PDP-11/45 RSTS/E boot problem

[Jon Elson]

On 02/04/2019 04:28 AM, Noel Chiappa via cctalk wrote:
>
> First oddity - the problem is dependent on the location of the command in main
> memory! If Fritz says "sleep 360 &", to run a trivial command in the
> background, and _then_ says 'ls' - it works (so we know the binary of 'ls' on
> disk is OK)! We _think_ this is because the process executing the 'sleep'
> takes up a chunk of main memory, and thus changes the location of the process
> executing the 'ls'.
>
>
OK, the classic Heisenbug.  Is this truly a fault given by 
the memory management system, or some other kind of fault 
(Unibus timeout or memory parity error)?  If really related 
to MMU, then maybe there is a bad bit in the MMU that is 
causing it to reference the wrong segment entry or somehow 
thinking the setting of that segment entry is invalid.  Does 
the MMU classify what the error condition was, or just 
assume the trap handler can figure it out be looking at the 
registers?

Anyway, is it possible to borrow an MMU from somebody else?  
I can easily imagine that the diags can't test every 
possible bit combination while the diags are ALSO running in 
memory.
So, a somewhat cryptic bug could go undetected.

If this fault could be caused by memory, then it may be a 
pattern-sensitive error, and ls is just the perfect pattern 
to trip it up.

Jon