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Unit 3: Using Operating System

Notes
Figure 3.7: Windows XP & Suse Linux on Mac OS X with Parallels Desktop

Management systems are implicitly designed to be used by multiple users, typically one
system administrator or more and an end-user community.
It’s important to differentiate between multi-user operating systems and single-user
operating systems that support networking. Windows 2000 and Novell Netware can each
support hundreds or thousands of networked users, but the operating systems themselves
aren’t true multi-user operating systems. The system administrator is the only “user”
for Windows 2000 or Netware. The network support and all of the remote user logins the
network enables are, in the overall plan of the operating system, a program being run by the
administrative user.

3.2.4 Multiprogramming
Multiprogramming is a rudimentary form of parallel processing in which several programs
are run at the same time on a uniprocessor. Since there is only one processor , there can be
no true simultaneous execution of different programs. Instead, the operating system executes
part of one program, then part of another, and so on. To the user it appears that all programs
are executing at the same time.
If the machine has the capability of causing an interrupt after a specified time interval, then
the operating system will execute each program for a given length of time, regain control,
and then execute another program for a given length of time, and so on. In the absence of
this mechanism, the operating system has no choice but to begin to execute a program with
the expectation, but not the certainty, that the program will eventually return control to the
operating system.

If the machine has the capability of protecting memory, then a bug in one program is
less likely to interfere with the execution of other programs. In a system without memory
protection, one program can change the contents of storage assigned to other programs or
even the storage assigned to the operating system. The resulting system crashes are not
only disruptive, they may be very difficult to debug since it may not be obvious which
of several programs is at fault.

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