> From: Fritz Mueller
> I've had a bit of time in front of the machine to repro this and take a
> look. What I actually see is:
> R0 177770
> R1 0
> R2 0
> R3 0
> R4 0
> R5 34
> R6 141774
> PC 000254
Argh. (Very red face!)
I worked out the trap stack layout by looking at m40.s and trap.c, and
totally forgot about the return PC (that's the 0444) from the call to
0001740 000013 141756 022050 000013 000000 000000 000000 000034
0001760 000444 000031 177760 000000 030351 177770 010210 170010
I clearly should have looked at core(V) in the V6 manual!
The R6 you have recorded is correct for just after the trap; that's
the kernel mode SP, which points to the top of the kernel stack,
in segment 6 (in the swappable per-process kernel area, which runs
So there is no R5 mystery, I was just confused. Back to the other two!
> From: Wayne S
> it might be a wonky filesystem. ...
> The corruption probably came because the entire disk was going bad.
This theory is contradicted by the fact (mentioned several times, including in
the message you were replying to) that doing a plain 'ls' bombs, but 'sleep
300 &; ls' works fine.
> From: Jay Jaeger
> This sort of situation, where DEC diagnostics run OK but UNIX has issues
> was reported to be not all that uncommon - to the point where the urban
> legend was that some DEC FE's would fire up Unix V6 as a sort of system
Amusing! Never heard that; our -11's were never under maintenance, so DEC FE's
never worked on them.
> Make a copy of ls, and see if the copy also fails
It acts just like the original; fails when run by itself, runs OK when 'sleep'
is also running (in the background).
> From: Bob Smith
> We finally had the cpu backplane replaced
Ow. Not an option for Fritz, I expect. (I dunno - anyone have a spare /45
> From: Paul Koning
> Is there any way to attach a logic analyzer to various data paths on
> this machine?
I had suggested to Fritz that the symptoms led me to believe that it was time
to deploy a LA, especially since the MM trap only occurs once between him
typing 'ls' and the process failing - i.e. easy to trigger on.
He offered me the options of look at the IR or at the UNIBUS - I opted for
the IR so we can see _exactly_ what the machine _thinks_ it is doing! No
report back yet, though.
> From: Jon Elson
> Does the MMU classify what the error condition was
Yes, there are a series of bits in SSR0 to indicate the particular error:
'non-resident', 'length', 'read-only', etc (and also the segment number the
error's from). As my message mentioned, we're seeing the 'length' error bit
on, and for segment 1 (which the instruction isn't using).
> is it possible to borrow an MMU from somebody else?
Fritz does have a spare board, but it has known errors. We haven't thought about
borrowing one yet, it may come to that.
> If this fault could be caused by memory
Well, _some_ of it could. E.g. if the 'jsr r5, csv' is read as 'jsr r4, csv',
dropping the '1' bit in the register number, that would explain the wonky R5
content - R4 does contain the 034 that should be in R5, I have just noticed.
But that doesn't explain the bogus MM trap. Although I suppose there could be
several different problems, all at the same time.
> does this machine have a cache?
Here's a question for someone who has been around long enough to
Why did none of the available PDP-11 Unixes support the TU-58?
I have looked at Ultrix-11, V7M and BSD 2.11 (didn't try 2.9
but I suspect if it isn't in 2.11 it wasn't in 2.9) and none
of them had support for the TU-58. Seems to me it would have
been a rather simple device to handle as it ran over a serial
I'm restoring a PDP-8/a with the help of some
friends. The CPU is now passing the MAINDECs I've
thrown at it. The memory is a modern semiconductor
board my friend Anders Sandahl made.
This machine is pieced together from several others
and the limited function panel I got does not match
the backplane I have.
My theory is the DEC simplified the design of the
boardto cut costs and simpler design is not
compatible. Mine is labeled (on the PCB):
"LIMITED FUNCTION BD.
And the one I need is:
However, the picture I have of the other is not so
good. I may have read the numbera wrong.
I would very much like to buy one to finish this