Front cover - IBM Redbooks

360 Lotus Security Handbook
Operating system functions
In short, the operating system must provide two main functions. The OS must:
► Manage the resources available to the computer system
► Provide a reliable, stable, secure, and consistent interface for applications to
access the computer's resources
The first function is critical because it defines how applications access the
system's resources. By controlling the various aspects of how hardware and
software are used, the OS ensures that every application gets a chance to use
the processor.
The second function defines the methods by which an application can access
these resources. Because the OS often acts as a buffer between an executing
program and the hardware, it needs to provide some means of allowing
applications to access resources without needing to know the details of each and
every unique computer system.
Operating system types
There are four main types of operating systems, classified according to the types
of programs they support and the way these programs interact with users:
1. Real-time operating system: This operating system is most often found in
robotic machinery and scientific devices (QNX is a good example). It doesn't
provide much room for user operation, with the exception of some
configuration changes. Typically, this operating system contains highly
polished timing mechanisms due to the impact even the slightest error could
have in automated production or measurements.
2. Single-user, single task operating system: This type of operating system is
used by devices such as PDAs or other miniature computers (Palm OS is a
good example). It basically allows one user to operate one program at a time.
If another program is needed, the user must close the currently executing
application.
3. Single-user, multitasking operating system: This type of operating system is
most familiar because it includes most Microsoft Windows systems. In this
operating system, a user can open multiple programs and jump back and
forth between applications as required. In fact, there is much debate that
although Windows Server Operating Systems appear to be multiuser
systems, they're actually single-user, multitasking operating systems (with the
exception of Terminal Services).
4. Multi-user operating system: A true multi-user operating system allows many
users to access the computer's resources simultaneously. A common
example of this type of OS is Linux. In this type of system, the OS manages