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VIRTUAL MACHINES

In its broadest sense, "virtual machine" refers to the
universality of the computer: any computer can simulate any
other.  We can make any computer assume the identity of any other
machine simply by loading it with the software simulator of the
other.  No other machine has this capability.

Within the computing field, "virtual machine" has been used in
several related, but different senses.  You need to learn them
and distinguish among them.

(1) IBM's operating systems all offer virtual machines.  In this
case the virtual machine offered is an exact replica of the
actual hardware, except that it has less memory than the real
machine and that sensitive instructions are not part of its
instruction set.  The IBM Virtual Machine Monitor partitions the
real memory and allocates each part to one virtual machine.  It
runs each virtual machine in the "user" mode so that the
sensitive instructions are not executable; an attempt by a
program to execute a sensitive instruction causes an interrupt,
allowing the VMM to simulate that instruction in a way that does
not disturb the computing environments of any other virtual
machine.  IBM uses the virtual machine system to obtain perfect
partitioning -- applications in different virtual machines cannot
interfere with one another.  OS developers can test new versions
of an OS without fear of crashing the system or losing anyone's
data -- the worst effect is to crash the virtual machine.  IBM
spent a great deal of time designing the archtecture to achieve
these properties.  The system is even recursive: you can run the
VMM inside one of the virtual machines and partition it.   Note
that these virtual machines run at nearly the same speed at the
real machine because most of the instructions are directly
executable; only the sensitive instructions trap to the VMM.

(2) Many PCs run software that simulates the instruction set of
another computer, allowing programs for the other computer to be
run on the PC.  An example is the program "Virtual PC" that runs
on MacIntosh computers.  It allows you to run Windows on the Mac
and run all applications that run on Windows.  Unlike the IBM
simulation, the OS traps every instruction of the virtual machine
and simulates it with instructions from the PC instruction set.
These virtual machines run much slower than their real
counterparts.

(3) Many components of operating systems are formulated as
"abstract machines".  An abstraction machine encapuslates the
implementation knowledge about a set of operations applicable to
a class of objects.  The user of the abstract machine gets to use
the objects through a high-level (usable) interface and does not
have to be concerned with inner details of implementation.  The
functions of an abstract machine are software routines contained
inside the machine's perimeter; to the user, these functions look
like extensions to the instruction set of the machine.  Abstract
machines can be nested, and the outer machine may hide some of
the instructions of the inner one.  The principle of abstraction
as used here was formally defined in the 1960s, when it was a
widely practiced method; it was called "data abstraction".  It
eventually matured into object oriented programming and design.
Some authors refer to these abstract machines also as virtual
machines.

(4) The user-level process in Unix is defined as a "program
running on a simulated computer".  Thus this process is actually
a virtual machine.  Unix defines a standard form for the user
process.  Invoking any program is simply a matter of loading the
program into a user process and starting it.  Since the user
processes all have the same external interface, it's easy to
connect them to a variety of input or output sources and to
pipelines.  This is the type of virtual machine that your
course project examines.

In our course project, we will be implementing the Unix-style
virtual machine as objects managed by a VMM; thus the VMM is an
abstract machine.  Be wary of the different uses of "virtual
machine" and don't allow yourself to be confused by them.