Virtual machines

Terrence W. Zellers
Thu, 22 May 1997 01:04:50 +0000

    I posted the following to a few days
ago.  Bill House said I may find this list interesting and I 
have, though I know very little about lisp per se and have only
a limited general knowledge of machine architecture.  (BTW.
thanks Bill).

    Apologies if this is old ground, I've only gotten through
some of the archives thus far (is there a FAQ?)


   I'd like to jump in here.  Let me say at the outset that I haven't
even *looked* at the JVM architecture and know nothing about it save
passing mentions I've encountered.   But as a more-or-less expert on
the original real "VM" from IBM I'd like to make a suggestion to
anyone *thinking* about what an idealized virtual machine should do.

   IMO it ***MUST*** be capable of fully virtualizing itself in 
execution, not just emulation.   This means it must have a paging
prefix algorithm, the ability to intercept any instructions which
deals with "real" rather than "virtual" hardware,  and the
interception and determination of any illegal program state (invalid
opcode, invalid address, etc).   Admittedly this is not trivial but it
buys the ability to completely virtualize a process as if it were
running on "real" hardware.   

    This in turn, buys the ability to create an OS which can run
indefinitely deep under itself. (I've reliable reports from IBM of VM
running 8 levels deep).  That means that you can test different
versions of the OS on one live running box.  You can concurrently test
different versions of the same program.  You can create safe "cells"
for users to run OS level software without danger of them corrupting
your "real" OS.  You can create and test software for hardware which
**doesn't exist on your real system**.

   As someone who knows VM very well, I'm afraid I'm not communicating
this very clearly, but it's part and parcel of what I work with every

   Anyway there is some discussion of this kind of thing, though not
as deep or detailed as I hoped it would be on the archive logs of
FREEVM-L on  The list is pretty much dead
I'm afraid, but anyone really interested ought to at least look at
what the real, honest, original VM has been doing for years.  The
mainframe may (or maybe not) be dying, but I think the idea of a
self-idealizing virtual machine is valuable enough to perpetuate.


    I think that in the argot of the present company I'm trying to
say that the architecture should be such that the OS itself can be
implemented as an instancable(sic?) object (forgive me, I'm fairly 
new to OO as well).

                                                   -- TWZ
| Terrence W. Zellers  | "Careful what you carry 'cause |
| | the man is wise, you ars still |
|                      | an outlaw in their eyes..."    |