PDP-11/45 RSTS/E boot problem

[Jon Elson]

On 02/06/2019 09:11 PM, Noel Chiappa via cctalk wrote:
>      > From: Jon Elson
>
>      > I'm thinking it is bad memory. ... I think it is just a bad memory chip
>
> Nothing so simple, I'm afraid! The memory actually contains:
>
>    PA:171600: 016162 004767 000224 000414 016700 016152 016702 016144
>
> and it's _supposed_ to be holding:
>
>    PA:171600: 110024 010400 000167 000016 010500 010605 010446 010346
>
> This together with Fritz's discovery of that first 'bad memory' pattern _elsewhere_
> in the binary for the command makes it look pretty likely that some sort of other
> error has wound up with stuff being put in the wrong location.
>
OK, now it is starting to look like an address problem.  
That could actually be several things.
Possibly something going wrong in DMA, and the disk data is 
being written into the wrong place in memory.  If the two 
places the same data show up are related by some simple 
binary transposition, maybe under some cases a write to 
memory gets written simultaneously into two banks of the 
memory.  A memory interference test OUGHT to pick up 
something like that.  It could also be a bus problem, or 
something going haywire in the MMU.

And, one other possibility is that the duplicate data is a 
disk buffer or cache that was then copied to the location to 
be executed.

Jon