Certainly it's a burden to understand how every piece of xdm configure
files interact each other. 

> I am having a hard time figuring out how to get xdm working.  I can start
> an X session using xinit (after I start the X font server, xfs, and have
> the port 7100 set to xfs in /etc/services) where my 
>
> .xserverrc file contains: exec X -fp tcp/localhost:7100
> and
> .xinitrc file contains:   xterm -g 80x24+0+0 &
>                          fvwm
>
> My impression from the xdm man pages were that I needed the 
> /usr/X11R6/lib/X11/xdm/Xserver file to be the same as the .xserverrc file
> above, and the Xsession file to be much the same as the .xinitrc

No.  'Xservers' file should look something like;

:0 local /usr/X11R6/bin/X -terminate -fp tcp/localhost:7100

And you need not to modify the system-global 'Xsession' file at all.  What
you need is '.xsession' at your home directory.  At very least, you can
start an X window session without '.xsession' file because XDM will provide
you 'default (yes, least atractive) environment' with TWM & XTERM.

You can take traces and make analysis running XDM manually with
-nodeamon and -debug N options.  You would prefer to invoke the
debugging sessions via rlogin/telnet.

This question belongs to 'how to configure & fine-tune my X11 environment,'
rather than DECstation specifics. 

Tohru Nishimura
Information Technology Centre
Nara Institute of Science and Technology