3/60+CG4 strangeness -- Explained!

[Gene Spafford]

A while back, I posted a request concerning running the X11R3 server
with Purdue+ patches on a Sun 3/60+CG4.  I had been having problems
with "bitrot" that no one else seemed to get, and I couldn't find the
reason why.

I found out why.

The 3/60+CG4 combo has a firmware bug not present in other systems.

Yup, you find this only on Sun 3/60's with a CG4, and the bug only
occurs if you are trying to do a 24 bit write  to the frame buffer
memory with the bfins instruction.  How's that for bizarre?  I was told
that I am the first person in the 2 years since the machine was
released who has found a way to exercise the bug.  

I don't know the exact cause of this bug, although I have some informed
speculation based on flags & bus writes.  I do know there is no way to
fix it in software -- it will require a hardware fix.  This means that
our soon-to-be-released set of Purdue/Purdue+ patches using inline
assembly must have a special set of macros for that particular
combination.  It also means that anyone with a Sun 3/60+CG4 cannot take
full advantage of the speedups we've developed.

The problem has been filed as Sun bug report 1016963.  If you have a
Sun 3/60+CG4, you can ask your field service creatures if an ECO has
been issued to fix the problem.   If enough people ask, maybe they'll
come up with one :-)

In the meantime, thanks to everyone who responded, including a number
of people at Sun who asked that I not mention their names  (either
because they're humble or because Sun penalizes employees for helping
customers?), folks at CSRI in Toronto who checked the code on lots of
different Sun configurations, and especially Rusty Sanders @ Megatek
who came up with a program that could reliably recreate the problem.

Watch comp.windows.x for an announcement of the Purdue 2.0 patches.
-- 
Gene Spafford
NSF/Purdue/U of Florida  Software Engineering Research Center,
Dept. of Computer Sciences, Purdue University, W. Lafayette IN 47907-2004
Internet:  spaf at cs.purdue.edu	uucp:	...!{decwrl,gatech,ucbvax}!purdue!spaf