Odd memory error in PDP-11/04

Noel Chiappa jnc at mercury.lcs.mit.edu
Wed Sep 7 07:35:11 CDT 2016

    > 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
	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.

    > 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?


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