Thanks to dsg's kind help, I now have the precompiled X distribution from
ftp.unit.no on my DECstation 3100. Unfortunately,

shardik# startx

couldn't open /dev/mouse

Fatal server error:
no screens found
XIO:  fatal IO error 32 (Broken pipe) on X server ":0.0"
      after 0 requests (0 known processed) with 0 events remaining.
      The connection was probably broken by a server shutdown or KillClient.
kbd_mode: not found
shardik# ls -l /dev/mouse
crw-------  2 root  wheel    8,   0 Jul 13 17:44 /dev/mouse
shardik# ls -l /usr/X11R5/bin/X
lrwxr-xr-x  1 root  wheel  8 Jan 26 01:20 /usr/X11R5/bin/X -> Xcfbpmax

Has anyone seen this before? It looks to me vaguely that there's no
X server for the video... what do pmaxes use for video? I was given it,
and I know next to nothing about the architecture. There's no special
frame buffer in it; it's just whatever the standard pmax color video
is. dmesg is out of sync with my kernel so I can't paste in the probe
data...

Thanks so much! It'd be really great to get X working.
	-Ben