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EMM386 and VDS: Not Quite Working

by eaguyhn on 2/28/20, 1:11 PM with 5 comments

• by userbinator on 3/1/20, 6:17 AM

  I've always found DOS esoterica --- and how it behaves with Windows (from Windows/386 up through the 9x line) very interesting in that it's one of the examples of how virtualisation can be a powerful tool. Windows could multitask DOS applications by effectively virtualising the hardware, and the whole VXD driver model was based around the notion that DOS applications --- and even Windows ones via userspace driver libraries --- could continue to be written as though they accessed the hardware directly, but the virtual device driver (VXD) that runs in ring0 would catch the accesses and manage sharing the physical device between different "virtual machines".

  Of course it wasn't perfect in practice, but I thought it was a rather elegant solution to the problem of both backwards compatibilty and performance (since virtual devices often only needed to handle multiplexing the physical device, which could be as simple as saving and restoring state when switching between VMs, there is no additional API layer and applications could still act like they access hardware directly.)

• by chkaloon on 3/1/20, 7:05 AM

  Seeing terms like EMM386 and DesqView conjured up some strange flashbacks from the late 80s for me. Thanks for that!

• by anonsivalley652 on 3/1/20, 6:02 AM

  Virtual memory manipulation maybe making it look like magic. I would verify in a good emulator with complex, conditional breakpoints that the page tables weren't being modified and TSS wasn't being switched within one of these API calls.