OPERATING SYSTEM©
An operating system is a software program designed to act as an interface between a user and a computer. It controls the computer hardware, manages system resources, and supervises the interaction between the system and its users. The OS also forms a base on which application software are developed and executed.

“Some sort of layer of programs is required that will hide the complexity of the underlying hardware, manages all parts of the system and present the user with an interface. This layer or set of programs is the OS.”

Functions of Operating System:
1. Command Interpretation 2. Peripheral Management 3. Run Time Memory Management 4. Disk Management 5. Process Management 6. Supervise the Computer System 7. Controlling input and output peripherals 8. Directing the flow of data 9. Maximize the use of hardware resources 10. To assure that number of programs can be run without human intervention 11. Scheduling and loading of programs 12. Provide appropriate response to the events 13. Handling errors and using corrective routines 14. Protecting hardware, software and data from improper use 15. Furnishing the complete records of all events and activities

Types of Operating System
Single-user OS: • • It is used for stand alone Computer (PC) It is further classified into two types: (1) Single-user Single-tasking (MS DOS) (2) Single-user Multi-tasking (MS Windows)

Multi-user OS / Network Operation System: • • • It handles multiple users as well as multiple peripheral devices simultaneously. It also handles multiple tasks at the same time and therefore known as multi-user multi-tasking OS. Examples: UNIX, Linux, Windows (NT or Network Edition)

Functions of Network Operating System:
NOS do everything a stand alone OS does and more in much more complex environment. It is the heart and soul of the computer network. Apart from functions of stand alone OS, an NOS performs following functions:

1. Directing data traffic throughout the network 2. Security and authority (password and access right) 3. Managing flow of data between variety of different work stations 4. Managing users’ requests 5. Managing communication between the network and remote sites 6. Make its services as transparent as possible to each user 7. Support software metering and address massing 8. Provide GUI support and file backup operations

Internal vs. External DOS Commands
DOS comes with a built-in command interpreter called COMMAND.COM. This COMMAND.COM file is loaded during the boot process, which means that COMMAND.COM is resident in memory at all times, and the commands that it understands are available to the user at all times. These commands contained within COMMAND.COM are known as Internal DOS commands. Not all DOS commands are understood by COMMAND.COM. There are commands called external DOS commands that reside in separate files on your hard drive, and must be called specifically for you to use them. DOS could only address a very limited amount of memory (1MB total). Since COMMAND.COM is loaded into memory at the beginning of the boot process, and stays resident in memory at all times, it would not make sense to load commands that you would only use infrequently, or to load commands that only certain users would ever need. So these commands were placed in external files where they could be accessed if needed. If you look in your DOS directory on your hard drive, you will see these external commands represented by files that are either *.EXE or *.COM files. You won't see the internal commands here, though, because these commands are all contained within COMMAND.COM. Internal DOS Commands These commands are contained within one common file COMMAND.COM. These commands are take very less memory space. These commands are always stay resident in memory and therefore available to the users at all times. These commands are used frequently by maximum users. These commands are not available in DOS directory because they contained within COMMAND.COM. List of Internal DOS Commands: cd (chdir), cd.., cd\, md (mkdir), dir, rd (rmdir), copy, del, erase, ren, type, break, cls, date, time, exit, prompt, ver, vol, etc. External DOS Commands These commands are reside in separate files. These commands are large and take up a lot of space. These commands are not available all the times in memory and specifically called by users to use them. These commands are used infrequently & only certain users would ever need them. These commands represented by either *.EXE or *.COM files & available in DOS directory. List of External DOS Commands: attrib, backup, restore, tree, deltree, undelete, xcopy, chkdsk, diskcomp, diskcopy, edit, find, format, unformat, help, mem, print, scandisk, sort, etc.

UNIX What is UNIX? Why UNIX? What are shell and Kernel? Describe the features of UNIX.©

What is UNIX? UNIX was developed at AT & T by Dennis Ritchie with the intention of creating a portable Network Operating System (NOS). It is finally written in C, and now able to runs on all machines. UNIX is a NOS that can be used by several users concurrently. In other words, a single copy of the OS installed on a single disk can serve the needs of hundreds of user. UNIX is security conscious and can be used only by those persons who maintain the account with the computer system.

Why UNIX? 1. PORTABILITY: The UNIX OS were mainly written in assembly and ‘C’ languages that used less memory and executed instructions faster than those written in other languages. The UNIX system can be placed on different machines in relatively easy way. Its design has a very small portion that depends on actual hardware of underlying machine. 2. PORTABLE APPLICATION PROGRAMS: A program written on one machine can be put to another machine and executed there without great difficulty. 3. BACKGROUND PROCESSING: With UNIX user can execute more jobs simultaneously and thus, increase productivity. 4. HIERARCHICAL FILE SYSTEM: With UNIX, a user can organize the information in structured fashion so that he/she can handle with ease or confidence. 5. COMMAND INTERPRETER: UNIX in addition to execution of commands typed by the user, acts as a programming language, so the user can store a number of commands to be executed in command file and execute all of them by giving the name of the file.

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6. PIPES: By piping user can offer the output of one command as the input to the another. Thus, various commands can be combined in useful ways. 7. UTILITIES: UNIX provides a large collection of useful programs that are already written by different people. What are shell and Kernel? Whenever any thing is typed through the keyboard, it is thoroughly examined and processed by a separate agency. The command will be executed only when the examiner finds it suitable for execution. In this case, the examiner, in the course of processing, decided that all multiple spaces are to be reduced to a single one. This agency is called the ‘Shell’. No command can be executed unless it obtains the clearance of the shell.

The Shell, when analyzing the command and its associated arguments, often modifies and simplifies its input before it forwards it to another agency that actually executes the command. This agency is called the ‘Kernel’. UNIX permits the use of complex command structures that can’t be understood by the Kernel, the Shell has to take on the role of interpreter to translate them into form that the Kernel can understand.

Different types of Shell: A number of shells have been developed in response to different needs of user. Some of the popular shells are listed below: Bourne Shell: This is the original command processor developed at AT & T named after its developer, Stephen R. Bourne. This is the official and most widely used shell at present. It is fasted UNIX command processor available and can be used on all UNIX systems. It used the dollar sign ($) as a prompt. The executable file name is sh. C Shell: This is developed by William Jay and other at the University of California at Berkeley, and gets its name from its programming language, which resembles the C programming language in syntax. It used the percent Sign (%) with a counter that keeps track of how many command have been executed. The executable file name is csh. Korn Shell: Developed by David Korn, this combines the best features of both the above shells. Although very few systems currently offer this shell, it is slowly gaining popularity. The executable file name is ksh. Restricted Shell: This is a restricted version of the Bourne Shell. It is typically used for guest logins- (users who are not part of the system). The executable filename is rsh.

Describe the features of UNIX. 1. A multi user system: In UNIX, the resources are actually shared between all users, be it the memory (RAM), the CPU or the hard disk. The computer breaks up a unit of time into several segments, and each user is allotted a segment. So, at any point of time, the machine will be doing the job of a single user. The moment the allocated time expires, the previous job is kept in a background, and the next user’s job is taken up. This process goes on till the clock has turned full circle and the first user’s job is taken up once again. The Kernel does this several times in one second and keeps all ignorant and happy. 2. A multi tasking system: In UNIX, a single user can run multiple tasks concurrently. You can switch jobs between background and foreground, suspend, or even kill them. This is done by running on job normally and the others in the background. 3. Pattern Matching: This feature of UNIX makes it attractive to the end user. It easily matches number of file names, number of similar patterns, etc. 4. The tool Kit: UNIX is a collection of tools, which lets you dispense with programming work for many applications. 5. Programming facility: UNIX is highly programmable, it was designed for a programmer, not a casual end user. The UNIX shell programming language has all the necessary ingredients like control structures, loops and variables that establish it as a programming language in its own right. 6. System calls and Libraries: UNIX is written in C. Though there are a couple of hundred commands handling specialized functions, they all do that by using a handful of functions call system call. These calls are built into the Kernel and all library functions and utilities are written using them. 7. Windowing Systems: Currently, UNIX is a fully commercial product used by different users with varied background and aptitudes. Now UNIX had to come up with its own GUI. 8. Documentation: Today, there is no feature of UNIX on which a separate textbook is not available.

The two powerful concepts: “Files have places and processes have life” Files are situated in space that makes it Processes have parents, children and easy to locate them with reference to a pre- grandchildren; those are born, even reborn, determined place. File can be placed at a and die. Like files, processes also follow a specific location in the file system and also separate hierarchical free structure and ‘move’ from one place to another. because a process is simply the ‘time image’ of an executable file, you can access and control it with greatest ease.

UNIX *UNIX itself comprises hundreds of files that are provided along with a UNIX-based system. *UNIX is larger in terms of storage requirement. *A hard disk created for use on UNIX has a much larger hierarchical structure.

DOS *While MS-DOS consists of only 3 files

*MS-DOS is much smaller than UNIX.

*A hard disk that is created for use on MSDOS has only one directory—the root directory denoted by \. * UNIX itself forces its users to work in a * MS-DOS provides the facility to create hierarchical structure directories but does not enforce it on its users

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