Re: [ros-dev] (Free)DOS subsystem

[Imre Leber]

> -----Original Message-----
> From: Myria [mailto:[e-mail]]
> Sent: Tuesday, August 29, 2006 10:14 AM
> To: 'ReactOS Development List'
> Subject: Re: [ros-dev] (Free)DOS subsystem
>
> I've wanted to write an NTVDM for ReactOS, but I'm not sure when I'd ever 
> have time.
>
> Windows NT's DOS subsystem, NTVDM, is a user-mode program that runs on top 
> of Win32.  It runs DOS programs as Windows processes, using v86 mode to run 
> the programs.  DPMI programs are supported by asking the kernel to allocate 
> LDT segments and running directly.  (If you write a DOS32 program and know 
> the correct addresses, you can call MessageBoxW in user32.dll and it will 
> work.)
>
> This differs significantly from DOSBox, which is effectively a machine 
> emulator rather than an API translator.  DOSEmu, in comparison, is 
> essentially the same design as NTVDM but for Linux.
>
> FreeDOS in DOSBox works very well, because the FreeDOS kernel is its normal 
> self and is unaware that it's inside a VM.  However, this doesn't work for 
> NTVDM/DOSEmu.  In these, the FreeDOS kernel will need to be heavily 
> modified, particularly because the NT kernel handles file I/O.  Programs 
> inside the virtual DOS environment use illegal opcodes to talk to NTVDM.

FreeDOS works just fine in DOSemu. Actually that is how it got debugged by the kernel maintainer who was also the DOSemu maintainer. But no code of FreeDOS is actually special for DOSemu.

The invalid opcode handler is probably similar to int e6 in DOSemu. It is an extra API by which DOS software can use services of DOSemu.

What probably is needed is an implementation of the int 21 interface, and maybe others, in the V86 monitor.

For what I understood from reading a litle on the web is that it more or less works like this:

You have v86 code in the kernel. This can be mapped using memory paging to point to a certain address, where a DOS program is loaded.

When an interupt occurs, the V86 mode is exited (the only way a V86 task can be exited) and the V86 monitor, in this case ntvdm takes over. This looks at the state of the interrupt and acts upon this. For example when calling an int 16 function it may return the status of a key. After the interrupt has been serviced the registers are loaded with the right values, or memory is filled correctly, or something like that and the v86 task is allowed to run until the next interrupt. Of course the software APIs can also be serviced by the V86 monitor and this is probably what is done in windows.

ntvdm is special in that, unlike DOSemu, it serves as the V86 monitor for all the V86 tasks running in the session.

Imre
> Melissa
>
> ----- Original Message ----- 
> From: "James Tabor" [e-mail]
> To: "ReactOS Development List" [e-mail]
> Sent: Monday, August 28, 2006 14:19
> Subject: Re: [ros-dev] (Free)DOS subsystem
> > Imre Leber wrote:
> > > Hi,
> > >
> > > I guess i am new to this list.
> > >
> > > I have been a maintainer on the FreeDOS project for more then 7 years.
> > >
> > > I have also noticed that you are still looking into building a DOS 
> > > subsystem in reactos.
> > >
> > > After the release of FreeDOS version 1, I would like to see wether I 
> > > could not take up the task to having it running under reactos.
> > >
> > > My main idea involves using a port of DOSemu.
> > >
> > > Is there any documentation regarding the running of a DOS subsystem in 
> > > windows that I should study beforehand?
> > >
> > > Imre
> >
> > Hi Imre!
> > Welcome to ReactOS! 8^D
> >
> > The idea is still open.
> >
> > We've tested Qemu under ReactOS running FreeDOS at one time and it does 
> > work. Another project that
> > was tested, DOSBox, it works like DOSemu, http://dosbox.sourceforge.net/
> >
> > Running a dos-subsystem and docs? Maybe one of the other developers can 
> > help here. You can look at
> > the OS2 subsystem at http://svn.reactos.ru/svn/reactos/trunk/os2/ , it's 
> > incomplete, this would be
> > your best starting point. There is allot more to it. You need to dive into 
> > the code and start
> > reading. I do not recall any docs or books dealing with rolling your own 
> > subsystem. The best book
> > you can get for understanding Windows or ReactOS is, Windows Internals. 
> > Understanding the Csrss
> > subsystem would help with the os2 one.
> >
> > Anyway, some projects that could be good examples on how to do this, Posix 
> > subsystem,
> > http://sourceforge.net/projects/winntposix ,and running linux bins in 
> > windows,
> > http://sourceforge.net/projects/keow . One of our developers is currently 
> > working with CoLinux,
> > http://www.colinux.org/ .
> >
> > Sorry I couldn't help you more here.
> >
> > Thanks,
> > James
> >
> > Ros-dev mailing list
> > [e-mail]
> > http://www.reactos.org/mailman/listinfo/ros-dev
>
> Ros-dev mailing list
> [e-mail]
> http://www.reactos.org/mailman/listinfo/ros-dev

[Nate DeSimone]

Not that I'm trying to rain on anyones parade, but it would be really 
cool if for each program you could choose to run it in either an 
emulation environment or in a V86 mode (ala WinNT NTVDM.EXE,) that way 
older DOS games could be used.

Imre Leber wrote:
> > -----Original Message-----
> > From: Myria [mailto:[e-mail]]
> > Sent: Tuesday, August 29, 2006 10:14 AM
> > To: 'ReactOS Development List'
> > Subject: Re: [ros-dev] (Free)DOS subsystem
> >
> > I've wanted to write an NTVDM for ReactOS, but I'm not sure when I'd ever 
> > have time.
> >
> > Windows NT's DOS subsystem, NTVDM, is a user-mode program that runs on top 
> > of Win32.  It runs DOS programs as Windows processes, using v86 mode to run 
> > the programs.  DPMI programs are supported by asking the kernel to allocate 
> > LDT segments and running directly.  (If you write a DOS32 program and know 
> > the correct addresses, you can call MessageBoxW in user32.dll and it will 
> > work.)
> >
> > This differs significantly from DOSBox, which is effectively a machine 
> > emulator rather than an API translator.  DOSEmu, in comparison, is 
> > essentially the same design as NTVDM but for Linux.
> >
> > FreeDOS in DOSBox works very well, because the FreeDOS kernel is its normal 
> > self and is unaware that it's inside a VM.  However, this doesn't work for 
> > NTVDM/DOSEmu.  In these, the FreeDOS kernel will need to be heavily 
> > modified, particularly because the NT kernel handles file I/O.  Programs 
> > inside the virtual DOS environment use illegal opcodes to talk to NTVDM.
>
> FreeDOS works just fine in DOSemu. Actually that is how it got debugged by the kernel maintainer who was also the DOSemu maintainer. But no code of FreeDOS is actually special for DOSemu.
>
> The invalid opcode handler is probably similar to int e6 in DOSemu. It is an extra API by which DOS software can use services of DOSemu.
>
> What probably is needed is an implementation of the int 21 interface, and maybe others, in the V86 monitor.
>
> For what I understood from reading a litle on the web is that it more or less works like this:
>
> You have v86 code in the kernel. This can be mapped using memory paging to point to a certain address, where a DOS program is loaded.
>
> When an interupt occurs, the V86 mode is exited (the only way a V86 task can be exited) and the V86 monitor, in this case ntvdm takes over. This looks at the state of the interrupt and acts upon this. For example when calling an int 16 function it may return the status of a key. After the interrupt has been serviced the registers are loaded with the right values, or memory is filled correctly, or something like that and the v86 task is allowed to run until the next interrupt. Of course the software APIs can also be serviced by the V86 monitor and this is probably what is done in windows.
>
> ntvdm is special in that, unlike DOSemu, it serves as the V86 monitor for all the V86 tasks running in the session.
>
> Imre
> > Melissa
> >
> > ----- Original Message ----- 
> > From: "James Tabor" [e-mail]
> > To: "ReactOS Development List" [e-mail]
> > Sent: Monday, August 28, 2006 14:19
> > Subject: Re: [ros-dev] (Free)DOS subsystem
> > > Imre Leber wrote:
> > > > Hi,
> > > >
> > > > I guess i am new to this list.
> > > >
> > > > I have been a maintainer on the FreeDOS project for more then 7 years.
> > > >
> > > > I have also noticed that you are still looking into building a DOS 
> > > > subsystem in reactos.
> > > >
> > > > After the release of FreeDOS version 1, I would like to see wether I 
> > > > could not take up the task to having it running under reactos.
> > > >
> > > > My main idea involves using a port of DOSemu.
> > > >
> > > > Is there any documentation regarding the running of a DOS subsystem in 
> > > > windows that I should study beforehand?
> > > >
> > > > Imre
> > >
> > > Hi Imre!
> > > Welcome to ReactOS! 8^D
> > >
> > > The idea is still open.
> > >
> > > We've tested Qemu under ReactOS running FreeDOS at one time and it does 
> > > work. Another project that
> > > was tested, DOSBox, it works like DOSemu, http://dosbox.sourceforge.net/
> > >
> > > Running a dos-subsystem and docs? Maybe one of the other developers can 
> > > help here. You can look at
> > > the OS2 subsystem at http://svn.reactos.ru/svn/reactos/trunk/os2/ , it's 
> > > incomplete, this would be
> > > your best starting point. There is allot more to it. You need to dive into 
> > > the code and start
> > > reading. I do not recall any docs or books dealing with rolling your own 
> > > subsystem. The best book
> > > you can get for understanding Windows or ReactOS is, Windows Internals. 
> > > Understanding the Csrss
> > > subsystem would help with the os2 one.
> > >
> > > Anyway, some projects that could be good examples on how to do this, Posix 
> > > subsystem,
> > > http://sourceforge.net/projects/winntposix ,and running linux bins in 
> > > windows,
> > > http://sourceforge.net/projects/keow . One of our developers is currently 
> > > working with CoLinux,
> > > http://www.colinux.org/ .
> > >
> > > Sorry I couldn't help you more here.
> > >
> > > Thanks,
> > > James
> > >
> > > Ros-dev mailing list
> > > [e-mail]
> > > http://www.reactos.org/mailman/listinfo/ros-dev
> >
> > Ros-dev mailing list
> > [e-mail]
> > http://www.reactos.org/mailman/listinfo/ros-dev
>
> Ros-dev mailing list
> [e-mail]
> http://www.reactos.org/mailman/listinfo/ros-dev

[ShadowFlare]

Hmm, I'm not sure if it is really even necessary to directly support DOS
programs.  One thing I thought of that I should mention here is that
Microsoft doesn't even support DOS programs at all with their latest Windows
versions (Windows XP and Server 2003 x64 Editions).  However, that isn't
really a problem since there are alternatives to Windows' DOS support (which
wasn't really even that great in some ways in Windows 2000 or XP to begin
with anyway).

I don't know why, but for some reason I'm thinking it would just kind of be
a waste of time to implement NTVDM for ReactOS if the developers working on
it would be able to work on other parts of ReactOS.  Hmm, it might be at
least somewhat worthwhile to work on a NTVDM alternate that could run on
both ReactOS and Windows (possibly even x64 editions), though.

Well, just my opinions on the matter.

-ShadowFlare

On 8/29/06, Nate DeSimone [e-mail] wrote:
> Not that I'm trying to rain on anyones parade, but it would be really
> cool if for each program you could choose to run it in either an
> emulation environment or in a V86 mode (ala WinNT NTVDM.EXE,) that way
> older DOS games could be used.
>
> Imre Leber wrote:
> > > -----Original Message-----
> > > From: Myria [mailto:[e-mail]]
> > > Sent: Tuesday, August 29, 2006 10:14 AM
> > > To: 'ReactOS Development List'
> > > Subject: Re: [ros-dev] (Free)DOS subsystem
> > >
> > > I've wanted to write an NTVDM for ReactOS, but I'm not sure when I'd
>
> ever
> > > have time.
> > >
> > > Windows NT's DOS subsystem, NTVDM, is a user-mode program that runs on
>
> top
> > > of Win32.  It runs DOS programs as Windows processes, using v86 mode to
>
> run
> > > the programs.  DPMI programs are supported by asking the kernel to
>
> allocate
> > > LDT segments and running directly.  (If you write a DOS32 program and
>
> know
> > > the correct addresses, you can call MessageBoxW in user32.dll and it
>
> will
> > > work.)
> > >
> > > This differs significantly from DOSBox, which is effectively a machine
> > > emulator rather than an API translator.  DOSEmu, in comparison, is
> > > essentially the same design as NTVDM but for Linux.
> > >
> > > FreeDOS in DOSBox works very well, because the FreeDOS kernel is its
>
> normal
> > > self and is unaware that it's inside a VM.  However, this doesn't work
>
> for
> > > NTVDM/DOSEmu.  In these, the FreeDOS kernel will need to be heavily
> > > modified, particularly because the NT kernel handles file
>
> I/O.  Programs
> > > inside the virtual DOS environment use illegal opcodes to talk to
>
> NTVDM.
> > >
> >
> > FreeDOS works just fine in DOSemu. Actually that is how it got debugged
>
> by the kernel maintainer who was also the DOSemu maintainer. But no code of
> FreeDOS is actually special for DOSemu.
> > The invalid opcode handler is probably similar to int e6 in DOSemu. It
>
> is an extra API by which DOS software can use services of DOSemu.
> > What probably is needed is an implementation of the int 21 interface,
>
> and maybe others, in the V86 monitor.
> > For what I understood from reading a litle on the web is that it more or
>
> less works like this:
> > You have v86 code in the kernel. This can be mapped using memory paging
>
> to point to a certain address, where a DOS program is loaded.
> > When an interupt occurs, the V86 mode is exited (the only way a V86 task
>
> can be exited) and the V86 monitor, in this case ntvdm takes over. This
> looks at the state of the interrupt and acts upon this. For example when
> calling an int 16 function it may return the status of a key. After the
> interrupt has been serviced the registers are loaded with the right values,
> or memory is filled correctly, or something like that and the v86 task is
> allowed to run until the next interrupt. Of course the software APIs can
> also be serviced by the V86 monitor and this is probably what is done in
> windows.
> > ntvdm is special in that, unlike DOSemu, it serves as the V86 monitor
>
> for all the V86 tasks running in the session.
> > Imre
> > > Melissa
> > >
> > > ----- Original Message -----
> > > From: "James Tabor" [e-mail]
> > > To: "ReactOS Development List" [e-mail]
> > > Sent: Monday, August 28, 2006 14:19
> > > Subject: Re: [ros-dev] (Free)DOS subsystem
> > > > Imre Leber wrote:
> > > > > Hi,
> > > > >
> > > > > I guess i am new to this list.
> > > > >
> > > > > I have been a maintainer on the FreeDOS project for more then 7
>
> years.
> > > > > I have also noticed that you are still looking into building a DOS
> > > > > subsystem in reactos.
> > > > >
> > > > > After the release of FreeDOS version 1, I would like to see wether I
> > > > > could not take up the task to having it running under reactos.
> > > > >
> > > > > My main idea involves using a port of DOSemu.
> > > > >
> > > > > Is there any documentation regarding the running of a DOS subsystem
>
> in
> > > > > windows that I should study beforehand?
> > > > >
> > > > > Imre
> > > >
> > > > Hi Imre!
> > > > Welcome to ReactOS! 8^D
> > > >
> > > > The idea is still open.
> > > >
> > > > We've tested Qemu under ReactOS running FreeDOS at one time and it
>
> does
> > > > work. Another project that
> > > > was tested, DOSBox, it works like DOSemu,
>
> http://dosbox.sourceforge.net/
> > > > Running a dos-subsystem and docs? Maybe one of the other developers
>
> can
> > > > help here. You can look at
> > > > the OS2 subsystem at http://svn.reactos.ru/svn/reactos/trunk/os2/ ,
>
> it's
> > > > incomplete, this would be
> > > > your best starting point. There is allot more to it. You need to dive
>
> into
> > > > the code and start
> > > > reading. I do not recall any docs or books dealing with rolling your
>
> own
> > > > subsystem. The best book
> > > > you can get for understanding Windows or ReactOS is, Windows
>
> Internals.
> > > > Understanding the Csrss
> > > > subsystem would help with the os2 one.
> > > >
> > > > Anyway, some projects that could be good examples on how to do this,
>
> Posix
> > > > subsystem,
> > > > http://sourceforge.net/projects/winntposix ,and running linux bins in
> > > > windows,
> > > > http://sourceforge.net/projects/keow . One of our developers is
>
> currently
> > > > working with CoLinux,
> > > > http://www.colinux.org/ .
> > > >
> > > > Sorry I couldn't help you more here.
> > > >
> > > > Thanks,
> > > > James
> > > >
> > > > Ros-dev mailing list
> > > > [e-mail]
> > > > http://www.reactos.org/mailman/listinfo/ros-dev
> > >
> > > Ros-dev mailing list
> > > [e-mail]
> > > http://www.reactos.org/mailman/listinfo/ros-dev
> >
> > Ros-dev mailing list
> > [e-mail]
> > http://www.reactos.org/mailman/listinfo/ros-dev
>
> Ros-dev mailing list
> [e-mail]
> http://www.reactos.org/mailman/listinfo/ros-dev

[fd0man™—The Magical Floppy Man]

On Tue, 2006-08-29 at 19:34 -0600, ShadowFlare wrote:
> Hmm, I'm not sure if it is really even necessary to directly support
> DOS programs.  One thing I thought of that I should mention here is
> that Microsoft doesn't even support DOS programs at all with their
> latest Windows versions (Windows XP and Server 2003 x64 Editions).
> However, that isn't really a problem since there are alternatives to
> Windows' DOS support (which wasn't really even that great in some ways
> in Windows 2000 or XP to begin with anyway).
>
> I don't know why, but for some reason I'm thinking it would just kind
> of be a waste of time to implement NTVDM for ReactOS if the developers
> working on it would be able to work on other parts of ReactOS.  Hmm,
> it might be at least somewhat worthwhile to work on a NTVDM alternate
> that could run on both ReactOS and Windows (possibly even x64
> editions), though.
>
> Well, just my opinions on the matter.
>
> -ShadowFlare

Doesn’t NTVDM also handle WOW (Windows on Windows) sessions with 16-bit
Windows applications?  I seem to recall when I worked at AT&T having
(quite a few) applications that the only way they could be killed was to
kill NTVDM and another process (old[something]app or something?) and
then restart the application.  In any case, there are still quite a bit
of software applications that are available online that would make use
of these types of subsystems—and it would be nice, I think, to continue
supporting something that Microsoft does not, and improve upon it,
because that would just be another point in favor of getting people to
use free software.  Right?  :-)

— Mike

--
The fd0man™—The Magical Floppy Man! ([e-mail]) 
"One world, one web, one program"  —Microsoft promotional ad 
"Ein Volk, ein Reich, ein Fuehrer"  —Adolf Hitler

[ShadowFlare]

On 8/29/06, fd0man™—The Magical Floppy Man [e-mail] wrote:
> On Tue, 2006-08-29 at 19:34 -0600, ShadowFlare wrote:
>
> Hmm, I'm not sure if it is really even necessary to directly support DOS
> programs.  One thing I thought of that I should mention here is that
> Microsoft doesn't even support DOS programs at all with their latest Windows
> versions (Windows XP and Server 2003 x64 Editions).  However, that isn't
> really a problem since there are alternatives to Windows' DOS support (which
> wasn't really even that great in some ways in Windows 2000 or XP to begin
> with anyway).
>
> I don't know why, but for some reason I'm thinking it would just kind of
> be a waste of time to implement NTVDM for ReactOS if the developers working
> on it would be able to work on other parts of ReactOS.  Hmm, it might be at
> least somewhat worthwhile to work on a NTVDM alternate that could run on
> both ReactOS and Windows (possibly even x64 editions), though.
>
> Well, just my opinions on the matter.
>
> -ShadowFlare
>
> Doesn't NTVDM also handle WOW (Windows on Windows) sessions with 16-bit
> Windows applications?  I seem to recall when I worked at AT&T having (quite
> a few) applications that the only way they could be killed was to kill NTVDM
> and another process (old[something]app or something?) and then restart the
> application.  In any case, there are still quite a bit of software
> applications that are available online that would make use of these types of
> subsystems—and it would be nice, I think, to continue supporting something
> that Microsoft does not, and improve upon it, because that would just be
> another point in favor of getting people to use free software.  Right?  :-)
> — Mike

Heh, I still think that an actual NTVDM implementation might not necessarily
be the best solution, though.  Well, if it could run on x64 editions of
Windows then it would be fine, but if it couldn't, then that's why I was
suggesting an alternative.  Of course, the alternative still wouldn't have
to be just for DOS programs either, it could also be for 16-bit Windows
applications as well.

Hmm, if there could be an implementation with no kernel-mode code or at
least something that could somehow work when running AMD64 CPUs in a 64-bit
OS, then that would be a good thing to work toward.  Then if ReactOS was
later made to support the API for Windows x64, then it could have 16-bit app
support, while Microsoft's 64-bit OS doesn't. :)

-ShadowFlare

[fd0man™—The Magical Floppy Man]

On Tue, 2006-08-29 at 20:10 -0600, ShadowFlare wrote:
> Heh, I still think that an actual NTVDM implementation might not
> necessarily be the best solution, though.  Well, if it could run on
> x64 editions of Windows then it would be fine, but if it couldn't,
> then that's why I was suggesting an alternative.  Of course, the
> alternative still wouldn't have to be just for DOS programs either, it
> could also be for 16-bit Windows applications as well.
>
> Hmm, if there could be an implementation with no kernel-mode code or
> at least something that could somehow work when running AMD64 CPUs in
> a 64-bit OS, then that would be a good thing to work toward.  Then if
> ReactOS was later made to support the API for Windows x64, then it
> could have 16-bit app support, while Microsoft's 64-bit OS
> doesn't. :)

Well, as I understand it, in long mode the v86 Mode functionality of
32-bit protected mode systems is no longer available, though that does
not, I do not think, preclude creating a new NTVDM type system that is
compatible with the 32-bit current NTVDM.  The only real difference is
that you would more likely than not have to have the v86 processes
running in an actual virtual machine with a monitor, though it should
(in theory) be possible to do that and retain compatibility, possibly at
a slight loss of speed.

Of course, there are a whole slew of choices that are available then;
should it be a virtual machine, or should it perhaps have some sort of
dynamic recompliation mechanism that would "turn" it into a native
executable, perhaps even ahead of time?  IANAP (at least, not a good
one, yet) and so I am not sure what would really be the more feasible
option, and other decisions would need to be made after that.  But, yes,
it would be nice to have a "one-up" of sorts on Microsoft.

— Mike

--
The fd0man™—The Magical Floppy Man! ([e-mail]) 
"One world, one web, one program"  —Microsoft promotional ad 
"Ein Volk, ein Reich, ein Fuehrer"  —Adolf Hitler

[Myria]

> Hmm, I'm not sure if it is really even necessary to directly support DOS
> programs.  One thing I thought of that I should mention here is that
> Microsoft doesn't even support DOS programs at all with their latest 
> Windows
> versions (Windows XP and Server 2003 x64 Editions).  However, that isn't
> really a problem since there are alternatives to Windows' DOS support 
> (which
> wasn't really even that great in some ways in Windows 2000 or XP to begin
> with anyway).

The 2 main reasons to bother with them are to support old games and install 
programs.  DOS application programs like WordPerfect 5.1 have long since 
been replaced.  Games, however, never really become obsolete.

I think a large part of why Windows didn't run DOS programs too well was 
that they didn't implement what games needed.  Games need Sound Blaster, 
VESA, and joysticks.  These can be done, but they didn't bother.  I'm not 
blaming Microsoft - I doubt that spending time on it would've helped their 
bottom line.
> I don't know why, but for some reason I'm thinking it would just kind of 
> be
> a waste of time to implement NTVDM for ReactOS if the developers working 
> on
> it would be able to work on other parts of ReactOS.  Hmm, it might be at
> least somewhat worthwhile to work on a NTVDM alternate that could run on
> both ReactOS and Windows (possibly even x64 editions), though.

NTVDM is basically a "marketing" project.  If eventually ReactOS runs old 
DOS games better than NT, then that becomes a "selling" point.  Of course, 
that's a long way off...

NTVDM for Win64 is basically impossible for exactly one reason: Win64 x86-64 
does not implement the LDT.  It does "xor eax, eax \ lldt eax" during 
startup and that's the end of it.  NtSetLdtEntries is a stub, the trap frame 
doesn't have an LDT field, and the context-switching code does not have code 
to switch LDT values.

If you tried to implement it by patching the kernel, you'd run into 
PatchGuard.  If you break PatchGuard, your code would only work until the 
next second Tuesday.

Finally, even if you got around PatchGuard, Vista64 requires drivers to be 
signed, and such a system is antithetical to open-source.  People wouldn't 
be able to compile and run the code themselves (unless they used test 
signing, which would disable playback of HD-DVDs).

The issue that V86 mode doesn't work in Long Mode isn't a big deal.  Modern 
machines can run the programs that ran in real mode in an emulator faster 
than they ran on their original hardware.  DOS32 games require more 
performance, but you can run those directly.
> Well, just my opinions on the matter.
>
> -ShadowFlare

Melissa

[Peter Dolding]

http://sourceforge.net/projects/ntvdm  Requirement with Xp to install 
this stuff to get the NTVDM functioning anywhere near good.

NTVDM was never fully functional as it could have been from the starting 
line.

At least some of out NTVDM is aquireable from other opensource projects.

Peter Dolding
Myria wrote:
> > Hmm, I'm not sure if it is really even necessary to directly support DOS
> > programs.  One thing I thought of that I should mention here is that
> > Microsoft doesn't even support DOS programs at all with their latest 
> > Windows
> > versions (Windows XP and Server 2003 x64 Editions).  However, that isn't
> > really a problem since there are alternatives to Windows' DOS support 
> > (which
> > wasn't really even that great in some ways in Windows 2000 or XP to begin
> > with anyway).
>
> The 2 main reasons to bother with them are to support old games and install 
> programs.  DOS application programs like WordPerfect 5.1 have long since 
> been replaced.  Games, however, never really become obsolete.
>
> I think a large part of why Windows didn't run DOS programs too well was 
> that they didn't implement what games needed.  Games need Sound Blaster, 
> VESA, and joysticks.  These can be done, but they didn't bother.  I'm not 
> blaming Microsoft - I doubt that spending time on it would've helped their 
> bottom line.
> > I don't know why, but for some reason I'm thinking it would just kind of 
> > be
> > a waste of time to implement NTVDM for ReactOS if the developers working 
> > on
> > it would be able to work on other parts of ReactOS.  Hmm, it might be at
> > least somewhat worthwhile to work on a NTVDM alternate that could run on
> > both ReactOS and Windows (possibly even x64 editions), though.
>
> NTVDM is basically a "marketing" project.  If eventually ReactOS runs old 
> DOS games better than NT, then that becomes a "selling" point.  Of course, 
> that's a long way off...
>
> NTVDM for Win64 is basically impossible for exactly one reason: Win64 x86-64 
> does not implement the LDT.  It does "xor eax, eax \ lldt eax" during 
> startup and that's the end of it.  NtSetLdtEntries is a stub, the trap frame 
> doesn't have an LDT field, and the context-switching code does not have code 
> to switch LDT values.
>
> If you tried to implement it by patching the kernel, you'd run into 
> PatchGuard.  If you break PatchGuard, your code would only work until the 
> next second Tuesday.
>
> Finally, even if you got around PatchGuard, Vista64 requires drivers to be 
> signed, and such a system is antithetical to open-source.  People wouldn't 
> be able to compile and run the code themselves (unless they used test 
> signing, which would disable playback of HD-DVDs).
>
> The issue that V86 mode doesn't work in Long Mode isn't a big deal.  Modern 
> machines can run the programs that ran in real mode in an emulator faster 
> than they ran on their original hardware.  DOS32 games require more 
> performance, but you can run those directly.
> > Well, just my opinions on the matter.
> >
> > -ShadowFlare
>
> Melissa
>
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