After intensive head scratching and hair pulling (accompanied by appropriate curses and a bit of scope and logic analyzer work ;) I finally found the problem with my 8/A this afternoon. First, of course, it was necessary for me to learn exactly how an M8315 CPU card works! The 7483 adder I suspected was not at fault since the logic analyzer always showed the correct outputs vs. inputs. I finally figured out that if the AC incremented properly, which it did, and was not data pattern sensitive, which it was not, that that ruled out almost all of the register circuitry except for a few gates unique to the MA (address) lines. For the curious, it was a bad 74367 (8097) hex buffer chip at E104 - actually only one section was bad, pins 14 -> 13 (which drives the ADD IN 4 line). Although both output enables were sitting low, every output matched its respective input except one... Even though the bit 4 line was a logic 1 (0 volts on that internal bus), the output was stuck at +5V (logic 0), explaining why on the next CPMA load that bit would always clear itself. Eureka... There was no hard short found, and connecting that line through a milliammeter to ground confirmed that only 5 ma would bring it to below .1 volts, so that current was mostly due to the 1K pullup resistor. The inevitable conclusion was that the output of that 74367 section was not able to pull low at all. I managed to change the chip without destroying the board too much and now all addresses increment properly again. Simple TTY print and keyboard echo programs work too :) Incidentally, I have pretty much given up trying to make a workable image with more than one 2 Mb partition on my RL02 drive. Every attempt to build one and boot it either hangs SIMH entirely, or worse yet, causes it to halt simulation with an error message, for some reason that's beyond my understanding. What the heck, 2 Mb is more than enough just to play around with OS/8 and run text adventure games anyway. If anyone can figure this out, I can email you a copy of my (currently working) image for reBUILDing. Of course the system now still won't boot (I can hear the drive head move, and then the RL02 FAULT light comes on) but it's likely the OS/8 image got clobbered when the chip failed and the CPU started writing to odd places. You can't write-protect the pack either, since OS/8 swaps in and out frequently to maintain such a tiny "footprint" in core. Anyhow I'm leaving tonight for three weeks of vacation in sunny Crete so it can just wait until I get home! -Charles