[ros-dev] (Free)DOS subsystem
imre.leber at telenet.be
Tue Aug 29 19:18:02 CEST 2006
>From: Myria [mailto:myriachan at cox.net]
>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
>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
>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.
>----- Original Message -----
>From: "James Tabor" <jimtabor at adsl-64-217-116-74.dsl.hstntx.swbell.net>
>To: "ReactOS Development List" <ros-dev at reactos.org>
>Sent: Monday, August 28, 2006 14:19
>Subject: Re: [ros-dev] (Free)DOS subsystem
>> Imre Leber wrote:
>>> 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?
>> 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
>> http://sourceforge.net/projects/winntposix ,and running linux bins in
>> 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.
>> Ros-dev mailing list
>> Ros-dev at reactos.org
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