[vox-tech] definition of a virtual machine

[Michael Wenk]

On Monday 31 March 2003 12:39 pm, Peter Jay Salzman wrote:
> can someone tell me if this is a formally correct explanation of what a
> virtual machine is?
>
> pete
>
>
> A "real computer" provides an operating system many things, including a
> CPU, I/O channels, memory, a BIOS to provide low level access to
> motherboard and I/O resources, etc.  When an operating system wants to
> write to a hard drive, it communicates through a device driver that
> interfaces directly with the hardware device memory.
>
> However, it's possible to give a program all the hardware resources it
> needs.  When it wants to access a hard drive, give it some memory to
> write to.  When it wants to set an IRQ, give it some bogus instructions
> that lets it think it set an IRQ.  If you do this correctly, then in
> principle, there's no way for the application to know whether it's
> really accessing hardware or tricked by being given resources which
> simulate hardware.  A virtual machine is the environment which tricks
> applications into believing they're running on a real computer.  It
> provides all the services that a real computer would provide.
>
> VM's were used initially in the 1960's to emulate time shared operating
> systems, but these days we use them to run software which was written
> for foreign operating systems, or more commonly, an entire operating
> system.  Because of the nature of the VM, the foreign OS can't tell the
> difference between operating in a VM or in a "real" machine.
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Well, I can't find my old CS book, but I prefer the Java definition for a 
VM: 

"An abstract specification for a computing device that can be implemented in 
different ways, in software or hardware. ..." 

It goes on to more java and jvm specific information.  I think the 
important keyword is abstract.  You're definition is more specific, but 
a VM can describe just about any computing device, and arguably 
every computing device may not have the components you listed.  

Mike

--
wenk@praxis.homedns.org
Mike Wenk