Odd memory error in PDP-11/04

Ethan Dicks ethan.dicks at gmail.com
Wed Sep 7 09:46:48 CDT 2016

On Wed, Sep 7, 2016 at 8:35 AM, Noel Chiappa <jnc at mercury.lcs.mit.edu> wrote:
>     > From: Ethan Dicks
>     > What's happening now . is when I change one location .. it
>     >  echoes across multiple locations...
>     > ...
>     > 1) Depositing any value is echoed 000020 later.
>     > ...
>     > Does this sound like a dodgy CPU, dodgy RAM or both?
> It could be either. One possible cause of the symptoms you're seeing is that
> address line A04 on the bus is being held to one value (high or low) by
> something on the bus, so that whether the CPU tries to set it to 0 or 1, it
> has no effect. Also, of course, there could be some fault in the CPU, so that
> when it tries to do something with address 0, it gets 020 (or vice versa).
> And similarly for the memory.
> I'd try to write a small (two instruction) loop that sets that address line
> high/low (e.g.:
>         5037    CLR     @#1020
>         1020
>         775     BR      .-4
> and look and see if that address bit is flickering on/off on the UA11 (it will
> be on, but dully; constant assertion is bright on, constant de-assertion is
> full off). If so, the problem is almost certainly in the memory; if not, it
> could be either.

Once I get the console ODT to show up again, I will try that.
Something appears to have died while I was powered-on and testing last
night and now, the run light goes off right away after hitting boot,
and I don't see the address lines or the data lines flickering.

>     > 2) Setting D10 in location 000000 results in D10 set in all the
>     > locations
> Sorry, didn't follow that? Did you mean that if you store 02000 in location
> 0, all other locations now report the 02000 bit set?

Only 04000, but, yes.  If I set that bit in location 0, or other
locations, it gets set in all locations.  If I clear that bit, it

I won't be able to enter 5037 / 1020 / 775... it will read back as
1037 / 1020 / 775 based on my previous work (depositing 1020 will
"turn" 5037 to 1037...)


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