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SubjectRe: Wlinux vs. LWin95, looking at the alternative
nuke@bayside.net wrote:
>
> On Tue, 14 Apr 1998, Rik van Riel wrote:
>
> > On Thu, 9 Apr 1998, Richard Jones wrote:
> > > As I understand it, there are only a few commands that
> > > can't be properly virtualized on the i386 processors. Is
> >
> > They will trap with 'invalid instruction', and can (in
> > theory) be deassembled and emulated JIT...
> > [well, _can_ they??? I don't know enough about x86 to really know]
>
> anything can be emulated. don't ever think otherwise. there is currently a
> working nintendo 64 emulator for win95. *anything* can be emulated on
> *any* hardware (provided the display is suitable).
>
> > > it possible that there are only a few tiny patches to the
> > > Win95 `binary' that need to be made to make it behave itself
> > > in a virtualized 386 environment? Perhaps someone who knows
> > > more about this than me can comment ...

Sure, anything can be *emulated*, but the real problem is
running the emulated binary at near to full speed as possible.
This is why virtualizing the 386 is a better approach (given
that writing a really good JIT compiler for 386 opcodes is
difficult).

I don't know the full intricacies of this, so this should be
taken with a pinch of salt, but as I understood it, various
instructions which should cause traps don't. An example is
popping a stack word into the flags register which could alter
certain system-specific flags in kernel mode. The hypothesis
(not proven) is that Win95 probably doesn't do such things
in many places, and so it might be feasible to patch the binary
where it does this to make it behave in a virtualized 386
environment.

Of course, you are then open to MS changing the way future
releases of Windows work to make these patches more and more
difficult to achieve, particularly if this approach were
to be successful and everyone started running their Win
programs under Linux :-)

Rich.

[Paul K. - I'm cc-ing this discussion on Linux-kernel to you, since
it might be in your field]

--
Richard Jones rjones@orchestream.com Tel: +44 171 460 6141 Fax: .. 4461
Orchestream Ltd. 262a Fulham Rd. London SW10 9EL. "you'll write in
PGP: www.four11.com telegraphic, or you won't write at all" [Céline]
Copyright © 1998 Richard W.M. Jones

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