Information Technology Reference
In-Depth Information
system designer depend on the physical capabilities of the hardware, and on
the application and user demand for that hardware. Here are some examples of
operating systems that the reader might have used recently:
Desktop, laptop, and netbook operating systems. Examples in-
clude Windows 7, MacOS, and Linux. These systems are characterized by
having a single user at a time, many applications, and many I/O devices.
One might think that with only one user, there is no need to design the
system to support sharing, and indeed the initial personal computer oper-
ating systems took this approach. They had very limited ability to isolate
different parts of the system from one another. Over time, however, it
became clear that stricter fault isolation was needed to improve system
reliability and resilience against computer viruses.
Smartphone operating systems. A smartphone is a cellphone with
an embedded computer capable of running third party applications. Ex-
amples of smartphone operating systems include iOS, Android, Symbian,
WebOS, Blackberry OS and Windows Phone. Obviously smartphones
have only one user, but they still must support many applications. Be-
cause applications downloaded off the Internet might have viruses that
attempt to surreptitiously take control over the phone, the operating sys-
tem must be designed to protect itself from misbehaving applications and
to protect applications from each other.
Embedded systems. Over time, computers have become cheap enough
to integrate into any number of consumer devices, from cable TV set top
boxes, to microwave ovens, to the control systems for automobiles and
airplanes, to LEGO robots, and to medical devices such as MRI machines
and WiFi based intravenous titration systems. Embedded devices typi-
cally run a customized operating system bundled with the task-specific
software to control the device. Although you might think these systems
as too simple to merit much attention, software errors can have quite dev-
astating effects. One of the earliest documented examples of a computer
causing human deaths was the Therac-25 in the mid-1980's. The problem
occurred because of programming errors in the operating system code for
a medical device, errors that could have been prevented had the system
developers followed the design paradigms outlined in this topic.
Virtual machines. As we mentioned, a virtual machine monitor is an
operating system that can run another operating system as if it were an
application. Example virtual machine monitors include VMWare, Xen,
and Windows Virtual PC. Virtual machine monitors face many of the same
challenges as other operating systems, with the added challenge posed by
coordinating a set of coordinators. The operating system running inside a
virtual machine makes resource allocation and fault isolation decisions as
if it is in complete control of its resources, even though it is in fact sharing
the system with other operating systems and applications.
