For the impatient ones, who don't want to read: ntvdmpatch\doc\autobuild.txt should be the fastest way to get NTVDMx64 compiled.

Known defects

Please be aware that starting with Windows 11 22H2, support for 16bit Windows applications (i.e. Windows 3.11 applications) is no longer possible due to removal of crucial functions (UserRegisterWowHandlers) from USER32.DLL and the whole Win16 support relies on this. You can still run DOS applications without issues, but for Win16, please use WineVDM instead. You may want to upvote issue https://aka.ms/AAl858b in MS Feedback hub to convince Microsoft to put back these functions.

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 OR if Windows is in Testsigning mode, which therefore is a requirement for this all to work. Starting with Windows 11, "Secure Boot" cannot be disabled out of the box, therefore we use AppCert.DLL approach on Win 11 loader. Fortunately, the Windows 11 loader can also be used under Windows 10, so it is now possible to also use ntvdmx64 without Secure Boot. 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 invade the console server and inject into created console applications from there (WinEvent hook). 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 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 (Google and web.archive.org are your friends) if you want to try it out. Depending on the build flavour you choose, you also need the appropriate SDK package. 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. :-(

There are various "flavours" of NTVDM that can be built:

• old-src [!DO NOT USE!] The original NT4 NTVDM. Maybe the easiest version to build, but it doesn't have multilanguage and lacks some features like LFN This has been the default repository until Feb/2020 where development has been switched to MINNT tree. This build method therefore is DEPRECATED in favour of the MINNT