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Run Microsoft Windows NTVDM (DOS) on 64bit Editions
…nymore as CreateProcess hook was removed and so symbol update process never triggered anymore
Latest commit b45ffc3
Jun 17, 2018
What is it? =========== NTVDMx64 is a port of the well-known Windows NTVDM, which is used by windows to run DOS-Applications, to 64bit Windows, so that you can run your DOS-Applications on 64bit Windows too. This is a proof-of-concept that it is possible to run the NTVDM on 64bit Windows and Microsoft was just too lazy to port it over and have disabled it for unknown reasons. More specifically, it's a set of patches to the sourcecode of original Microsoft NTVDM from NT4 in order to work on 64bit Windows. Motivation ========== The first question that may come now is: Why porting NTVDM when DosBox is working even better and more accurate in x64 Windows. There are mainly 2 reasons: 1) DosBox doesn't have a good interaction with the windows console. Specifically it's not possible to start Windows Applications from within the DOS-programs, redirect their output, and other useful interactions that are possible with Windows' own NTVDM. Over time my DOS-Applications have grown to interact with various Windows Applications and Interfaces and that wouldn't work with DosBox for instance. 2) Simply as a proof-of-concept that it would technically be possible and to encourage Microsoft to maybe take my patches and incorporate them into future Windows releases in order to bring back proper NTVDM Support on x64 (still x32 is a lot faster because of V86 mode, but on machines with lots of RAM, which may be required to run x64, there may be use for this). How does it work? ================= I never thought that it would be possible at all, as NTVDM on Win32 uses V86 mode of the CPU for fast code execution which isn't available in x64 long mode. However I stumbled upon the leaked Windows NT 4 sourcecode and the guys from OpenNT not only released the source but also patched it and included all required build tools so that it can be compiled without installing anything but their installation package. The code was a pure goldmine and I was curious how the NTVDM works. It seems that Microsoft bought the SoftPC solution from Insignia, a company that specialised in DOS-Emulators for UNIX-Systems. I found out that it also existed on MIPS, PPC and ALPHA Builds of Windows NT 4 which obviously don't have a V86 mode available like Intel x86 has. It turned out that Insignia shipped SoftPC with a complete emulated C-CPU which also got used by Microsoft for MIPS, PPC and ALPHA-Builds. For V86 mode, Microsoft implemented some facility called "Monitor" which is the piece of Software that interacts with the V86 part in the NT Kernel. However they assumed that i386 build = built with Monitor, therefore the sourcecode had to be patched to distinguish between a i386 build with CCPU and one with MONITOR. But there are a lot of other pitfalls too, which are to overcome. One major problem is that NTVDM uses calls to the Console and CSRSS Subsystem which are only used by the NTVDM and are therefore not working out of the box on x64 systems. Also stuff like Page allocation granularity poses a problem to EMM memory management on x64 systems. And last but not least, the way NTVDM is invoked is rather complicated, because it is done by the loader code of kernel32/kernelbase which registers the NTVDM and the requested application with the console server and the builds a commandline to execute NTVDM with the appropriate parameters (when I have more time, I may write how this all works in more detail). But fortunately as there still are 32bit Versions of Windows, Microsoft still has the code available in their x64 Windows Versions, as they are compiling the same codebase as on Win32. This enables us to use these facilities also on 64bit Windows, but structures have to be converted forward and back between 32bit and 64bit structures as NTVDM is 32bit code and therefore runs inside WOW64. WOW64.dll generally has transformations for a lot of calls by 32bit applications, but Microsoft didn't implement the ones that are necessary for NTVDM (as they are just used by this one application). Additionally they broke some functions on x64 builds simply by introducing bugs or not properly converting them, i.e. for console graphics. So all these difficulties require the system to be patched in order to support the calls of NTVDM and let it load properly with the CCPU. One solution would be binary patches of the Windows system files, but this is problematic as these files can change with various upgrades and there is also a nasty Patchguard in Windows which doesn't allow system files to be modified. Therefore the way to go here is a DLL that gets loaded into every process and patched the operating system DLLs in memory right after loading the application. The patcher also has to replicate between 32bit and 64bit processes which is another challenging task. The method that has been chosen by me is creating 2 DLLs, one for 32bit and one for 64bit (as they required different code for replication and modification). These DLLs get loaded into every process as AppInit-DLL, which is a nice mechanism by Windows that allows DLLs to be injected into every GUI process. Unfortunately these DLLs nowadays have a signature enforcement in order to get loaded, but this can be turned off in registry. As far as I know this only works if "Secure Boot" is off, which therefore is a requirement for this all to work (however I don't know what this exactly is, I guess it is some UEFI-stuff). AppInit-DLLs only get injected to GUI processes by the CSRSS, but injection is also required into CLI processes as these are even more likely to launch a DOS application. Therefore the AppInit-DLL also has to hook CreateProcess() functions in order to determine if the called process is a Commandline process and if so, inject the process via remote thread injection. Of course this mess could be all avaoided if Microsoft would implement all the required patches from the AppInit-DLL into their WOW64 and into NTDLL (which wouldn't hurt anyway). Further technical details will be provided later. With the help of the mentioned AppInit hook loader it is possible to start the NTVDM when a user tries to launch a DOS-Application. The NTVDM.EXE has to reside within SYSTEM32 and SysWow64 directories, as it gets searched and called from 32- and 64bit appliations. As NTVDM.EXE is a 32bit application, all its support files need to be in SysWow64 directory as this is the SYSTEM32 directory seen by 32bit applications. Futhermore the way how the windows console works changed with Windows 7. In earlier times, this was all managed by the CSRSS, but nowadays it is done with the conHost.exe Console host process, which processes the calls to the console. The calls are transparent for the caller, so this shouldn't be a problem, but there are 2 different ConHost DLLs that implement 2 versions of the console server. Only version 1 of the Console supports direct console I/O by the NTVDM. On Windows 10 for instance, a registry entry forces the conhost to use Console V2, so this has to be changed to use V1 console so that NTVDM can interact with it, but this fortunately is just a matter of setting a registry key. Some registry keys also need to be set to show that the NTVDM is present and can get called. So this is all rather complex, but it sort of works (except DPMI-stuff, which doesn't seem to work too well or still crash for unknown resons) with some patches to the operating system. It's enough as a proof-of-concept and to use it for most business applications and even some games. Cool, where can I get it? ========================= Redistribution of the compiled NTVDM is probematic, as it is based on leaked Windows NT 4 sourcecode, which is (C) by Insignia and Microsoft, so I'm legally not allowed to redistribute the code. However I published the code and compiled versions of the loader that enables you to run the patched NTVDM, as this is my own code. As the OpenNT project released the sourcecode, copyright issues are up to them, I can just say that you need the old-src.trunk.r687.20150728.7z source package released by them and the old-sdk.trunk_r57.20150728.7z SDK package (Google and web.archive.org are your friends) if you want to try it out. I'm providing patchfiles that patch the sourcecode to compile your own version of NTVDM which works on x64, publishing patches shouldn't be a copyright issue, I hope. And therefore you can try to patch the source and compile it yourself. Of course it would be nice if Microsoft would do that themselves to provide x64 Windows users a NTVDM again, but I'm not very positive that this will happen, they seem to forget about us DOS-users. :-( Here is a short howto on how to compile this: 1) Prepare your machine according the the OpenNT project build instructions: a) Unpack contents of old-src directory from old-src.trunk.r687.20150728.7z to your working directory You do not need to extract the .svn folder in it b) Create directory nt\public\sdk in your working directory and unpack the contentes of old-sdk directory from old-sdk.trunk_r57.20150728.7z to it. You do not need to extract the .svn folder in it c) Ensure that you don't have w: drive already assigned and then setup w: drive to point to the working directory by typing SUBST W: . inside the working directory and then switch to W: 2) Now extract the ntvdmpatch directory from this source repository to the root of W: drive 3) Run the patch.cmd file inside the ntvdmpatch directory to patch the sourcecode tree accordingly 4) Run zSHtst.cmd to enter build shell where you then type: a) bld-ntos.cmd to build base components that are required for building NTVDM. errors after build can be ignored b) bld.cmd to build now patched NTVDM "Build error" in the end isn't a problem as long as ntvdm.exe is built. 5) Run mkrelease.bat to create a redistributable package in the release\ subdirectory 6) Pack release-Directory as installation package for target machine. How to install ============== Now you can install the NTVDM on the destination machine: Run install.bat so that all files will be installed on the target machine and Registry patches will get applied. Afterwards you may need to reboot the machine to get the AppInit-DLL loaded into explorer, but it should already get loaded into the next GUI process you start (if you don't have secure boot enabled which would prevent proper operation anyway), so if you start a DOS-Application from this process, it should also work. You must also ensure, that the machine is connected to the Internet during the first run of a DOS-Application, because the loader code needs to fetch symbols from the Microsoft Symbol server so that it can call OS internal functions in order to properly start NTVDM. When the symbols are once fetched for the current user, they won't be fetched again, until either the Temp-folder is cleared or the OS files are upgraded. Debugging ========= Name of debugger Master Yoda is. Compiling frontend in mvdm\softpc.new\debugger you do. Like Gandalf, debugger master Yoda smokes pipe in \\.\pipe\softpc Therefore writing REG_SZ Key named PIPE containing \\.\pipe\softpc to HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\WOW\CpuEnv you need. After running vdmdebug.exe, you will be enlighted by running vdmdebug. Do not call Yoda, Yoda will call you. Known bugs ========== Currently there are crashes with some DPMI programs like i.e dpmi.exe, but most programs should work, I hope. Tested on ========= Windows 7 x64 Windows 8 x64 Windows Server 2008 x64 Windows 10 x64 You are invited to improve this project to bring back DOS to Windows.