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

[Robert Armstrong]

>Is the RM and possibly Group Mark valid in a P or Q address and if so, what would be the 

>value in the address? Could the RM be a 0 and the GM (1111) be a 5 using modulo 10.

 

  My guess would be that an RM (or any other invalid digit) does nothing special when entered into the MARS, but that it would cause a MAR CHECK stop if or when it ended up in the MAR.   That means fetching instruction with an invalid P or Q digit wouldn’t cause any error, but attempting to reference that address would.  There are some instructions that don’t use the P or Q fields or don’t use them as memory addresses and for those instructions I’d expect that an invalid P or Q digit would be ignored.

 

  But you said it was a TD instruction and AFAIK that uses both operands as memory addresses.  I’d expect that to halt the machine with a MAR CHECK assuming the corresponding check stop switch was turned on.  That last part may be the key – a real machine wouldn’t halt on this error if the switch was off.

 

  That’s my best guess…

 

Bob

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