Doubts and Queries

If you have any doubts or queries or you want to understand any topic related with OS please comment below. Will solve all ur queries. 

Disk Scheduling

FCFS Scheduling

The simplest disk scheduling algorithm is First Come First Serve Scheduling algorithm.

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SSTF Scheduling

SSTF stands for Shortest Seek Time First algorithm. The SSTF selects the request that have smallest seek time from the current head position. The algorithm have better performace over FCFS scheduling algorithm

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SCAN Scheduling

In SCAN algorithm, disk arm starts moving from one end of the disk to other end. While moving, it services along the way. On reaching other end, direction of head movement is reversed and while coming back, it again services the request in the path. This algorithm is also known as elevator algorithm.

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C-SCAN Scheduling

C-SCAN stands for Circular SCAN Scheduling. In this scheduling algorithm like SCAN algorithm, head moves from one end to other and services the request  along the way. But on reaching the other end, the head immediately returns to the beginning of the disk without service request while coming back.

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Linux Directory Structure

In a hierarchical inverted tree like structure, the Linux provides following directories under the /directory to store a specific type of file.

1. / – Root

    Every single file and directory starts from the root directory.    

    Only root user has write privilege under this directory.

     /root is root user’s home directory.

2. /home – Home Directories

    Contains all the HOME directories of users.

    Home directories for all users store particular user personal files.

    For example: /home/teachers, /home/students, /home/mhcollege

3. /bin – User Binaries

    Contains many utilities as binary executables.

    The utilities are common linux commands you need to use in single-user modes

    Commands used by all the users of the system are located here.

    For example: ps, ls, ping, grep, cp.

4. /dev – Device Files

    Contains device files.

    These include terminal devices, USB, or any device attached to the system.

    For example: /dev/tty1, /dev/usbmon0

5. /etc – Configuration Files

    Contains configuration files required by all programs.

    Stores system related data which users and the system need to refer.

    This also contains startup and shutdown shell scripts used to start/stop individual programs.

    For example: passwd, /etc/resolv.conf, /etc/logrotate.conf

6. /lib – System Libraries

    Contains library files for the compilers installed in the system 

    Library filenames are either ld* or lib*.so.*

    For example: ld-2.11.1.so, libncurses.so.5.7

7. /sbin – System Binaries

     Just like /bin, /sbin also contains binary executables.     

     But, the linux commands located under this directory are used by system aministrator, for system            maintenance purpose.

      For example: iptables, reboot, fdisk, ifconfig, swapon

8. /boot – Boot Loader Files

    Contains boot loader related files.

    Kernel initrd, vmlinux, grub files are located under /boot

    For example: initrd.img-2.6.32-24-generic, vmlinuz-2.6.32-24-generic

9. /usr – User Programs

    Contains operating system files which are not involved in the boot process. 

    /usr/bin contains binary files for user programs. For example: at, awk, cc, less, scp

    /usr/sbin contains binary files for system administrators. For example: useradd, userdel

    /usr/lib contains libraries for /usr/bin and /usr/sbin

    /usr/local contains users programs that you install from source. 

10. /var – Variable Files

    var stands for variable files.

    Content of the files that are expected to grow can be found under this directory.

     Includes system log files (/var/log); packages and database files (/var/lib); emails (/var/mail); print         queues (/var/spool); lock files (/var/lock); temp files needed across reboots (/var/tmp);

11. /tmp – Temporary Files

    Contains temporary files created by system and users.

    Files under this directory are deleted when system is rebooted.

12. /media – Removable Media Devices

    Temporary mount directory for removable devices.

    For examples, /media/cdrom for CD-ROM; 

                           /media/floppy for floppy drives; 

                           /media/cdrecorder for CD writer

vi Editor

The vi editor is visual editor used for creating and editing text files containing data, document or programs. 

It is like Notepad we use in Windows.

It display content of files on the screen and allows user to add, insert, delete or change text. 

Some of the other editors for Linux are

ed (line editor)

emacs(edit macros editor)

red (restricted line editor) etc.

Starting vi editor

SYNTAX: vi filename <Enter>

EXAMPLE: [root@localhost student]# vi msg.txt

- - - - - - - - - - “msg.txt” [New File]

             To end insertion or append mode, Press <Esc> and type vi editors command.

Commands of vi editor

The vi editor works in two mode insert mode and append mode. After starting vi editor, Press I to add text. The other commands are

:wq<Enter>                         save all changes and quit

:w<Enter>                         save file

:e<filename><Enter>    open specified file

yy or y                     marks current line as line to be copied

p                              paste marked text after current cursor position

P                            paste marked text before current cursor position 

Functions of vi editor

1. Vi editor allow user to create a file and write data to it.

2. It allow user to open existing file, modify content and save changes in file.

3. It allows user to copy and paste text from one position in a file to another.’

4. It allow searching for a particular expression or word.

5. Large amount of data can also be handled using vi editor.

The History of Linux

1991: Linus Torvalds announced Linux

Linux Torvalds, a student of Helsinki University, Finland, introduced linux in 1991. He worked on the Linux project and write the source code for the Linux kernel. He made Linux available on the Internet. Many programmers added to the code, changed it and built in support for all kinds of hardware. Linux 0.02 was introduced in 1991.

On 25 August 1991 sent a mail to a newsgroup on Usenet. He talked about developing a free operating system.

1992: Linux became Open Source

Linux was not always Open Source. The first few licenses of Linux forbid commercial redistribution. It was with version 0.12 in February’92 that Linux Kernel was released under GPL.

1993: Slackware Linux released

The oldest currently (as of 2018) existing Linux distribution, Slackware version 1.0 was released for the first time on 17th July 1993. Later in the same year, the Debian project is established. Today it is the largest community distribution.

1994: Someone registered Linux trademark and it was not Linus Torvalds

Torvalds and hundreds of developers from across the world worked on it and in March 1994, version 1.0 of Linux kernel was released.

Linux trademark was registered in 1994 by a William R. Della Croce, Jr.

1995: First Linux Expo

First Linux specific tradeshow and conference series was launched by people at North Carolina State University. This became one of the most attended annual Linux show for next several years.

Linux is also ported to the DEC Alpha and to the Sun SPARC.

1996: Tux gets to be the symbol of Linux

Linus Torvalds recommended a penguin as the mascot of Linux. Tux was created by Larry Ewing in 1996. Tux has been unchanged for last 19 years.

Version 2.0 of the Linux kernel is released. The kernel can now serve several processors at the same time using symmetric multiprocessing (SMP), and thereby becomes a serious alternative for many companies.

1997: GNOME Project is born

GNOME is one of the most successful open source projects. It has been crucial to the spread of desktop Linux. It has given us a number of programs that we use today on desktop Linux.

1998: KDE 1.0 released. Many major companies such as IBM, Compaq and Oracle announce their support for Linux.

2000: Dell announces that it is now the No. 2 provider of Linux-based systems worldwide and the first major manufacturer to offer Linux across its full product line.

2002: Red Hat Enterprise Linux released

The first commercial Linux for Business IT was released in the year 2002. RHEL is one of the few Linux distributions that changed Linux forever.

2003: Attempt to install backdoor in Linux kernel

An attempt was made to insert a backdoor in the Linux kernel source. Disguised as an innocuous error checking routine, the backdoor was designed to obtain root privileges under specific conditions. Linux Kernel maintainers caught it before it could made to mainline Linux kernel.

2004: Ubuntu 4.10 released

On 20th October 2004, Ubuntu 4.10 was released. This new Linux distribution marketed itself as Linux for human being.

2005: Linus Torvalds created Git

In 2005, BitKeeper decided to not provide the free version to the community anymore. This forced Linus Torvalds to work on his own version control system and thus Git was born.

2006: Oracle releases its own distribution of Red Hat Enterprise Linux. Novell and Microsoft announce cooperation for a better interoperability and mutual patent protection.

2007: Linux powered netbook arrived

In a time when Windows came pre-installed on majority of personal computers, Asus launched Eee PC, a lower end, lightweight netbook series. It came preinstalled with a custom version of Linux called Xandros. Dell also started distributing laptops with Ubuntu pre-installed on them.

2008: Android version 1.0 released

With the release of Android, a mobile operating system based on Linux Kernel, Linux took the first step in the world of mobile OS. While desktop Linux might not have been that big a sucess, with Android, Linux is dominating the world of mobile OS.

2009: Google announced Chrome OS

Google announces its own  desktop operating system Chrome OS, based on Linux kernel. Later on, Google also started releasing Chromebooks, dedicated devices to run Chrome OS. Chromebooks have grown in popularity in recent years and last year it outsold MacBooks in the US.

Red Hat's market capitalization equals Sun's, interpreted as a symbolic moment for the "Linux-based economy".

2010: Red Hat became first billion dollar open source company

Red Hat Linux became the first billion dollar open source company in 2010. The success of Red Hat breaks the myth that open source companies cannot make money.

2011: Version 3.0 of the Linux kernel is released.

2012: The aggregate Linux server market revenue exceeds that of the rest of the Unix market.[60]

2013: Ubuntu Phone announced

2014: Ubuntu claims 22,000,000 users.

Microsoft’s new CEO Satya Nadella shocked the tech world with his “Microsoft loves Linux” remark. As we see later that this was just the beginning of Microsoft’s grand design to rule cloud world. However, this actually brought a shift in Microsoft policies and for the first time Microsoft started open sourcing its products and bringing it to Linux.

2015: Version 4.0 of the Linux kernel is released

Microsoft has its own version of Linux. A of software meant for network switches that required Linux.

2019: Version 5.0 of the Linux kernel is released.

Source: Wikipedia, Timeline of Linux

Page Replacement Algorithm

When a process executes and the desired page is not in memory, page fault occurs. After locating desired page in memory, OS finds free frame. BUT OS finds that there are no free frames. The OS at this point have multiple options

1.       Terminate the user process

2.       Swap out the process and freeing all its frames

3.       Page Replacement

When no frame is free, process of finding a page that is not currently being used and free it is known as page replacement. The page replacement causes the following sequences of steps to occur

1.       Find the location of the desired page on the disk

2.       Find a free frame

a.       If frame is free, use it

b.      If there is no free frame, use page replacement algorithm to find a page that is not currently being used to select victim frame

c.       Write victim frame back to disk

d.      Update page and frame table

3.       Issue a disk read operation to a free frame

4.       After completing disk read, update page table to show desired page is in the memory now.

5.       Restart the user process

FIFO Page Replacement Algorithm

The simplest page replacement algorithm is First In First Out (FIFO) algorithm. The algorithm associates with each page the time it was brought into the memory.  It replaces that has been in memory for the longest period of time( brought in memory first).  A queue is used to implement FIFO algorithm. A new page is inserted at the rear end whereas page is replaced from the front end of the queue.

The algorithm is easy to understand and program. But performance is not good.

Optimal Page Replacement Algorithm

Optimal Page Replacement algorithm replaces the page that will not be used for the longest period of time.

This algorithm gives lowest possible page fault rate for a fixed number of frames. This algorithm on the other hand is difficult to implement because it requires future knowledge of reference string.

LRU Page Replacement Algorithm

LRU Page replacement algorithm replaces the page that has not been used for the longest period of time. The algorithm associates with each page the time of the page last use.

LRU can be viewed as Optimal Page replacement algorithm in backward  direction and therefore doesnot require future knowledge of reference string.

The algorithm can be implemented in two ways

Keep stack of page numbers. When page is referred, it is popped 

Demand Paging

Consider a menu driven program that has 50 options. Depending upon user choice one option will execute. But to execute a program it must be loaded into the memory.

One way is to load the complete program in physical memory at program execution time. This makes inefficient use of memory because out of 50 options available, code under one option will be executed.

Alternatively, we can load pages only when they are needed. This is known as demand paging and used in virtual memory management.

Demand Paging is an efficient memory management technique in which page is loaded only when demanded during program execution. Pages that are never accessed, never loaded into the memory. 

This technique has following advantages

i.                     No limitation on size of physical memory available.

ii.                   Each program takes less memory as a result more programs run at same time which in turn increases CPU utilization and throughput.

iii.                  Less number of swapping occurs therefore program runs faster

In demand paging, each process resides in secondary memory(disk). When a process execute instead of loading entire process in physical memory, the page that is needed will only be swapped. As compared to swappere who swaps all the pages of process in memory, we use a lazy swapper that swap only those pages that are need. In addition, since each process is a collection of pages therefore we call it a pager not swapper. Pager guess which page will be used and therefore instead of swapping whole process, pager brings only those pages in memory that is needed.

To distinguish between the pages that are in memory or on the disk, page table uses valid invalid bit scheme is used. If the page is legal and in memory, then the bit is set to valid. The bit is set to invalid, if the page is in the disk or page doesn’t belong to logical address space of the process.

When a process executes and access only pages that are in memory (known as memory resident pages), execution occurs normally. BUT when a process requires a page that was not in memory (bit is invalid), page fault occurs. The following steps is required to handle page fault

1.       Trap sent to the operating system

2.       If page reference was legal and determine the location of the page on the disk to brought the page into the memory.

3.       Find a free frame.

4.       Issue a disk read operation to a free frame

5.       After completing disk read, update page table to show desired page is in the memory now.

6.       Restart the instructions that were interrupted by the trap.

Virtual memory

Virtual memory is a storage allocation scheme in which secondary memory can be treated as a part of main memory.

Virtual memory gives an illusion of having large memory when in reality only smaller system’s RAM (physical memory) is available.

Virtual Memory is implemented using Demand Paging.

Paging

Paging is a memory management scheme that permits physical address space of a process to be non contiguous. 

In paging, physical memory is partitioned into fixed size blocks called FRAMES and logical memory is partitioned into block of same size called PAGES. The size of page is equal to frame size. Every CPU generated address is now divided into two parts 

i. Page Number (p): Used as an index into page table. 

ii. Offset

In paging, it is not necessary for a free frame to be contiguous. It can be allocated to a process that needs it. Thus there is no external fragmentation and there is no need for compaction.

Page table contain address of each page in physical memory. That is page table contain frame number in which page is available.  The base address plus page offset gives physical memory address. 

To provide memory protection, page table consists of additional bit known as valid or invalid bit. The bit is set to valid if page is in logical address space and is valid. It is set to invalid if page is not in the logical address space. Valid and Invalid bit is set by OS for each page. 

When a process arrives, its size in pages is examined. Each page of process needs one frame. Process of n pages requires that n frames are available. If frame is available, first page is loaded into one of the available frame and entry of frame number is made in the page table. The next page is now loaded and entry is made in page table. The process is repeated for all the pages of the process. 

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In addition to page table, OS maintains a frame table. Frame table has one entry for each frame indicating whether it is free or allocated. If frame is allocated, frame table stores process name and its page number. 

Structure of Page Table

Some the methods to structuring the page table is as follows

1. Hierarchical Paging: In a 32 bit logical address space and page size of 4 KB (212), page table consist of 232/212 entries. The page table becomes very large. One solution is to divide the page table into smaller pieces. When we divide page table in two parts it is called two-level paging algorithm.  The table is also known as forward mapped page table because address obtained by moving for outer page to inner page.

In a 32 bit logical address space and page size 4KB (212), 12 bits consist off set. The remaining 20 bits page number is divided into two parts:  p1=10 bits for page number and p2=10 bits for offset. P1 is index number of outer page table and p2 is displacement of outer page table.

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2. Hashed Page Tables: In hashed page tables the hash value is the page number. Each entry in a hash table contains a linked list of elements that hash to same location. Each element consist of three parts

i. Logical page number

ii. Value of mapped page frame

iii. A pointer to next element of linked list

The logical page number is compared with the field 1 of first element in the linked list. If there is a match, the corresponding frame number present in field 2 is used to form the desired physical address. If no match, next entries are searched. 

Hashed Page Table

3. Inverted Page tables: An inverted page table has one entry for each frame of memory.  Each entry consist of logical address of the page and the information of a process to which the page belongs; i.e. process id. It means there is only one page table is in the system.  

Here, logical address consist of three parts

i. Process Id

ii. Page Number 

iii. Offset

The inverted page table is sorted by physical address but searching is done on logical address. When memory reference occurs, page table is searched for a match. If match is found at position x, then the physical address is (x,offset). If no match is found, an illegal address is accessed. 

Fragmentation

Fragmentation is a phenomenon in which storage space is used inefficiently. Fragmentation occurs when main memory is available but space is not sufficient to load any other process.  Due to swapping and dynamic memory allocation, user process swap in and swap out of the main memory. As a result there are free space but we cannot load new process because available size is smaller than size of the process.

Types of Fragmentation:

Internal Fragmentation occurs when memory allocated to a process is slightly larger than the actual memory requirement of the process. The difference is known as internal fragmentation. Internal fragmentation is actually unused memory internal to a process partition.

Example: Suppose, four process arrives and P1 requires 35 KB, P2 requires 70 KB, nd P4 requires 28 KB. The new process P5 requires 54KB. Memory requirement of process P5 is not fulfilled because internal fragmentation occurs in this case.

REASON: Total Free Memory = 50 + 2 + 5= 57 KB. But we cannot allocate to process P5 because free memory is within allocated process memory area.

2   External Fragmentation occurs when there is a sufficient area within main memory to satisfy current process memory requirement. BUT memory cannot be allocated because available memory is not contiguous.

Example: Suppose a new process arrives and needs 100 KB. In this case, external fragmentation occurs and memory is not allocated to new process.

REASON: The total free memory available is 30+20+60+10=120KB. But we cannot allocate memory to new process because available free memory is not contiguous.

 

The solution for external fragmentation is compaction and non contiguous memory allocation

Compaction is a process to shuffle memory content so that all free memory forms one large block. Practically this is not possible every time.

Another way to solve external fragmentation is allowing logical address space to be non contiguous. The two methods used are Paging and Segmentation.

What is Operating System ?

An operating system is a program designed to run other programs on a computer. It is defined as system software that control and coordinate all the working of computer system. It is considered the backbone of a computer, managing both software and hardware resources. It acts as an intermediary between a user of a computer and the computer hardware.

Operating systems are responsible for everything from the control and allocation of memory to recognizing input from external devices and transmitting output to computer displays.

•Operating System is a software that manages computer hardware.

•An operating system as the programs that make the hardware useable.

•An operating system is the set of programs that controls and coordinates the operation of various Input, Output and Process devices; i.e. a computer

 

 

Objectives of OS

•To hide details of hardware

–Examples

•Operating systems provide subroutines called device drivers that perform operations on behalf of programs for example, input/output operations.

•Files so that programs do not have to deal with disks.

•Each application appears to have the entire machine to itself.

–OS transforms the computer hardware into multiple virtual computers, each belonging to a different program. E

  

•Resource Allocator

–To allocate resources to processes so that multiple users share resources fairly, efficiently, safely and securely.

–Example

•Multiple processes share one processor.

•Multiple programs share one physical memory

•Multiple users and files share one disk.

 

•Provide a pleasant and effective user interface

–The user interacts with the operating systems through the user interface.

–Common Interfaces are

•Command line User Interface (CUI)

–Commands that are given to the operating system via command statements that execute

•Graphical  User Interfaces (GUI)

–User employs a mouse based window and menu system

ऑपरेटिंग सिस्टम एक डिज़ाइन प्रोग्राम है जो कंप्यूटर पर अन्य प्रोग्राम चलाने के लिए बनाया गया है. यह एक सिस्टम सॉफ्टवेयर है जो नियंत्रण और कम्प्यूटर प्रणाली के सभी काम करता है। यह एक कम्प्यूटर की रीढ़ माना जाता है, दोनों सॉफ्टवेयर और हार्डवेयर के संसाधनों का प्रबंधन है. यह एक user और कंप्यूटर हार्डवेयर के बीच एक मध्यस्थ के रूप में कार्य करता है.
ऑपरेटिंग सिस्टम नियंत्रण और स्मृति के बाह्य उपकरणों से इनपुट पहचानने और कंप्यूटर को दर्शाता है उत्पादन प्रसारण के लिए आवंटन से सब कुछ के लिए जिम्मेदार हैं.

Operating System are designed to

1. Convenience: Operating system makes computer more convenient to use.

2. Efficiency: Operating system allows computer resources to be used in the efficient manner.

3. Ability to evolve: Operating system permits efficient development, testing and introduction of new system without interfering with current provided services.

आपरेटिंग सिस्टम design किया गया
1. सुविधा: ऑपरेटिंग सिस्टम कंप्यूटर का उपयोग अधिक सुविधाजनक बनाता है.

2. क्षमता: ऑपरेटिंग सिस्टम कम्प्यूटर संसाधनों का प्रभावी ढंग से प्रयोग की जाने की अनुमति देता है.

3. विकसित करने की योग्यता: ऑपरेटिंग सिस्टम&gt; वर्तमान प्रदान की गई सेवाओं के साथ हस्तक्षेप किए बिना कुशल विकास, परीक्षण और नई प्रणाली का परिचय दे सकते हैं.

EVOLUTION OF OPERATING SYSTEM

Operating system has evolved through various stages of its development like Serial Processing, Batch Processing, Multi Programming and Time Sharing System.

1. Batch Systems: In this system, the user did not interact directly with the computer system. The user prepared a job which consisted of the program, the data and some control information and submitted it to the computer operator. This job is in the form of punch cards. After some time, the output appeared. To speed up processing, operators batched together jobs with similar needs and ran them as a group. In this environment, the CPU is often idle because the speeds of the I/O devices are slower than CPU.

इस प्रणाली में हो, तो user सीधे कंप्यूटर system के साथ संपर्क नहीं किया है. user एक job जो प्रोग्राम, डाटा और कुछ नियंत्रण की जानकारी शामिल है और यह कम्प्यूटर ऑपरेटर को submit किया जाता है . यह job पंच कार्ड के रूप में है. कुछ समय के बाद, output दिखाई देता है . Processing Speed increase करने के लिए , ऑपरेटरों सारे job जो एक सामान है एक समूह (batch) के रूप में execute करते है. इसमे CPU अक्सर idle है, क्योंकि I/ O device, सीपीयू की तुलना में धीमी गति से काम करते है.

2.  Multiprogrammed Systems: A single user cannot keep either CPU or the I/O devices busy all the times. In multiprogrammed system, the operating system keeps several jobs in memory simultaneously. When one program needs I/O, that job is suspended and the other job whose data is ready and in main memory and waiting for CPU is taken up for execution. In this way, multiprogramming, organizes job so that CPU always has one to execute and thus increases CPU utilization.
Multiprogrammed सिस्टम: एक single user CPU को हमेशा व्यस्त नहीं कर सकते. Multiprogrammed प्रणाली में, ऑपरेटिंग सिस्टम memory में कई jobs एक साथ रहते है। जब एक प्रोग्राम मैं I/O की जरुरत होती हे, उस काम को निलंबित कर दिया जाता है और दूसरे काम जिसका डाटा तैयार है और main memory में है और CPU के इंतज़ार कर रही है उसे CPU दे दिया जाता है. इस तरह, multiprogramming, jobs organizes करता है जिसमे हमेशा एक job exeute होता है और सीपीयू उपयोग बढ़ता है।

3. Time Sharing Systems: Time Sharing or Multitasking is an extension of Multiprogramming. In this, CPU executes multiples jobs by switching among them. A time-shared operating system allows many users CPU scheduling and multiprogramming to provide each user with a small potion of time shared computer. Each command in a time shared system tends to be short know as quantum only little CPU time is needed for different process. As the system switches rapidly from one user to the next, each user is given the impression that the entire computer is dedicated to her use even though it is shared among many users process.
Time Sharing operating systems are more complex than multiprogramming operating systems. To ensure orderly execution of process, the system must provide mechanisms for concurrent execution of process , process synchronization and communication.
Time Sharing या Multitasking, multiprogramming का ही विस्तार है. इसमें, सीपीयू jobs में स्विच करके सारे jobs execute करता है।एक बार ऑपरेटिंग सिस्टम साझा कई प्रयोक्ताओं की अनुमति देता है CPU समयबद्धन और समय का एक छोटा सा औषधि कंप्यूटर साझा के साथ एक उपयोगकर्ता प्रदान बहु क्रमादेशन. एक साझा प्रणाली के समय में प्रत्येक आदेश को कम ही थोड़ा CPU समय मात्रा के रूप में जानते अलग प्रक्रिया के लिए जरूरी है जाता है. के रूप में प्रणाली एक उपयोगकर्ता से अगले, प्रत्येक उपयोगकर्ता धारणा है कि पूरे कंप्यूटर उसे इस्तेमाल करने के लिए समर्पित हालांकि यह प्रक्रिया कई प्रयोक्ताओं के बीच साझा किया है दी गई है तेजी से स्विच.
समय साझा ऑपरेटिंग सिस्टम विविध ऑपरेटिंग सिस्टम की तुलना में अधिक जटिल है. प्रक्रिया को व्यवस्थित करने के क्रियान्वयन सुनिश्चित करने के लिए प्रणाली की प्रक्रिया के समवर्ती निष्पादन, प्रक्रिया तुल्यकालन और संचार तंत्र के लिए प्रदान करनी चाहिए.

Types of Operating System

1. Single User OS
–Single-user operating systems have no facilities to distinguish users, but may allow multiple programs to run in tandem
2. Single Tasking OS
–A single-tasking system can only run one program at a time
•Features of Single User and Single Tasking OS
–MS-DOS and Palm OS
–Take up little space on disk
–Run on inexpensive computer

3. Multi Taskin (Time Sharing) OS
   Multiprocessing and Multitasking are used interchangeable BUT
–multiprocessing sometimes implies that more than one CPU is involved.
–The ability to execute more than one task at the same time is called as multitasking.

Multi-tasking operating system allows more than one program to be running in concurrency.


In multitasking, only one CPU is involved, but it switches from one program to another so quickly that it gives the appearance of executing all of the programs at the same time.

This is achieved by time-sharing, dividing the available processor time between multiple processes that are each interrupted repeatedly in time slices.

Examples: Linux, Scolaris, Windows NT, AmigaOS





4. Multi User OS
A multi-user operating system extends the basic concept of multi-tasking with facilities that identify processes and resources belonging to multiple users, and the system permits multiple users to interact with the system at the same time.

Gives illusion that each user has his own machine.

Each user has a unique session.

5. Distributed System



A distributed operating system manages a group of distinct computers and makes them appear to be a single computer.

A distributed computer system is a collection of autonomous computer systems capable of communication and cooperation via their hardware and software interconnections.

Advantages of distributed System
Resources Sharing
Computation speed up
 load balancing
Scalability
Reliability
Fail-Safe

6. Real time systems
Real time systems are used in time critical environments where data must be processed extremely quickly because the output influences immediate decisions.

Real time systems are used for space flights, airport traffic control, scientific experiments industrial processes, sophisticated medical equipments, telephone switching etc

Each process is assigned a certain level of priority that corresponds to the relative importance of the event that it services.

The processor is normally allocated to the highest-priority process among those that are ready to execute.

7. Embedded Systems
OS embed on the System itself
Fast, Application specific
Examples Processor in modern washing machines, Cell phones, Control systems etc…

Booting Process

Booting may be defined as the process of loading the Operating System onto the RAM

Once the computer system is turned on,the following are the steps that a typical boot sequence involves.

• The BIOS performs Power on Self Test (POST) that checks to see whether the devices in the system are in perfect order. 

• After the BIOS is done with the pre boot activities, it searches for the Master Boot Record in the first physical sector of the bootable disk. When it finds one, it implies that it is the bootable partition, and, the Operating System loader, also called the boot strap loader is loaded from that partition onto the memory. A boot strap loader is a special program that is present in the boot sector of the bootable drive.

• MS DOS Operating system comprises of the following files

1. IO.Sys
2. MSDOS.Sys
3. Command.Com
4. Config.Sys
5. Autoexec.bat

Note: The first three files are mandatory while the rest two are optional. Further, the first three files should be present in the bootable drive of the disk

• The boot strap loader first loads the IO.SYS file. The IO.Sys file as the name suggests, is responsible for Input Output in the DOS environment. 

• The next file that is loaded is the MSDOS.SYS which is the core of the DOS operating system. The MSDOS.Sys file is  responsible for Memory management and Processor Management in the DOS environment. 

• The MSDOS.SYS file now searches to find the name of the command interpreter in the CONFIG.SYS file and when it finds one, it loads the same onto the memory. 

• If no command interpreter is specified in the CONFIG.SYS file, the COMMAND.COM file is loaded as it is the default command interpreter of DOS Operating system.

• The last file to be loaded and executed is the AUTOEXEC.BAT file that contains a sequence of DOS commands.

• Now, the prompt is displayed and you can see the drive letter of the bootable drive displayed on your screen indicative of the fact that the Operating System has been loaded successfully from that drive.

Command Interpretation

Command Interpreter is the interface between the user and the operating system. Many commands are given to the operating system by control statements. when a new job is started, or when user logs on to the system, a program that reads and interprets control statements is executed automatically. This program is called interpreter or command interpreter. It is also know as shell. The function is to get the next command and execute it.

The Command Interpreter must be user friendly. One style of user friendly interface is the mouse based window in Microsoft Windows. The mouse is moved to position the mouse pointer on images known as icon. Depending upon the position of mouse pointer, clicking a button can open program, select a file or folder or pull down menu that contains commands.

IO.SYS

IO.SYS is an essential part of MS-DOS. It contains the default MS-DOS device drivers  and the DOS initialization program.

In the PC booting process, the first sector of the boot disk is loaded into memory and executed. If this is the DOS boot sector, it loads the first three sectors of IO.SYS into memory and transfers control to it. IO.SYS then:

1. Loads the rest of itself into memory.
2. Initializes each default device driver in turn.
3. Loads the DOS kernel and calls its initialization routine. The kernel is stored in MSDOS.SYS with MS-DOS and in IO.SYS with Windows 9x.
4. Processes the MSDOS.SYS file with Windows 9x.
5. Processes the CONFIG.SYS file.
6. Loads command.com
7. Displays the bootsplash in Windows 9x.

MSDOS.SYS

MSDOS.SYS an important system file on MS-DOS and Windows 9x systems. It is run after IO.SYS. In MS-DOS, it contains the core operating system code, the kernel. The Windows Setup program creates a file called Msdos.sys in the root folder and sets the file with the Read-Only, System, and Hidden attributes. Unlike the Msdos.sys file in MS-DOS, this file is a text file. It contains a [Paths] section that lists the locations for other Windows files (such as the registry) and an [Options] section that you can use to personalize the boot process.

By default the file is located in the root directory of the bootable drive/partition (normally C:\) and has the hidden, read-only and system file attributes set.

CONFIG.SYS

CONFIG.SYS is the primary configuration file for the DOS and OS/2 operating systems. It is a special file that contains setup or configuration instructions for the computer system.

The commands in this file configure DOS for use with devices and applications in the system. The commands also set up the memory managers in the system. After processing the CONFIG.SYS file, DOS proceeds to load and execute the command shell specified in the COMMAND.COM. The command shell in turn is responsible for processing the AUTOEXEC.BAT file.

In DOS, CONFIG.SYS is located in the root directory of the drive from which DOS was booted.

CONFIG.SYS डॉस और ओएस ऑपरेटिंग सिस्टम के लिए प्राथमिक configuration फाइल है. यह एक विशेष फ़ाइल है जिसमे कम्प्यूटर के सेटअप या configuration details  शामिल है.

 उपकरणों और प्रणाली में अनुप्रयोगों के साथ प्रयोग के लिए इस फ़ाइल को विन्यस्त डॉस में हासिल है. भी व्यवस्था में सेट कमांड स्मृति प्रबंधक. बाद CONFIG.SYS फ़ाइल प्रसंस्करण, डॉस आय और लोड करने के लिए आदेश COMMAND.COM में विनिर्दिष्ट खोल निष्पादित अगर ऐसी कोई रेखा है. बदले में कमांड खोल AUTOEXEC.BAT फ़ाइल प्रोसेसिंग के लिए जिम्मेदार है.

इस फाइल के कोम्मोंड्स डॉस को configure करते है दूसरी devices and applications को इस्तेमाल करने के लिए. इसके कमांड्स memory managers को भी सेट उप करते है. की प्रोसेसिंग के बाद डॉस लोड और execute करता है command shell जो   COMMAND.कॉम में स्टोर है. command shell AUTOEXEC.BAT file. की प्रोसेसिंग के लिए जिम्मेवार होता है.

डॉस में CONFIG.SYS root directory में स्टोर होती है जहा से डॉस लोड होता है.