Operating System Assocition

7/31/2019 Operating System Assocition

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WHAT IS AN OPERATING

SYSTEM ? An operating system (OS) is a set of programs that

manage computer hardware resources and provide common

services for application software.

The operating system is the most

important type of system software in a

computer system.

A user cannot run an application program on thecomputer without an operating system


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TYPES OF OPERATING

SYSTEMS

REAL-TIME

MULTI-TASKING Vs SINGLE-TASKING

MULTI-USER Vs SINGLE-USER

DISTRIBUTED

EMBEDDED


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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 thesystem resources are used to solve the computingproblems of the users

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

Users People, machines, other computers


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Four Components of a ComputerSystem


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Operating System Definition

OS is a resource allocator Manages all resources

Decides between conflicting requests forefficient and fair resource use

OS is a control program Controls execution of programs to prevent

errors and improper use of the computer


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No universally accepted definition

Everything a vendor ships when you order anoperating systemis good approximation

But varies wildly

The one program running at all times on thecomputeris the kernel. Everything else iseither a system program (ships with theoperating system) or an application program


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Computer-System Operation

I/O devices and the CPU can execute concurrently. Each device controller is in charge of a particular

device type. Each device controller has a local buffer. CPU moves data from/to main memory to/from

local buffers I/O is from the device to local buffer of controller.

Device controller informs CPU that it has finished itsoperation by causing an interrupt.


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Operating-System Structures

Simple Structure

MS-DOS Layer Structure

Layered Approach UNIX

Microkernel system Structure

Modules


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Simple Structure

MS-DOS written to provide the mostfunctionality in the least space

Not divided into modules Although MS-DOS has some structure, its

interfaces and levels of functionality are

not well separated


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MS-DOS Layer Structure


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Layered Approach

The operating system is divided into anumber of layers (levels), each built on top oflower layers. The bottom layer (layer 0), isthe hardware; the highest (layer N) is theuser interface.

With modularity, layers are selected suchthat each uses functions (operations) andservices of only lower-level layers


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Layered Approach Structure


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UNIX

UNIX limited by hardware functionality, theoriginal UNIX operating system had limitedstructuring. The UNIX OS consists of twoseparable parts

Systems programs The kernel

Consists of everything below the system-call interface and

above the physical hardware Provides the file system, CPU scheduling, memory

management, and other operating-system functions; a largenumber of functions for one level


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UNIX System Structure


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Microkernel System Structure

In computer science, a microkernel is the near-minimum amount of software that can provide themechanisms needed to implement an operating

systems. Benefits:

Easier to extend a microkernel Easier to port the operating system to new architectures More reliable (less code is running in kernel mode)

More secure Detriments:

Performance overhead of user space to kernel spacecommunication


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Modules

Most modern operating systems implementkernel modules Uses object-oriented approach

Each core component is separate

Each talks to the others over known interfaces

Each is loadable as needed within the kernel

Overall, similar to layers but with moreflexible


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Process management is an integral part of any modern day

operating system (OS). The OS must allocate resources toprocesses ,enable processes to share and exchangeinformation, protect the resources of each process from otherprocesses and enable synchronization among processes.

Process Concept Process Scheduling Operations on Processes Co-operating Processes Interprocess Communication

Communication in Client-Server Systems

Processes


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Process Concept

An operating system executes a variety of programs: Batch system jobs Time-shared systems user programs or tasks

Textbook uses the termsjob and process almostinterchangeably Process a program in execution; process execution

must progress in sequential fashion A process includes:

program counter

Stack data section


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Process State

As a process executes, it changes state

new: The process is being created

running: Instructions are being executed

waiting: The process is waiting for some eventto occur

ready: The process is waiting to be assigned to aprocessor

terminated: The process has finished execution


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Diagram of Process State


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Process Control Block (PCB)

Information associated with each process Process state

Program counter CPU registers

CPU scheduling information

Memory-management information Accounting information

I/O status information


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Process Scheduling Queues

Job queueset of all processes in the system Ready queueset of all processes residing in

main memory, ready and waiting to execute Device queues

set of processes waiting for

an I/O device Processes migrate among the various queues


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Schedulers

Long-term scheduler(or job scheduler) selects which processes should bebrought into the ready queue

Short-term scheduler(or CPU scheduler)selects which process should beexecuted next and allocates CPU


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Short-term scheduler is invoked very frequently (milliseconds)(must be fast)

Long-term scheduler is invoked very infrequently (seconds,

minutes)

(may be slow)

The long-term scheduler controls the degree of multiprogramming

Processes can be described as either: I/O-bound processspends more time doing I/O than computations,

many short CPU bursts CPU-bound processspends more time doing computations; few very

long CPU bursts


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Process Creation

Parent process create children processes, which, in turn create otherprocesses, forming a tree of processes Resource sharing

Parent and children share all resources Children share subset of parents resources Parent and child share no resources

Execution Parent and children execute concurrently Parent waits until children terminate

Address space Child duplicate of parent Child has a program loaded into it

UNIX examples fork system call creates new process execsystem call used after a fork to replace the process 'memory space

with a new program


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Process Termination

Process executes last statement and asks the operatingsystem to delete it (exit) Output data from child to parent (via wait)

Process 'resources are deal located by operating system Parent may terminate execution of children processes

(abort) Child has exceeded allocated resources Task assigned to child is no longer required

If parent is exiting ome operating system do not allow child to continue if its parent

terminates All children terminated -cascading termination


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Deadlocks

A deadlock is a situation wherein two or morecompeting actions are each waiting for the other tofinish, and thus neither ever does.

The Deadlock Problem

System Model Deadlock Characterization Methods for Handling Deadlocks Deadlock Prevention

Deadlock Avoidance Deadlock Detection Recovery from Deadlock


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The Deadlock Problem

Waiting to acquire a resource held by another processin the set.

Example

System has 2 disk drives. P1 and P2 each hold one disk drive and each needs another one.

Example semaphoresA and B, initialized to 1

P0 P1wait (A); wait(B)wait (B); wait(A)


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Methods for Handling Deadlocks

Ensure that the system will never enter adeadlock state.

Allow the system to enter a deadlock stateand then recover.

Ignore the problem and pretend that

deadlocks never occur in the system; usedby most operating systems, includingUNIX.


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Deadlock Prevention

.

Mutual Exclusionnot required for sharableresources;

must hold for non-sharable resources.

Hold and Waitmust guarantee that whenever aprocess requests a resource, it does not hold anyother resources. Require process to request and be allocated all its

resources before it begins execution, or allow process torequest resources only when the process has none.

Low resource utilization; starvation possible.


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No Preemption

If a process that is holding some resources requestsanother resource that cannot be immediately allocated toit, then all resources currently being held are released.

Preempted resources are added to the list of resources forwhich the process is waiting.

Process will be restarted only when it can regain its oldresources, as well as the new ones that it isrequesting.

Circular Waitimpose a total ordering of all resourcetypes, and require that each process requests resourcesin an increasing order of enumeration.


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Thank You..!

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Operating System Assocition