Operating Systems

Operating System: Basic software of a computer, that is executed as the first program. Main tasks:

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hides the hardware

• user clicks on a window instead of programming input channels,
• handles files instead of blocks on disks.

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runs other programs

• user starts text processor, browser and a sound player,
• the operating system runs them without giving up control.

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distributes resources

• text processor, browser and sound player share the memory, the screen and the processor.

• Different kinds of operating systems:

single tasking OS: runs only one application at a time. A new one starts only, after the last one has finished.

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MS-DOS

multi tasking OS: allows several applications to run simultaneously.

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Windows95/98/NT, UNIX

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A multi tasking OS assigns a certain amount of time (a ``time slice'') to each running program. After that time it switches to the next one, usually in a round robin way or according to some priorities.

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Since the time slices are rather short (typically 1/50s), the user has the impression of several programs running at the same time.

single user OS: only one user (at a time) can work with the computer.

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MS-DOS, Windows95

multi user OS: allows several users to work with a computer at the same time. This implies multi-tasking, of course.

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UNIX, e.g. Linux, HP-UX, Solaris

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Important tasks of a multi user OS:

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identifies user

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shields data and applications from other users

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distributes resources between users in a ``fair'' way

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First operation when working with a multi-user system: identify yourself (``log in''). Usually done by providing a user name and a password.