Operating System: Basic software of a computer, that is executed as
the first program. Main tasks:
- hides the hardware
- user clicks on a window instead of programming input channels,
- handles files instead of blocks on disks.
- runs other programs
- user starts text processor, browser and a sound player,
- the operating system runs them without giving up control.
- 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.
multi tasking OS: allows several applications to run simultaneously.
- Windows95/98/NT, UNIX
- 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.
- 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.
- MS-DOS, Windows95
multi user OS: allows several users to work with a computer at the
same time. This implies multi-tasking, of course.
- UNIX, e.g. Linux, HP-UX, Solaris
- Important tasks of a multi user OS:
- identifies user
- shields data and applications from other users
- distributes resources between users in a ``fair'' way
- First operation when working with a multi-user system: identify yourself
(``log in''). Usually done by providing a user name and a password.
Peter Junglas 8.3.2000