VAX-11's compatibility mode

[yadran at ucla-cs.UUCP]

I'm trying to use the vax-11' compatibility mode on an object code written for
a pdp-11/45 computer, and getting the message "file compiled with separate i/d
space".
Is there anybody out there who knows about the compatibility mode and the
meaning of that message?
What should I do about it?
Where can I get good information about the compatibility mode?

Thanks,
            Yadran