Operating SystemI, Definition and typesThe operating system (OS) can be considered as the most important program that runs on a computer. Everygeneral-purpose computer must have an operating system to provide a software platform on top of which otherprograms (the application software) can run. It is also the main control program of a computer that schedulestasks, manages storage, and handles communication with peripherals. The central module of an operatingsystem is the 'kernel'. It is the part of the operating system that loads first, and it remains in main memory.Because it stays in memory, it is important for the kernel to be as small as possible while still providing all theessential services required by other parts of the operating system and applications. Typically, the kernel isresponsible for memory management, process and task management, and disk management.In general an application software must be written to run on top of a particular operating system. Your choice ofoperating system, therefore, determines to a great extent the applications you can run. For PCs, the mostpopular operating systems are Windows 95/98, MS-DOS (Microsoft-Disk Operating System), OS/2, but othersare available, such as Linux, BeOS…For large systems, the operating system has even greater responsibilities and powers. It is like a traffic cop: itmakes sure that different programs and users running at the same time do not interfere with each other. Theoperating system is also responsible for security, ensuring that unauthorized users do not access the system.From this point of view, operating systems can be classified as follows:§ Multi-user: Allows two or more users to run programs at the same time. Some operating systems permithundreds or even thousands of concurrent users.§ Multiprocessing: Supports running a program on more than one CPU.§ Multitasking: Allows more than one program to run concurrently.§ Multithreading: Allows different parts of a single program to run concurrently.§ Real-time: Real time operating system (RTOS) responds to input instantly. General-purpose operatingsystems, such as DOS and UNIX, are not real-time.An OS is a 16-bit operating system if it processes 16 bits of data at once, e.g.: DOS. On the other hand,Windows 98 and OS/2 Warp are 32-bit operating systems because they can process 32 bits of data at once.A network operating system (NOS) is an operating system which makes it possible for computers to be on anetwork, and manages the different aspects of the network. Some examples are Windows for Workgroups,Windows NT, AppleTalk, DECnet, and LANtastic…SoftwareSystem ApplicationDevelopment Execution General Purpose Specific PurposeII, Storage managementTo manage storage, the operating system uses files. A file is a collection of data or information that has a name,called the filename. There are many different types of files: data files, text files, program files… Different typesof files store different types of information and therefore the type of the file reflects its usage. In DOS,Windows 95 and some other operating systems, one or several letters are added at the end of a filename andthey are called the extension. Filename extensions usually follow a period (dot) and indicate the type ofinformation stored in the file. For example, in the filename EDIT.COM, the extension is COM, which indicatesthat the file is a command file.The operating system has a file management system to organize and keep track of files. Although all theoperating systems provide their own file management system, you can install another file management systeminteracting with the operating system and providing more features, such as improved backup procedures andstricter file protection. The most commonly used file systemis the hierarchical one that uses directories to organize filesinto a tree structure. There can be one tree structure perstorage unit or one tree structure including all the storageunits and representing by some special directories.A directory is a special kind of file used to organize otherfiles. It contains bookkeeping information about files thatare, figuratively speaking, beneath them. You can think of adirectory as a folder or cabinet that contains files and,perhaps, other folders. In fact, many graphical user interfacesuse the term folder instead of directory.Computer manuals often describe directories and file structures in terms of an inverted tree. The files anddirectories at any level are contained in the directory above them. The topmost directory is called the rootdirectory. A directory that is below another directory is called a subdirectory. A directory above a subdirectoryis called the parent directory.To access a file, you may need to specify the names of all the directories above it, that is to say specify itsaccess pathname (short: path). The absolute path of a file is le list of the names of all the directories above itfrom the root. The operating system also keeps track of the directory in which you are currently working.Pathnames that do not start with the root directory are assumed by the operating system to start from theworking directory; they are relative paths. Each operating system has its own rules for specifying paths. In DOSsystems, for example, the root directory is named '\', the parent directory can be referred as '..' , and eachsubdirectory is separated by an additional backslash. In UNIX, the root directory is named /, and eachsubdirectory is followed by a slash. In Macintosh environments, directories are separated by a colon.So for example valid DOS absolute paths for the two files of the figure 1 are:\DOS\Example.txt and \Other\Mine\Account.xlsNow if the working directory is '\Other', DOS relative paths are: ..\DOS\Example.txt and Mine\Account.xlsA wildcards character is a special symbol that stands for one or more characters. Many operating systems andapplications support wildcards for identifying files and directories. This enables you to select multiple files witha single specification. For example, in DOS and Windows, the asterisk (*) is a wild card that stands for anycombination of letters and the question mark is a wild card that stands for any single letter. The filespecification 'm*' therefore refers to all files that begin with m. Similarly, the specification 'm*.doc' refers to allfiles that start with m and end with .doc. Many word processors also support wild cards for performing