OPERATING SYSTEMS

Dr.M.Kavitha

Assistant Professor

PG&Research Department of Commerce

Guru Nanak College, Chennai-42

What Is An Operating System?

An operating system in its most general

definition is the software that allows a

user to run crucial applications on his/her

computing device. It helps to manage a

computer’s hardware resources. It helps to

support basic functions like scheduling

tasks, and controlling peripherals.

DEFINITION OF OPERATION

SYSTEM

An Operating System (OS) is an

INTERFACE between a computer user

and computer hardware. An operating

system is a software which performs all

the basic tasks like file management,

memory management, process

management, handling input and output,

and controlling peripheral devices such as

disk drives and printers.

HISTORY

Early computers were built to perform a series of

single tasks, like a calculator. Basic operating

system features were developed in the 1950s,

such as resident monitor functions that could

automatically run different programs in

succession to speed up processing. Operating

systems did not exist in their modern and more

complex forms until the early 1960s

The first microcomputers did not have the

capacity or need for the elaborate operating

systems that had been developed for mainframes

and minis; minimalistic operating systems were

developed, often loaded from ROM and known

as monitors.

TYPES OF OPERATING SYSTEM

TYPES OF OS

Distributed Templated Real-time

Single- & multi-

user

Library

Single-tasking &

multi-tasking

COMPONENTS OF OPERATING

SYSTEM

KERNEL

With the aid of the firmware and device drivers, the kernel provides the most basic level of control over all of the computer's hardware devices.

PROGRAM EXECUTION

The operating system provides an interface between an application program and the computer hardware, so that an application program can interact with the hardware only by obeying rules and procedures programmed into the operating system.

INTERRUPTS

Interrupts are central to operating

systems, as they provide an efficient way

for the operating system to interact with

and react to its environment. The

alternative – having the operating system

"watch" the various sources of input for

events (polling) that require action – can

be found in older systems with very

small stacks (50 or 60 bytes) but is

unusual in modern systems with large

stacks.

MODES

In general terms, supervisor mode

operation allows unrestricted access to all

machine resources, including all MPU

instructions. User mode operation sets

limits on instruction use and typically

disallows direct access to machine

resources.

MEMORY MANAGEMENT

Among other things, a multiprogramming

operating system kernel must be responsible for

managing all system memory which is currently

in use by programs. This ensures that a program

does not interfere with memory already in use by

another program. Since programs time share,

each program must have independent access to

memory.

VIRTUAL MEMORY

The use of virtual memory addressing (such as

paging or segmentation) means that the kernel

can choose what memory each program may use

at any given time, allowing the operating system

to use the same memory locations for multiple

tasks

DEVICE DRIVERS

A device driver is a specific type of computer

software developed to allow interaction with

hardware devices. Typically this constitutes an

interface for communicating with the device,

through the specific computer bus or

communications subsystem that the hardware is

connected to, providing commands to and/or

receiving data from the device, and on the other

end, the requisite interfaces to the operating

system and software applications.

DISK ACCESS AND FILE SYSTEMS

Access to data stored on disks is a central feature

of all operating systems. Computers store data

on disks using files, which are structured in

specific ways in order to allow for faster access,

higher reliability, and to make better use of the

drive's available space

FUNCTIONS OF OPERATING

SYSTEM

Security –The operating system uses password protection to protect user data and similar other techniques. it also prevents unauthorized access to programs and user data.

Control over system performance –Monitors overall system health to help improve performance. records the response time between service requests and system response to have a complete view of the system health. This can help improve performance by providing important information needed to troubleshoot problems.

Job accounting –Operating system Keeps track of time and resources used by various tasks and users, this information can be used to track resource usage for a particular user or group of user.

Error detecting aids –Operating system constantly monitors the system to detect errors and avoid the malfunctioning of computer system.

Coordination between other software and users –Operating systems also coordinate and assign interpreters, compilers, assemblers and other software to the various users of the computer systems

Memory Management –

The operating system manages the Primary

Memory or Main Memory. Main memory is made

up of a large array of bytes or words where each

byte or word is assigned a certain address. Main

memory is a fast storage and it can be accessed

directly by the CPU. For a program to be

executed, it should be first loaded in the main

memory.

Processor Management –

In a multi programming environment, the OS

decides the order in which processes have access

to the processor, and how much processing time

each process has. This function of OS is called

process scheduling. An Operating System

performs the following activities for processor

management.Keeps tracks of the status of

processes.

Device Management –

An OS manages device communication via their

respective drivers. It performs the following

activities for device management. Keeps tracks of

all devices connected to system. designates a

program responsible for every device known as

the Input/Output controller. Decides which

process gets access to a certain device and for

how long. Allocates devices in an effective and

efficient way. Deallocates devices when they are

no longer required.

File Management –

A file system is organized into directories for

efficient or easy navigation and usage. These

directories may contain other directories and

other files.

ADVANTAGES OF OS

Allocating memory is easy and cheap

Any free page is ok, OS can take first one out of list it keeps

Eliminates external fragmentation

Data (page frames) can be scattered all over PM

Pages are mapped appropriately anyway

Allows demand paging and prepagingMoreefficient swapping

No need for considerations about fragmentation

Just swap out page least likely to be used

DISADVANTAGES

Longer memory access times (page table lookup)

Can be improved using TLB

Guarded page tables

Inverted page tables

Memory requirements (one entry per VM page)

Improve using Multilevel page tables and variable page sizes (super-pages)Guarded page tables

Page Table Length Register (PTLR) to limit virtual memory size

Internal fragmentation

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