Re: [ros-dev] DOS Subsystem.

[Jerry]

What I was thinking was to emulate the VGA bios/VESA by using DirectX or 
OpenGL calls (or possibly the Allegro Game Library).

I was more or less thinking of how to implement the VDM with security 
and stability in mind with a mininum set of things that the OS has to do.

Have you heard or read about the VIF (bit 19) and VIP (bit 20) flags in 
the EFLAGS register?


x86-64 in Long Mode does not have V86 mode. It probibly would be best to 
emulate the legacy x86 machine in x86-64 Long Mode

(maybe a bit off topic)
Hummm.......  maybe "x86-128" in "full mode" would loose 32 bit
support 
and drop 16 bit mode all together. If  this does happen then I see a 
trend comming.

KJKHyperion wrote:

>   TwoTailedFox wrote:
>
> >   If NTVDM/VDM is neither a service, nor a
> > Subsystem, then what 
> >  precisely is it?
> >   
> >  
>  a hack. A hack we don't have to support, since the VDM concept sucks, 
>  it has been used as a backdoor in the past and it remains a backdoor 
>  by design (direct hardware access for VGA, standard system calls 
>  bypassed in certain cases, own fault handling and other such niceties).
>
>  And intelligent emulation has worked much better despite theory would 
>  suggest a VDM to perform better - for one, most DOS applications are 
>  busy-waiting and when run in a VDM will continuously cause expensive 
>  context switches in a tight loop, as they raise interrupts, faulting 
>  into the kernel, bounced back to user mode, handled by the emulation 
>  layer which finally resumes the DOS code with another call into the 
>  kernel which switches back the CPU into V86 mode, give or take a 
>  context switch... this usually kills both the DOS application and the 
>  host system making both unusable. As opposed to an emulator that can 
>  simply recompile the DOS code into native code and execute it 
>  directly, with interrupts not being much more expensive than regular 
>  function calls, and with the chance to behave a lot more like real 
>  hardware, unlike V86 mode.
>
>  And this doesn't even take VGA emulation in account - Windows support 
>  for full-screen DOS applications is especially gruesome, adding a 
>  couple more context switches of the ugliest kind (TSS switching) to 
>  call into the real-mode 16-bit VGA BIOS. Again, emulation would be not 
>  only simpler, but also much faster, as the VGA BIOS virtually is dead 
>  code with performance that cannot even remotely be compared with what 
>  today's GPUs can do (more or less easily available through DirectX), 
>  and it's real-mode 16-bit code on top of that.
>
>  Also, Windows has successfully employed software emulation for DOS 
>  support, and the actual implementation of the user-mode VDM doesn't 
>  affect Win32 components relying on it, but it has a huge impact on 
>  kernel complexity and even security - software emulation is painless, 
>  has no special kernel requirements and is perfectly compatible. And 
>  makes for a self-contained implementation that can be easily 
>  customized, or completely removed from an installation or distribution 
>  without the need of a separate kernel build.
>
>  Finally, V86 support in the Windows kernel is virtually unused outside 
>  of the VDM, and completely inadequate for any kind of serious work if 
>  compared with V86 support on other operating systems.
>
>  In short, it's just ballast
>  _______________________________________________
>  Ros-dev mailing list
>  Ros-dev(a)reactos.org
>  http://www.reactos.org/mailman/listinfo/ros-dev
>