> > I suppose that you'll always need some basic support for a
> > disk device in the Mach kernel. Some other device drivers might be
> > moved out to
>
> Why would PCI bus have to be in the kernel? Why would you need support
> for a disk device in the kernel?
>
These are shared resources that are likely to have multiple processes
attempting to use them at the same time.
In a normal OS the kernel is responsible for assigning "ownership" of
these resources to a particular process based on some set of rules
(highest priority, shortest run-time, process that has been longest
dormant, etc).
But then again, I suppose a good user-space server that handled these
activities could perform much of the work.