I'd been having a weird problem trying to compile mkconf.c, as a first step toward rebuilding 6th Ed, and someone suggested that it might be a memory problem.  I dug out the XXDP disks I have, ran some diagnostics, and discovered that my illusions about my memory were ill-founded.  :-)  Fortunately, I had another MOS memory card, which I configured and installed.  Now cc completes with no problems, and I rebuilt the system!  It was weird that the system seemed to run fine, but a defect somewhere in the second 64k was causing a failure in cc. 
 
Of course, there's always a challenge.  Now I'm trying to build/install a filesystem on my big Plessey DD-11/80 drives; they seem something like an RP04, and use the CDC 9877 diskpacks, if anyone's heard of them.  I built my kernel using the hp driver, and that doesn't seem to work with it.  -- Ian