MaximumPC 2008 12

C

an a computer exist

without hardware?

It can if it’s a virtual

machine. A virtual machine

is soft ware that’s capable of

executing programs as if it

were a physical machine—it’s

a computer within a computer.

Virtual machines can be divid-

ed into two broad categories:

process virtual machines and

system virtual machines.

A process virtual machine

is limited to running a single

program. A system virtual

machine, on the other hand,

enables one computer to

behave like two or more

computers by sharing the

host hardware’s resources. A

system virtual machine con-

sists entirely of soft ware, but

an operating system and the

applications running on that

OS see a CPU, memory, stor-

age, a network interface card, and all the other

components that would exist in a physical

computer. For the remainder of this discussion,

we’ll use the term “virtual machine” to refer to

a system virtual machine.

Soft ware running on a virtual machine is

limited to the resources and abstract hardware

that the virtual machine provides. Since a virtual

machine can provide a complete instruction set

architecture (ISA, a defi nition of all the data types,

registers, address modes, external input/output,

and other programming elements that a given

collection of hardware is capable of working

with), a virtual machine can simulate hardware

that might not even exist in the physical world.

Using virtual machines, a computer can run

several iterations of an operating system—or

even several diff erent operating systems—with

each OS isolated from and oblivious to the exis-

tence of the others. The only requirement is that

each operating system must be capable of sup-

porting the underlying hardware. And, of course,

there must be enough resources (memory, hard

disk space, CPU cycles, and so

on) to support everything. You

could use a virtual machine to

run Linux on top of Windows, for

instance, or you could run two

versions of Windows and use one

as a sandbox for testing soft ware

you wouldn’t trust on a “real” machine.

MORE POWERFUL THAN A

SUPERVISOR

The soft ware that manages this trick is known

as a hypervisor. A Type 1 (native) hypervisor is a

program that runs directly on the host hardware,

i.e., as an operating system in and of itself. Mi-

crosoft ’s Hyper-V, formerly known as Windows

Server Virtualization, is one example of a Type

1 hypervisor. A Type 2 (hosted) hypervisor, such

as Microsoft ’s Virtual PC 2007, runs on top of

another operating system.

IBM developed the technology for its

big-iron mainframe computers in 1967, but

the Intel x86 architecture at the foundation

of IBM PC-compatible machines was not well

suited for running hypervisors. Achieving full

virtualization required exceedingly complex

code, which hampered runtime performance.

Although it remained a fi xture in mainframe

and midrange computer systems, virtual

machine technology saw very little progress

during the 1980s and 1990s.

In the last few years, however, AMD and

Intel both developed extensions to their x86

architectures that render newer CPUs much

more suitable for running hypervisors. AMD

has dubbed its extensions AMD Virtualiza-

tion (AMD-V); Intel calls its extensions Intel

Virtualization Technology (Intel VT). AMD-V is

present in many newer AMD CPUs, including

the Athlon 64 and Athlon 64 X2 (socket AM2

only), the Phenom X3 and X4, and second-

generation Opteron server parts.

IBM DEVELOPED VIRTUAL

MACHINE TECHNOLOGY FOR

ITS BIG-IRON MAINFRAME

COMPUTERS IN 1967....

70 |MAMAMAXIMXIMXIMXIMUUUUMMPPPCC|DEC 08 |www.maximumpc.com

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R&D^

EXAMINING TECHNOLOGY AND PUTTING IT TO USE

This software technology can make one computer do the work of several —MICHAEL BROWN

Virtual Machines

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Virtual Machines

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Two Types of Virtual Machines

HOW IT WORKS

System virtual machines fall into two categories: Type 1 hypervisors (left), which run directly on the host hardware,

and Type 2 hypervisors (right), which run on top of another operating system. Both are capable of running multiple

independent instances of one operating system or different operating systems, all of which behave as though they

are solely in control of the system.

Operating

System

Application

Operating

System

Operating

System

Application Application

Operating

System

Application

Operating

System

Operating

System

HYPERVISOR HYPERVISOR

HOST HARDWARE

HOST HARDWARE

Application Application

TYPE 1 HYPERVISOR TYPE 2 HYPERVISOR

OPERATING SYSTEM