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
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