Problems with Ultrix on DECStation

[Aaron Sloman]

My colleagues who have been porting Poplog to the DECStation 3100
running Ultrix have have come across some serious bugs.

The worst is that the sharable (text) segment is over-writeable: Poplog
provides some "fast" procedures for system programmers. If accidentally
misused these can over-write the shared segment on the DECStation. This
should be trapped by the memory management system and an error
generated. On DECStation Ultrix this is not detected and instead the
shared file gets corrupted, so that other users can suddenly find their
programs misbehaving. Protecting the executable file does not prevent
this as the pager presumably ignores protection, for speed.

Poplog does not have this problem on any other operating system as
far as I know.

It would be interesting to know whether anyone else has met the problem
and whether there are work-arounds. Unfortunately the problem is not
easy to reproduce in a small program, so it may be related to the size
of page-tables. (However, Poplog is not all that large - the problems
arise with an image of not much more than one megabute.)

We have reported the problem to Digital, but they have apparently not
come across the problem.

Comments and suggestions welcome.

When the problem is fixed (presumably when Ultrix is fixed!) Poplog will
be available on DEC RISC machines with Ultrix, providing incremental
compilers for Prolog, Common Lisp, Pop-11 and ML in a common environment
with X11 interface.

Aaron Sloman,
School of Cognitive and Computing Sciences,
Univ of Sussex, Brighton, BN1 9QH, England
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