Re: [ros-dev] 2GB Boundary

[Robert Köpferl]

on x86 NTVDM is a VDM = Virtual Machine int that it virtualizes the processor 
and the memory. In contrast to simulate or emulate the whole thing.
Since it is a "normal" process it mustn't access physical memory as all
others. 
It however can enable the VM-Bit to enable v86-mode. This and some registered 
HW-Exceptoion-handlers enables NTVDM to emulate privileged instructions. Such 
instr in usermode (v86 is usermode) raises an exception. NTVDM then can manage 
it's own GDT, LDT and map pages around to finnish up the missing part of a 
x86-processor inside NT.
NTVDM as its pendant in OS/2 can boot a DOS. At least in OS/2 it was possible 
to start a "DOS-Box" from a floppy in A: ...
The Bios is specially made for such a "realmode"-Box. It invokes system calls 
instead to access HW.

Asking me, I'd ignore all this fascinating but difficult stuff which was good 
at 386's times and use the project DOS-Box on nowerdays machines.

Alex Ionescu schrieb:

>   Barubary wrote:
>  
> > >   This is
> > > not 100% correct. The lowest 64k of the process _virtual_ 
> > >  address
> > >  space is is reserved and protected in order to detect NULL pointer 
> > >  access. 
> >
> >
> >  Do you know how NTVDM in NT/2000/XP/2003 gets around this?  NTVDM 
> >  allocates this area somehow for creation of the V86 IDT.
> >
> >  Melissa
> >  _______________________________________________
> >  Ros-dev mailing list
> >  Ros-dev(a)reactos.com
> >  http://reactos.com:8080/mailman/listinfo/ros-dev
> >  
>  Are you sure about that?
>  
>  In any case, NTVDM is a completely separate subsystem that actually 
>  "Boots up" with its own ROM/BIOS and has its own memory handlers, ivt, 
>  idt, gdt, ldt, etc. It emulates everything it can, but sometimes it 
>  can't and it raises exceptions (which is why many DOS apps won't work). 
>  So, by being an emulator, it has full control over its internal virtual 
>  memory. However, I'm still not convinced that the actual ntvdm.exe 
>  process allocates, in Windows, the first 0x10000 bytes.
>  
>  Best regards,
>  Alex Ionescu
>  _______________________________________________
>  Ros-dev mailing list
>  Ros-dev(a)reactos.com
>  http://reactos.com:8080/mailman/listinfo/ros-dev
>