لخّصلي

THE CRUX OF THE PROBLEM:
HOW TO VIRTUALIZE RESOURCES
One central question we will answer in this book is quite simple: how
does the operating system virtualize resources? This is the crux of our
problem. Why the OS does this is not the main question, as the answer
should be obvious: it makes the system easier to use. Thus, we focus on
the how: what mechanisms and policies are implemented by the OS to
attain virtualization? How does the OS do so efficiently? What hardware
support is needed?
We will use the “crux of the problem”, in shaded boxes such as this one,
as a way to call out specific problems we are trying to solve in building
an operating system. Thus, within a note on a particular topic, you may
find one or more cruces (yes, this is the proper plural) which highlight the
problem. The details within the chapter, of course, present the solution,
or at least the basic parameters of a solution.
is called the operating system (OS)
3
, as it is in charge of making sure the
system operates correctly and efficiently in an easy-to-use manner.
The primary way the OS does this is through a general technique that
we call virtualization. That is, the OS takes a physical resource (such as
the processor, or memory, or a disk) and transforms it into a more general,
powerful, and easy-to-use virtual form of itself. Thus, we sometimes
refer to the operating system as a virtual machine.
Of course, in order to allow users to tell the OS what to do and thus
make use of the features of the virtual machine (such as running a program,
or allocating memory, or accessing a file), the OS also provides
some interfaces (APIs) that you can call. A typical OS, in fact, exports
a few hundred system calls that are available to applications. Because
the OS provides these calls to run programs, access memory and devices,
and other related actions, we also sometimes say that the OS provides a
standard library to applications.
Finally, because virtualization allows many programs to run (thus sharing
the CPU), and many programs to concurrently access their own instructions
and data (thus sharing memory), and many programs to access
devices (thus sharing disks and so forth), the OS is sometimes known as
a resource manager. Each of the CPU, memory, and disk is a resource
of the system; it is thus the operating system’s role to manage those resources,
doing so efficiently or fairly or indeed with many other possible
goals in mind. To understand the role of the OS a little bit better, let’s take
a look at some examples.
3Another early name for the OS was the supervisor or even the master control program.
Apparently, the latter sounded a little overzealous (see the movie Tron for details) and thus,
thankfully, “operating system” caught on instead.