[Simh] Non-numeric character in IBM 1620 P & Q addresses

[Bob Supnik]

There's nothing conditional about the user documentation. The 1965 
reference manual says, "MARS (Memory Address Register Storage) Check 
Light. This light is turned on when a digit in MARS has a parity error 
or an invalid address. These errors halt the machine immediately."

Now the key question is, what's an invalid address? Earlier, the user 
documentation talks about detecting an address that's too large (high 
order digit). But an invalid bit combination? Here's what the Customer 
Engineering Manual has to say: "Near the end of each memory cycle, MAR 
is checked for invalid digits in the ten-thousands position and for odd 
parity in each of the five positions." Nothing about checking for 
invalid bit combinations.

So it appears to me that the simulator's definition of "valid" (or 
invalid) addresses is too strict. Invalid bit combinations are not 
checked explicitly.

Then the question becomes, what does the 1620 actually do with an 
invalid bit combination (with valid parity) in MAR? I can't understand 
the core memory matrix selection logic in the CE Manual well enough to 
figure that out. The decode process is supposed to yield exactly one of 
10 select wires for each digit position. For invalid bits, does it still 
yield just one? More than one? None?

/Bob