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.
Jon
