support -Compilers, assemblers, debuggers andinterpreters sometimes provided.


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Program loading and execution-Absolute loaders, relocatableloaders, linkageeditors, and overlay-loaders, debugging systems for higher-level and

machine language.

Communications -Provide the mechanism for creating virtual connectionsamong processes, users, and computer systems.

o Allow users to send messages to one anothers screens, browse webpages, send electronic-mail messages, log in remotely, transfer files

from one machine to another.

System structureSimple structure

MS-DOSwritten to provide the most functionality in the least space.o Not divided into modules

Although MS-DOS has some structure, its interfaces and levels offunctionality are not well separated.

MS DOS Layer Structure


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Layered Approach

The operating system is divided into a number of layers (levels), each builton top of lower layers. The bottom layer (layer 0), is the hardware; thehighest (layer N) is the user interface.

With modularity, layers are selected such that each uses functions(operations) and services of only lower-level layers.

Layered Operating System

UNIX

UNIXlimited by hardware functionality, the original UNIX operatingsystem had limited structuring. The UNIX OS consists of two separable

parts.

o Systems programso The kernel

Consists of everything below the system-call interface andabove the physical hardware.


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Provides the file system, CPU scheduling, memorymanagement, and other operating-system functions; a large

number of functions for one level.

UNIX System Structure

Microkernel System Structure

Moves as much from the kernel into userspace. Communication takes place between user modules using message passing. Benefits:

o Easier to extend a microkernelo Easier to port the operating system to new architectureso More reliable (less code is running in kernel mode)o More secure

Detriments:o Performance overhead of user space to kernel space communication.


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Mac OS X Structure

Modules

Most modern operating systems implement kernel modules.o Uses object-oriented approacho Each core component is separateo Each talks to the others over known interfaceso Each is loadable as needed within the kernel

Overall, similar to layers but with more flexible.Solaris Modular Approach


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Reference:

1.Operating System Principles-Abham Silberschatz

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