What is this panic?

[Bruce Nemnich]

In article <13700084 at uiucdcs.UUCP> irwin at uiucdcs.UUCP writes:
>To quote, "The 750s have a problem with translation buffer parity errors
>when running 4.2BSD, if the Rev 7 has not been installed. (tbuf panics)
>These errors go away if the machine is brought to Rev 7. In addition to
>this, 4.2BSD also has problems with cache memory parity errors, which
>also cause panic type crashes. These will be fixed with Rev 8, which will
>be available in the spring of '85. There are fixes in our software, both
>VMS and our version of UNIX, which gets around the bug, but not in 4.2."

He's being a little hard by half-blaming 4.2bsd for what is a DEC
hardware bug.  Pre rev 7 750s have a problem with tbuf par errors
regardless of the software they are running.  Some of the errors are
recoverable and some are not; the proportion and frequency depend on the
particular board set.

VMS does go through more contortions trying to recover from it; the last
I looked, it tries to recover, and if it can't, it terminates the
running process if in user mode or crashes if in kernel mode.  4.2
panics when it can't immediately recover.  My machine likes to take
simultaneous tbuf parity, bus, and cache errors, which will also cause
VMS to give up the ghost.

I can't speak for rev 7; DEC has failed to show up twice this week to
install it (but what's another few days, I was originally told to expect
it in August).  Actually, after going through 4 different L0003 modules,
we finally have one which rarely gives the tbuf problem.

There was also a bug in the original 4.2bsd tape in which a status word
was masked incorrectly to determine whether it was a tbuf error, causing
it to fail to try to recover on about half of the tbuf errors.
-- 
--Bruce Nemnich, Thinking Machines Corporation, Cambridge, MA
  ihnp4!godot!bruce, bjn at mit-mc.arpa ... soon to be bruce at godot.arpa