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</o:shapelayout></xml><![endif]--></head><body lang=EN-US link=blue vlink=purple><div class=WordSection1><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><div><div><p class=MsoNormal><span style='color:#1F497D'>></span>Is the RM and possibly Group Mark valid in a P or Q address and if so, what would be the <o:p></o:p></p></div><div><p class=MsoNormal><span style='color:#1F497D'>></span>value in the address? Could the RM be a 0 and the GM (1111) be a 5 using modulo 10.<span style='color:#1F497D'><o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'> 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.<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'> 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.<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'> That’s my best guess…<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Bob<o:p></o:p></span></p></div></div></div></body></html>