Operating Systems
An operating system (OS) is like a facilitator, providing an environment for application programs to run in. It provides an interface between computer hardware and the user of the application programs. Application programs are a stored set of instructions for responding to a specific request. Hardware are the physical devices that make up the system such as the monitor, keyboard, printer, ect. The OS typically includes anywhere from tens to thousands of built-in subroutines called functions which applications can call to perform basic tasks such as displaying a string of text on the screen or inputting a character from the keyboard. The OS allows the user to interact with and obtain information from the hardware through a series of layers. The command layer is the only part of the OS that interacts with the user and sometimes is referred to as the shell. It is the user's interface to the OS in that the user can request system services via a Job Control Language (JCL), which is the set of commands and their syntax requirements, or through a GUI (Graphical User Interface), the newest command language. The command interpreter decides which services must be executed to process the request and passes these to the service layer. The
One of an operating system's most important components is its file system - the part responsible for managing files stored on hard disks and other mass-storage devices. An operating system provides high-level functions that applications can call to control file I/O, which covers the opening, closing, creating, deleting, and renaming of files. To read a chunk of data from a file, the application calls an operating system function and passes the operating system the file handle (which uniquely identifies an open file), the address in memory where the data should be copied, and the number of bytes to copy. The OS has to identify each sector on the disk that holds a portion of the requested file and program the drive controller to retrieve the information, sector by sector. It has to do this efficiently so that the information is retrieved quickly. The OS finds the right locations on the disk because it keeps a map that contains the name of each file and a record of where it is stored. DOS and Windows95 use a file allocation table as a map to find the locations. The next fundamental duty of an operating system is to manage the memory - or storage of data - available to it and its applications. The goal in memory management is to give each application the memory it needs without infringing upon memory used by other applications. When a program is loaded and ran, the OS allocates a block of memory to hold the program's code and data. If a program requires additional memory, it can call a function in the operating system to request more. DOS does not handle memory very well because it runs in real mode, where only 1MB o
Some common words found in the essay are:
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Approximate Word count = 1102
Approximate Pages = 4 (250 words per page double spaced)
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