VxD-s technically allowed virtualizing devices, but only in a very limited way. The port IO instructions were easily intercepted but not much else. VxDs could u

VxD-s technically allowed virtualizing devices, but only in a very limited way. The port IO instructions were easily intercepted but not much else. VxDs could use page protection tricks to redirect MMIO but this was used sparingly. Really the only serious "virtualization" was in the interrupt handling. Code in Win9x that handles interrupts had to be 16-bit and located in the lower 640kb of RAM. Most VxDs worked like classic drivers, they encapsulated device-specific interfaces and provided more abstract application-level interfaces.