On 10 Jul 2000, at 21:30, Frank R. Brown wrote:
> I agree with your comment about the vdm.  But what limitation
> in dos/windows would prevent multiple vdm's from running?
> nt has permitted multiple vdm's since its early versions.

For win95 to be sucessful, MS had to ensure maximum
compatibility with older apps and the ability to run on limited
hardware (4 MB of RAM etc).
A robust VDM mechanism breaks both these requirements.

> I've never understood the complete story.  But if dos/windows
> has complete pre-emptive multitasking, how can a single rogue
> app consistently hang the whole os?  (In nt, this *rarely* happens

There is a big difference in OS architecture.

For example, on win9x, the running application thread has access
to the memory space used by the system VM.
On NT, what appears to the application as the ring 3 system
DLLs are in fact just proxy stub forwarders (which pass the calls to
the separate, isolated system VM).

This is one of the reasons why on NT you don't have the OS
crashes, lockups etc that you get with win98/95.

There are other vulnerabilities, such as the 16 bit GDI code in
win9x that allow a single app to hang the system.



Gerard R Thomas
Port of Spain,  Trinidad and Tobago
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