Perhaps I wasn't clear enough with my last message.   Regrettably,
there is no support for the DECstation 5100 in the NetBSD kernel.
You can't boot NetBSD on it.

The fact that you got as far as you did indicates that it probably has
a somewhat compatible boot PROM, which means that getting started on a
port shouldn't be too terribly hard, but without some documentation
for the hardware, it'd be a long, slow slog indeed.

I'd certainly be willing to help in getting you going on this, but you
*must* track down some hardware documentation first.

			       _MelloN_