PDP-11/45 RSTS/E boot problem

Jon Elson elson at pico-systems.com
Tue Feb 5 10:45:46 CST 2019

On 02/05/2019 07:36 AM, Noel Chiappa via cctalk wrote:
> One would hope that the DEC KT11 diagnostic would check for this... but just
> to be thorough, we have in fact written a short diagnostic which stores every
> possible value in each UISA register and checks that it's correct. So unless
> there is some sort of pattern sensitivity (e.g. when A is in UISAm and B is in
> UISAn), that's not it.
The MMU has to have some adders in it.  One adds the offset 
for the segment's beginning physical address to the supplied 
address from the CPU.  The other compares the requested 
address against the limit (size) of the segment, to make 
sure it doesn't exceed the segment size.  Either this adder 
or the comparator could be faulty.  I'd guess the diagnostic 
tries a few patterns to test for gross failure of this 
circuitry, but since it involves memory on a system running 
a program, it may not be able to exhaustively test these 
adders and comparators.


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